Diesel rev drop when braking on neutral

[rimpak]

So both my and friends rover acts strange when braking on neutral (bad habit, I know :) ) The revs drop to about 500~ rpms when braking on neutral gear. And then comes back to normal when releasing the brake pedal. This happens about 1 in 30 times, while on the other car it's more often.

The MAF sensor on my car is working good. The friends MAF sensor is faulty, but disconnecting it did not solve the problem.

Any thoughs?

[Jakg]

It's not a fault, it's a "feature".

http://www.the75andztclub.co.uk/forum/showthread.php?p=670049#post670049

[rimpak]

Thanks. Still a bit mistery to me, there are so many opinions about it :)

[Johnny2R]

Mine does something like this sporadically, and has done for years. It's definitely a fault but I've never been able to establish a pattern to exactly why and when it occurs. Sometimes I'll start off a journey and this will be happening, and then if I stop after 10 minutes, say, and start again, it will be OK again. What happens in my case is that the revs drop and the car shudders in a rather unpleasant way, as if some engine timing is out, and then it settles down to a normal idle.

[Jakg]

Mine does something like this sporadically, and has done for years. It's definitely a fault but I've never been able to establish a pattern to exactly why and when it occurs. Sometimes I'll start off a journey and this will be happening, and then if I stop after 10 minutes, say, and start again, it will be OK again. What happens in my case is that the revs drop and the car shudders in a rather unpleasant way, as if some engine timing is out, and then it settles down to a normal idle.
I can always reproduce it - drive along at 30, pop it in neutral and then brake to a stop. Just before you stop it'll drop to around 5-600 RPM and judder like crazy, and then go back to normal after a second or two.

[Mike Noc]

Yep, and if you let the revs reach idle speed before braking it doesn't do it.

[Sector-9]

Most likely emissions-related ECU strategy. If you wanted to test it, and had somewhere off-road to do so, disconnect the brake pedal switch and see if it makes a difference.

[Johnny2R]

I can always reproduce it - drive along at 30, pop it in neutral and then brake to a stop. Just before you stop it'll drop to around 5-600 RPM and judder like crazy, and then go back to normal after a second or two.

This is exactly what happens with mine. But as I say, only intermittently. Most of the time it doesn't do this, but every now and then it does, and I've never been able to work out what might be making the difference.

[COLVERT]

Something like this is happening.

When you slow with the car in gear the ECU cuts off the fuel supply.

The ECU monitors the engine revs and near tickover speed it re-commences to fuel the engine.

This might not be exactly the same revs every time.

If it cuts in a Fraction too late it might make the engine judder a little as it sorts itself out.---:eek::eek::eek:


Colvert.--;)

[COLVERT]

I can always reproduce it - drive along at 30, pop it in neutral and then brake to a stop. Just before you stop it'll drop to around 5-600 RPM and judder like crazy, and then go back to normal after a second or two.

I think that's to do with the disruption of the vacuum supply to the ECU. ( Electrically. )

Tickover produces very little vacuum from the pump.

Braking uses up this vacuum and thus upsets the ECU.--:eek::eek::eek:

When you stop the vacuum comes back up to normal and makes the ECU-:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D




Colvert.:getmecoat:

[Johnny2R]

So why might it be an intermittent problem on my car?

[Jakg]

So why might it be an intermittent problem on my car?

If your car is already idling when you brake, it doesn't.

If it's not, it does.

I don't know where the line inbetween - but that might explain it.

[Mike Noc]

I think that's to do with the disruption of the vacuum supply to the ECU. ( Electrically. )

Tickover produces very little vacuum from the pump.

Braking uses up this vacuum and thus upsets the ECU.--:eek::eek::eek:

When you stop the vacuum comes back up to normal and makes the ECU-:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D:D

Colvert.:getmecoat:

John the vacuum on our cars is only used for the brake servo and EGR valve. It doesn't make any difference to the ECU how much vacuum is available. :D

[Johnny2R]

If your car is already idling when you brake, it doesn't.

If it's not, it does.

I don't know where the line inbetween - but that might explain it.

No, the problem on my car is genuinely intermittent. Exactly the same driving behaviour will have different effects on different days. There's something which is influencing this, which I'd like to be able to put my finger on.

[COLVERT]

John the vacuum on our cars is only used for the brake servo and EGR valve. It doesn't make any difference to the ECU how much vacuum is available. :D

Could it be the EGR valve operating that alters the engine speed ???

[RogerHeinz57]

Hello guys, interesting thread this, guess what? my 75 has a similar habit, but irregular.
I have spoken to Phil T-4 about this, he has noticed the car doing this occasionally and he suspects the egr valve may be the cause. Although the egr valve has a fairly heavy sprung damper, if there is a disruption to the damping effect, the opening and closing may be happening too late in the proceedings.
He has recommended that I clean this out to a mirror cleanliness and ensure the damping action is smooth and uninterupted in its movements and clean all areas of the inlet tract that can be reached, also to check for vac leaks!
I still need to do this, everything is ready, but a bit too chilly and a lazy owner combination! Still runs like a dream so it can wait a bit longer!
Hope this may help somebody!
John H.

[DMGRS]

Could it be the EGR valve operating that alters the engine speed ???

I had no EGR on my first 75 Tourer, and it did it too.
It's probably an emissions-reducing feature, but I found it very annoying!

[Mike Noc]

Could it be the EGR valve operating that alters the engine speed ???

As Mat says John, even with the EGR disconnected it still does it.

If the EGR is sticking partially open that could affect the idle speed, but this drop in revs and then recovery is too controlled for that. At exactly the same time after the car stops the revs return to normal, and if you let it reach idle revs before braking the revs don't drop low.

[klarzy]

Not as simple as a short on the 12v to the lights, when braking the electrical load increases, stress on the alternator goes up and the revs drop?

[Dorset Bob]

I just think that it the car telling you that this is bad driving practice. ;)

[SD1too]

Are you all discussing this (http://www.the75andztclub.co.uk/forum/showthread.php?p=1557127#post1557127)?

If so, as you can see, it's not a problem but deliberate "fuel reduction strategy".

Simon

[Mike Noc]

The discussion was regarding manual cars Simon, but I think you are spot on and it is all part of the fuel reduction strategy.

[COLVERT]

The discussion was regarding manual cars Simon, but I think you are spot on and it is all part of the fuel reduction strategy.

POST number 9 maybe ???--:bowdown::bowdown::bowdown:

[Mike Noc]

Couldn't agree more John. :bowdown: