Revs drop while braking

[Jakg]

Wierd issue, this...

If I'm cruising along, and pop it out of gear, and start to brake, the revs obviously fall off - but rather than fall to idle (~800 RPM), it falls a bit further (~600 RPM), which makes the engine quite harsh (akin to being on the edge of stalling).

Once the car stops, the revs stay at ~600 for half a second and then it returns to normal idle.

And all of this is while out of gear :confused:

Any ideas? Car idles fine, if I pop it out of gear but let it coast it idles properly as well.

[COLVERT]

Wierd issue, this...

If I'm cruising along, and pop it out of gear, and start to brake, the revs obviously fall off - but rather than fall to idle (~800 RPM), it falls a bit further (~600 RPM), which makes the engine quite harsh (akin to being on the edge of stalling).

Once the car stops, the revs stay at ~600 for half a second and then it returns to normal idle.

And all of this is while out of gear :confused:

Any ideas? Car idles fine, if I pop it out of gear but let it coast it idles properly as well.

Why do you, 'Pop it out of gear and coast' ???? :shrug::shrug::shrug:

You are probably upsetting your vacuum pump by letting the revs drop and then braking which needs to use vacuum. :(:(:(

The decrease in vacuum upsets the calibration of the fuel input by the ECU. :mad::mad::mad:


Colvert. :D:D:D:getmecoat:

[COLVERT]

Not really a 'Weird issue' :D

More like weird behavior. PMSL. :bowdown::bowdown::bowdown:

[Jakg]

I highly doubt a loss of vacuum would cause the engines RPM to drop, when the brakes aren't connected to the engine as it's out of gear.

[crashmarks]

The vacuum brake servo is connected to both the brakes and the engine. It sounds to me like a vacuum issue.

[COLVERT]

The vacuum brake servo is connected to both the brakes and the engine. It sounds to me like a vacuum issue.
I second that. :bowdown::bowdown::bowdown:

[kiowa]

The vacuum brake servo is connected to both the brakes and the engine. It sounds to me like a vacuum issue.


I agree :hurray:

[John Hutch]

My son passed his test last year, and I was eventually banned from trying to teach him myself because things have changed since I passed my test (probably about 1960)
The main thing was that changing down when approaching lights etc is frowned upon - you are told to coast and brake, hence the probable reason behind the enquiry.
I was told that brake pads are cheaper to change than clutch or gearbox.
(I still go down the gears though!)
:D

[Mike Noc]

Me too!

Mike

[Tam]

Wierd issue, this...

If I'm cruising along, and pop it out of gear, and start to brake, the revs obviously fall off - but rather than fall to idle (~800 RPM), it falls a bit further (~600 RPM), which makes the engine quite harsh (akin to being on the edge of stalling).

Once the car stops, the revs stay at ~600 for half a second and then it returns to normal idle.

And all of this is while out of gear :confused:

Any ideas? Car idles fine, if I pop it out of gear but let it coast it idles properly as well.
Mines is doing exactly the same and i have seen it mentioned on here before, almost like the clutch doesn't disengage properly and the car gets close to stalling as you slow down. Don't know if related or not but my master cylinder for the clutch is shot as i've bled it twice in the last couple of months and still sometimes when you over use the clutch it sticks to the floor and engine has to be switched off to get it out of gear. Just ordered a new one today another lovely job to look forward to!!

[Robbberd]

Wierd issue, this...

If I'm cruising along, and pop it out of gear, and start to brake, the revs obviously fall off - but rather than fall to idle (~800 RPM), it falls a bit further (~600 RPM), which makes the engine quite harsh (akin to being on the edge of stalling).

Once the car stops, the revs stay at ~600 for half a second and then it returns to normal idle.

And all of this is while out of gear :confused:

Any ideas? Car idles fine, if I pop it out of gear but let it coast it idles properly as well.


Anybody ever get to the bottom of this , mines just started doing it yesterday ??????


weird ???:shrug::shrug::shrug::shrug::shrug::shrug:

[COLVERT]

I don't suppose you'll get an answer from the OP as he never did come back to tell us how he fixed the problem.
Maybe he ran out of brakes and had to use a tree to stop. :D:D:D




Colvert. ( Still plump for low vacuum though. ;) )

[Robbberd]

hi , how would that be checked ?
bearing in mind in a numpty at mechanincs lol

[stuartie007]

I don't believe it's a vacuum/popping it out of gear related problem. My car has been doing this for months, I don't pop it out of gear. It happens when idling at lights etc or sitting In traffic, many days when I start the car it idles low too.... It should idle at 625rpm but sometimes drops to 575, as if it's going to stall, but it doesn't. Yet to find out why my car has been doing this. My under bonnet stop went a couple of months ago so the in tank pump is probably knackered too, could this be causing a fuelling issue?

[Robbberd]

would , some injector cleaner etc etc help the issue , is it maf/map sensor related, egr is bypassed on mine too..

[Jakg]

I read somewhere this was some ECU anti-stall thing.

I never "fixed" it :(

[HarryM1BYT]

My son passed his test last year, and I was eventually banned from trying to teach him myself because things have changed since I passed my test (probably about 1960)
The main thing was that changing down when approaching lights etc is frowned upon - you are told to coast and brake, hence the probable reason behind the enquiry.
I was told that brake pads are cheaper to change than clutch or gearbox.
(I still go down the gears though!)
:D

As you slow you are supposed to stay in gear, not press the clutch, just use the brake alone. At the last moment before the engine stalls, you press the clutch and select what ever gear you need, or neutral if coming to a complete stop.

I was originally taught to use the gearbox to slow too, I now use a mix of methods - which ever suits the occasion. Generally I don't need to use the brakes much at all, relying upon planning well ahead.

[Robbberd]

As you slow you are supposed to stay in gear, not press the clutch, just use the brake alone. At the last moment before the engine stalls, you press the clutch and select what ever gear you need, or neutral if coming to a complete stop.

I was originally taught to use the gearbox to slow too, I now use a mix of methods - which ever suits the occasion. Generally I don't need to use the brakes much at all, relying upon planning well ahead.


even doing it as you have described , in that last instant as you clutch/brake /come to a stop... the revs drop quite low ( anti stall as someone mentioned ???), so low you think it will stall , and then they rise back up to idle..You still have to clutch when stopping , there is no escaping that one.
it is a weird one...

[Robbberd]

I read somewhere this was some ECU anti-stall thing.

I never "fixed" it :(
hmm , whats causing it to stall though ???
:(

[Jakg]

I think it sees revs dropping, not in gear, clutch not full engaged, your going to stall and does "something".

I think.

[BigBeere]

This happens to my 03 ZT 135. I just thought this was normal as my previous car, 58reg mondeo diesel, done exactly the same thing.

:shrug:

[HarryM1BYT]

even doing it as you have described , in that last instant as you clutch/brake /come to a stop... the revs drop quite low ( anti stall as someone mentioned ???), so low you think it will stall , and then they rise back up to idle..You still have to clutch when stopping , there is no escaping that one.
it is a weird one...

Other modern diesels I have driven, notice the car is still moving a increase the engine revs even when the car is in neutral. Revs hang a bit above 1000rpm, then fall to a normal tick-over as the car stops.

[Robbberd]

just been out in the car , noticing a bit of a "miss" every now and again, wonder if its related ???

[COLVERT]

Could be a fuel injector worn and giving a poor supply at low revs. The ECU picks up a potential stall and boosts the fuel supply to compensate. :shrug::shrug::shrug:

[denis]

Bottom line is it shouldn't do it. The revs should drop back to the adjusted idle and operation of the brakes or anything else shouldn't have an effect. Mine certainly doesn't play games like that.
If there is a so called vacuum problem and it affects the idle then it probably means the vacuum system has sprung a leak somewhere and is drawing in air when it shouldn't, which is destabilizing the idle.

[Mike Noc]

I don't think it's vacuum related. My daughter's Freelander TD4 had a split vacuum pipe that was so bad the brakes lost their servo assistance at low revs but the coasting idle was fine.

On the overrun the ECU shuts off fuel to the engine but has an anti stall strategy if the revs drop off. This appears to be what is happening.

So it could be due to the brake or clutch sensors on the pedals not registering so the ECU thinks the engine is being driven on the overrun when it isn't. I assume it's only manual cars that do this and not autos?

My car also does this, and the brake lights and cruise all work and shut off when they should, but there is a separate signal input from the brake pedal switch that goes directly to the ECM independent of the brake light and ABS ECU input. I wonder what would happen if this failed?

Mike










.

[Robbberd]

Just thinking , could it be a leak ? Just had a good route round engine and noticed bit of oil around intercooler hose , obviously o rings on way out .... ill have to order some from jules . But , could it be down to a pressure loss somehow ??? Just a thought .

[Robbberd]

I think it sees revs dropping, not in gear, clutch not full engaged, your going to stall and does "something".

I think.


ive read somewher it could be related to the cruise control switch ( is it still there if you havent got it ??) on the back of the pedal...

can anyone shed any light on this theory ??

[Jakg]

I have cruise - but it works fine.

[chris75]

Don't know about the diesel types, but on the v6 petrol versions it is worth dismantling the idle air valve and giving it a clean . This is mounted on the throttle body . Don't think the 1.8 still has one , although the 1.6 versions of the engine did !
Might help :shrug:

[Unclefista]

There's no IACV on the diesel as there's no throttle therefore no need to bleed air in so it will idle.

My car does this sometimes, not very often though, when coasting up to the lights in neutral, revs sometimes drop to 600-ish rather than the proper 650.
Noticeable when it usually idles at a rock steady 650rpm.

No idea why..

[James.uk]

The solution seems to be not to coast innit.... :shrug: :}
...

[Robbberd]

its not coasting though, it does it at the last instant that you put the clutch in ?????

more i look at it , beginning to think its the brakes ???? ,ie
clutch in then brake ... revs drop, lift off brake for split second an touch again , revs rise to norm ????



perplexed //:confused::confused::confused::confused::confused: :confused::confused:

[kissifer1972]

I am on my 3rd 75 CDT(i) and they have all done it. So long as it doesn't actually stall and the brakes work who cares?

Chris

[Robbberd]

Hi , I know what you mean , it all works etc , and to be honest theres a lot more needs doing to the car , Lol...

but it does sometimes really shake when doing it , revs drop quite low sometimes... its like having a really bad rattle you can find,, drives you mad..... like the voices in my head .....lol:drool4:

[kissifer1972]

LOL. I know exactly what you mean. It is annoying but I have just got used to it and had to stop it worrying me. It's another one of those diesel character traits.

[Trevor1975]

I thought that there is no connection between the engine air intake and brake servo on a diesel. It runs W.O.T. essentially so there is no significant vacuum when the engine idles - hence the need for a separate vacuum pump. :shrug:

It sounds more to me like the ECU is not governing engine revs when the car is moving, so it drops to a default fueling setting if the engine revs are below a set value (ay 1200 rpm or something similar). As soon as the car stops the ECU then takes over the governing function and maintains 780 rpm...

[Mike Noc]

I'm convinced this is part of the ECM fueling strategy. If you don't brake until after the engine has reached normal idle the revs don't drop any further.

If you brake before normal idle is reached the revs drop to an anti-stall idle, but if you then take your foot off the brake, let the engine reach normal idle and then brake again the revs don't drop further either.

The engine will also pick up to normal idle revs a second or so after you come to a halt, even if you keep your foot on the brake pedal. As this is repeatable the ECM would appear to be controlling the revs and letting them dip under certain conditions.

Would be interesting to find out why it is set up to do this though.

[pannono]

My first automatic Rover 75 diesel did the same.
When I started to brake everything was fine. Just before I stopped or in the moment when I stopped the idle dropped down suddenly with a rattle and shake and bounced back upper than normal. It was only a second or two; after that the idle went back to normal. It was like someone who has a breathing difficulty and suddenly cough or sneeze.
We used the car thousands of miles and I just used to it `cos no mechanic on the planet had any idea what could it be.
There were so many guessing and advise but all of them would have charged me a fortune.

[pannono]

Yall!
I forgot to tell ya everything is fine when the engine is cold. No rev dropping or bouncing, idle totally fine. As son as the engine temperature goes up to normal working temperature the play starts as I mentioned in my previous post.

[James.uk]

>>>when the brakes aren't connected to the engine as it's out of gear.<<<

Everything goes to the ECU, and that is the problem innit.

One must never upset the ECU..... :p:
...

[Mike Noc]

Yall!
I forgot to tell ya everything is fine when the engine is cold. No rev dropping or bouncing, idle totally fine. As son as the engine temperature goes up to normal working temperature the play starts as I mentioned in my previous post.

Another reason for it pointing to be part of the normal fuel strategy.

Everything goes to the ECU, and that is the problem innit.

Why is it a problem? It's just what the diesels do. :D

[Trevor1975]

I'm sure that the engine governer cannot be deployed with the brakes on whilst in motion as a safety feature. The brakes have to be able to stall the engine in case of an emergency... so a small pre determined fuelling, not affected by revs, would be applied by the ECU (Ie if the revs dropped the ECU would not inject more fuel to compensate when braking). This would be set to approximately the same speed as the governed idle. If anything isnt perfect however the revs may well be lower as a result - to be expected on an older car.... :shrug:

[Mike Noc]

There is no separate governor with these engines, just the fuel map in the ECM. And it does inject fuel to keep the engine at normal idle if you let the revs drop to normal idle before using the brakes. ;)

[Trevor1975]

There is no separate governor with these engines, just the fuel map in the ECM. And it does inject fuel to keep the engine at normal idle if you let the revs drop to normal idle before using the brakes. ;)

But the ECU acts as a governer without brakes on and goes to default minimum fueling when braking.

Try idling along in first or second gear and if you start up an incline the ecu tries to maintain 780 rpm (or whatever has been preset) by increasing the fuel injected....

Its obvious really. You cant have the engine fighting the brakes in an emergency...

With the brakes applied (and car moving), the ECU governer function (not a mechanical governor) is disabled and a default idle injection setting is held. This default setting can produce a variable idle speed depending upon engine conditions. A poor atomisation of fuel or extra load from the alternator, aircon, low compression etc etc can all affect then affect the un governed idle speed, as RPM is not compensated for by the ECU...

I'm sure I'm right. I'll try it on the way home :)

[pannono]

I think it is more than likely a fuelling issue.
A diesel specialist run a few diagnostic test on my car and all the fault codes were fuel or fuel pressure related. The car had no any other problem at all.
Quiet possible a dripping injector or any other injector or fuel pump fault as the diesel specialist said.

[Mike Noc]

But the ECU acts as a governer without brakes on and goes to default minimum fueling when braking.

Try idling along in first or second gear and if you start up an incline the ecu tries to maintain 780 rpm (or whatever has been preset) by increasing the fuel injected....

Its obvious really. You cant have the engine fighting the brakes in an emergency...

With the brakes applied (and car moving), the ECU governer function (not a mechanical governor) is disabled and a default idle injection setting is held. This default setting can produce a variable idle speed depending upon engine conditions. A poor atomisation of fuel or extra load from the alternator, aircon, low compression etc etc can all affect then affect the un governed idle speed, as RPM is not compensated for by the ECU...

I'm sure I'm right. I'll try it on the way home :)

Iceman it isn't a question of being right or wrong - just of getting to the bottom of why this happens. :D

There is no such thing as ungoverned idle - once idle is reached the ECM will keep the idle speed constant no matter what the normal load from the air con, power steering or alternator is.

When the engine is on the overrun the fueling is shut off, and the injectors will only start to fuel again as idle speed is approached.

You can check this for yourself if you put the radio on long wave with a weak radio station you can hear the injectors interfering with the radio. ;)

The only difference is if you brake while freewheeling before the engine has settled to normal idle - then it will go down to an anti-stall idle.

If you let it reach normal idle before you hit the brakes nothing happens at all - it remains at normal idle.

That's why it looks like the ECM is set up to do this to me.

I think it is more than likely a fuelling issue.
A diesel specialist run a few diagnostic test on my car and all the fault codes were fuel or fuel pressure related. The car had no any other problem at all.
Quiet possible a dripping injector or any other injector or fuel pump fault as the diesel specialist said.

The ECM can compensate for injector and fueling issues up to a point.

If it was a dripping injector or a pump fault then why do the revs pick back up to normal idle a second or two after the car has stopped?

And why don't the engine revs drop once normal idle has stabilised?

[Trevor1975]

Well I tried it in mine tonight (99v Cowley CDT with a 135 xpower remap) when moving (brakes on or off) it was idling around 793-806 rpm, as soon as the car stopped, straight to 778-780 dead consistant.

Braking, aircon, engine temperature 35 degrees up to 80 degrees, nothing made any difference to mine, only moving or not moving.

Also the 'governing function' of the ECU appears to be working with the brakes on too.

:shrug::shrug::shrug:

Mine has 155k on the clock. I have run 2 lots of injector cleaner through, given it a good few long drives and use the power often enough and have an EGR bypass. Either way it is the most consistent idling car I have ever owned and it runs really well. The OPs car should not be dropping revs... hope we get to the bottom of it.

[Jakg]

I don't think it's a fault with my car - as everyone else seems to have the same thing.

Try stopping from about 30, by putting the car in neutral and braking to a stop.

Just as you stop the RPM's should go down a lot.

[Trevor1975]

I don't think it's a fault with my car - as everyone else seems to have the same thing.

Try stopping from about 30, by putting the car in neutral and braking to a stop.

Just as you stop the RPM's should go down a lot.

As stated, when doing this in my car the revs increase (slightly) from 778/780 to 793/806.

But this is regardless of whether using the brakes or not.

The revs shouldn't be dropping. There seems to be something wrong. If the engine were to stall, you would loose PAS and brake vacuum from the pump :eek: I doubt it would idle at much less than 650 reliably...

[Mike Noc]

The revs shouldn't be dropping. There seems to be something wrong. If the engine were to stall, you would loose PAS and brake vacuum from the pump :eek: I doubt it would idle at much less than 650 reliably...

The engine starts from a cranking speed of around 200, so it won't stall at 650. ;)

I'm still not convinced anything is wrong here. This could just be part of the normal fueling strategy. :shrug:

[Trevor1975]

The engine starts from a cranking speed of around 200, so it won't stall at 650. ;)

I'm still not convinced anything is wrong here. This could just be part of the normal fueling strategy. :shrug:

Dont forget it fires from 200 rpm but is being driven through the compression strokes by the startermotor.

If it was idling unaided at 200 rpm it would almost certainly stop. I've stalled a few diesels, which seem to drop to 500 rpm or so then just stop dead, even though the clutch is only partially engaged. I dont know if this is an ECU thing or a flywheel / crank lack of inertia?

I've also hand started a number of 2 stroke diesel model aero engines, which can 'pop back' and even start backwards by firing slighlty b.t.d.c. I know its totally different (crank compression, cylinder porting, fuel air mix etc)

The point is the idle is set to 750 +/- 50 for a reason, smooth reliable idling for the engine set up as designed...

With a Massive flywheel it could be very smooth at lower idle rpm, but the bigger the flywheel, the more energy is lost when accelerating...

Either way, a low idle speed during braking still doesn't sound 'right' to me... it may be something very simple?

Perhaps a T4 session could bottom it out? :shrug:

[Mike Noc]

Perhaps a T4 session could bottom it out? :shrug:

Yes, even if it doesn't bottom it out it may well shed some light on it.

[brgcdti]

Surely this is exactly what you'd expect it to do ?.

1. It is not a vacuum fault as it's a diesel and the vacuum is provided by a pump and not the inlet manifold.
2. The engine ECU is receiving a road speed signal too , so as far as it's concerned it's not idling if the vehicle is moving, it's trying to provide engine braking when the throttle is released by lowering the minimum engine speed (without cutting out or full shut off on over run) and also provide easier gear changing, hence why when the vehicle comes to a complete stop the 'normal' idling speed is resumed.
3. Reduced emissions .

[Trevor1975]

Surely this is exactly what you'd expect it to do ?.

1. It is not a vacuum fault as it's a diesel and the vacuum is provided by a pump and not the inlet manifold.
2. The engine ECU is receiving a road speed signal too , so as far as it's concerned it's not idling if the vehicle is moving, it's trying to provide engine braking when the throttle is released by lowering the minimum engine speed and also provide easier gear changing, hence why when the vehicle comes to a complete stop the 'normal' idling speed is resumed.

On my car, which is running well, the default idling speed (default fuel setting for overrun, but under set rpm) gives me about the same idle speed as the 'governed' 780 when the car stops... I can see the difference when the IPK digital RPM is displayed but cant feel it or see it on the tachometer needle.

I think the rev drop on the OPs car is due to an inefficiency somewhere...

Blocked air filter? low compression? clogged injectors? etc... :shrug:

[brgcdti]

On my car, which is running well, the default idling speed (default fuel setting for overrun, but under set rpm) gives me about the same idle speed as the 'governed' 780 when the car stops... I can see the difference when the IPK digital RPM is displayed but cant feel it or see it on the tachometer needle.

I think the rev drop on the OPs car is due to an inefficiency somewhere...

Blocked air filter? low compression? clogged injectors? etc... :shrug:

My car does exactly the same and I put it down to CDTi and CDT .. mine has a switch on the clutch pedal but no cruise control, and although the same engine produces lower emissions due to finer tuning and fuel control. None of this is a 'new' idea.

[whitevanman]

Ours has always done it, when Phil gave us a healthcheck last year he noted that the HP fuel regulator was only allowing 530-550bar when it should have been 600Bar..

Car still runs fine but i'm just wondering whether this has an affect on low rev fueling, after all the ecu fires an injector for x milliseconds expecting 600bar to supply x amount of fuel, 530 bar maybe only supplies y amount of fuel. :shrug:

[Avulon]

Here's tuppence - Sticking/blocked EGR .


On the subject of coasting/braking/going down through the gears.

From an early age I had learned that the gear should be matched to the engine revs/road speed. Using the gears for braking was a hangover from early motoring and drum brake fade coupled with heavy vehicles.

On taking driving lessons in preparation for my driving test I was 'taught' to leave the gears alone when slowing down. This is probably a result of trying to prevent new drivers from learning to use the gears for braking which is bad for 'modern' (post 1960 ish) gearbox's and drive trains which were built to be much lighter than previously. However it does mean that you'll need to depress the clutch earlier when coming to halt and that should you need to accelerate you'll lose half a second finding the right gear - too slow in an emergency.

I find that the best method is to always be in a gear from which you can accelerate if required. Driving the diesel it's best to keep the engine in overrun (fuel cutoff) as long as possible while slowing down, so change down gear to prevent it dropping to idle speed as you slow, but not so soon as to cause the gearbox to do the braking. This is the most economic way to go, and will normally mean that the rev's won't drop below 11-12k revs while slowing: which as any cdt driver knows is enough to regain some speed in any of the first three gears just by squeezing the throttle a bit if you need it.

Coasting in neutral means that you'll have to match the engine revs if it turns out you don't have to actually stop and means that all the while you are coasting the engine will be on tickover (using fuel). Coasting with the clutch in (on the diesel) is just a quick way to ruin your clutch slave cylinder.

OK, that may be more than tuppence - but I'll let everyone else place any value on those words: it's just my opinion and what I've found / learnt.

If you're driving an auto ignore the above and just use the brake!

:getmecoat:

[Mike Noc]

Surely this is exactly what you'd expect it to do ?.

1. It is not a vacuum fault as it's a diesel and the vacuum is provided by a pump and not the inlet manifold.
2. The engine ECU is receiving a road speed signal too , so as far as it's concerned it's not idling if the vehicle is moving, it's trying to provide engine braking when the throttle is released by lowering the minimum engine speed (without cutting out or full shut off on over run) and also provide easier gear changing, hence why when the vehicle comes to a complete stop the 'normal' idling speed is resumed.
3. Reduced emissions .

:wot::iagree::wot: One other thing backs this up; if I let the engine revs drop to normal idle before braking then the revs don't drop any further.

This would suggest that the ECM is controlling the action and it isn't the result of a faulty component.

[Shuriken_57]

:wot::iagree::wot: One other thing backs this up; if I let the engine revs drop to normal idle before braking then the revs don't drop any further.

This would suggest that the ECM is controlling the action and it isn't the result of a faulty component.

The probem is the clutch pedale Switch.
Remove it ans try it , it will work.
But the problem IS if you have cruise control it will not work.
I have this problem on my two 75.
:furieux:

[Mike Trident]

I think 8 years on, they may have worked it out by now?

[Mike Noc]

The probem is the clutch pedale Switch.
Remove it ans try it , it will work.
But the problem IS if you have cruise control it will not work.
I have this problem on my two 75.
:furieux:

No it isn't - all you have done by removing the clutch switch is to disable the function, and as you say, disable the cruise control along with it. :getmecoat:


I think 8 years on, they may have worked it out by now?


Well I don't see it as a problem Mike, so nothing to work out. :}






.

[Shuriken_57]

I have two 75. I know this problem. The cruise switch is the problem . It's a setting problem because it's magnetic. Try to remove the switch and try to brake you will see the problem disappear.
Good bye

[Shuriken_57]

I have two 75. I know this problem. The cruise switch is the problem . It's a setting problem because it's magnetic. Try to remove the switch and try to brake you will see the problem disappear.
Good bye

But i don't know how to fit correctly the switch.
I have a cruise control on my second 75 and when i brake on neutral the problem come back.

[Mike Noc]

I think you have misunderstood what I posted. Yes maybe you can stop the revs dropping below normal idle if you unplug the clutch pedal sensor, but that doesn't mean the sensor is at fault or badly fitted, it just means the ECM sees the signal as missing and adjusts its fueling strategy accordingly.


The clutch pedal sensor behaves as a single switch input to the ECM, so it is either on or off.

The brake pedal has two signal wires, one going to the ABS ECU and light switch module, and the other directly to the ECM. If your brake lights work then the first signal must be working as well. So if you think you have a problem then I suppose you could check the second input that goes to the ECM.


As I have said before I don't see this as a problem, just the way the fueling strategy was programmed. That's only guesswork though. :}

[bl52krz]

Explained in layman’s terms. There is a speed sensor at the wheels. This communicates with the ecu. The ecu receives a message that the car is doing 20 mph but the throttle is telling the ecu that it only wants tickover revs. Result confusion of the ecu.So in effect the ecu does not know if it has to fuel for 20 mph or tickover revs. Hence the ‘stumbling’ revs. When the car slows down mph wise, to tick-over speed , the ecu then knows that fueling is required for tick-over speed, and resumes it’s normal duty. No secret recipe or dark forces, just two systems fooling each other.

[hogweed]

Explained in layman’s terms. There is a speed sensor at the wheels. This communicates with the ecu. The ecu receives a message that the car is doing 20 mph but the throttle is telling the ecu that it only wants tickover revs. Result confusion of the ecu.So in effect the ecu does not know if it has to fuel for 20 mph or tickover revs. Hence the ‘stumbling’ revs. When the car slows down mph wise, to tick-over speed , the ecu then knows that fueling is required for tick-over speed, and resumes it’s normal duty. No secret recipe or dark forces, just two systems fooling each other.


Aha... is this why my car gently "blips" the throttle when I'm coasting...?


I drive in a very relaxed fashion these days (Except when in the TF :devil:) and often knock the car into neutral when approaching lights or whatever, sub-30mph, and my rpm rises and falls slightly as I slow down, like I was regularly touching the throttle slightly...

[hogweed]

Actually... thinking about it, it hasn't done it since I got the front shockers replaced about 2 weeks ago. I wonder if it was to do with the ABS sensor getting disturbed :shrug:

[marinabrian]

It only occurs with cars fitted with non EOBD compliant ECM fitted, ie without the boost temperature sensor fitted in the top hose, and as the author of the thread had a ZT fitted with an NNN100692, that would make it perfectly normal for his car ;)

Petrol cars behave differently when the vacuum in the inlet tract decreases when the brakes are applied, however the ECM will adjust the position of the IACV to compensate.

Brian :D

[hogweed]

...as the author of the thread had a ZT fitted with an NNN100692, that would make it perfectly normal for his car


Hhmmm, think I've got one of them, with Brian's writing all over it :D

[marinabrian]

Hhmmm, think I've got one of them, with Brian's writing all over it :D

Nope, you have an NNN500340 covered in my scribble Roger ;)

Brian :D

[hogweed]

Nope, you have an NNN500340 covered in my scribble Roger


Well I got the letters right :duh:


I can see it all now, years from now on the Antiques Roadshow "Eeeeh, this takes me back. This comes from a long-lost relic called the Rover 75 tractor. And… what do we have here, it’s been autographed by the legendary Marinabrian – it was once said that he had nine Morris Marinas in his garage, but I’m sure it’s been exaggerated out of all proportion over the years. Well well well. For such a rare signed specimen, I’d expect it to fetch anywhere up to £20,000 at auction…”