75 1.8 manual; Surging deceleration and wavering idle!

[Rover Roy]

Has anyone experienced anything like surging deceleration and wavering idle between 600 & 1200 RPM. ????
When starting from cold the idle goes straight up to 2000 - 2500. Symptoms just started to show a few weeks ago so not related to the recent wet weather. Drain under EMU clear. Swapped the Idle air control valve off another 1.8 without fixing the prob. Dont know any Rover service in Hampshire. :confused: Any ideas?

[Keith]

A couple of snippets in reply to a similar question, I have never touched a 1.8 so don't know


"Part of the problem on no turbo 75s was the throttle body, If you have a plastic one, you have the problem part. My 75 non turbo had a plastic one and suffered from the same, it was changed for a "alloy" one and the problem was 90% gone. I think the problem was traced to the throttle body deforming when the air intake from the air cleaner was fitted to it, hence the throttle butterfly doesnt quite close, leaving the revs "hanging". The other 10% of the problem is engine tune related but far too complicated to explain. It is mostly to do with the fuelling during gearchanges which makes the revs stay a little higher to aid smooth gear changes."

And

"Once my TF was having the same symptoms but a bit worse, strange revs, up and down, and stalling, and doing that solved it!

On the F/TF it's turn ignition one click, fully press the loud pedal 5 times, turn off, then start.

Or something like that anyway.."

hth

[Rover Roy]

Are alloy throttle bodies expensive?

Is it easy to fit? Do i need to swap over all the butterfly, springs etc?

Is it worth trying for a used one?

[Christopher]

When I experienced that it turned out to be a dodgy HT lead, once changed it was sorted straight away.

[M47Rman]

Has anyone experienced anything like surging deceleration and wavering idle between 600 & 1200 RPM. ????
When starting from cold the idle goes straight up to 2000 - 2500. Symptoms just started to show a few weeks ago so not related to the recent wet weather. Drain under EMU clear. Swapped the Idle air control valve off another 1.8 without fixing the prob. Dont know any Rover service in Hampshire. :confused: Any ideas?

Not sure if this applies to the 1.8 K series, but I remember on the 1.4 K series R8, there was a problem with the idle switch, which I believe was located on the throttle pedal assembly, and it caused exactly the problems you are experiencing. It is possible I guess that the switch is now on the throttle body, and so it may or may not be related to the plastic throttle body issue that has already been mentioned in a previous reply.
Andy

[Rover Roy]

ordered a Haynes Rover 75 manual today. Hope it has a few hints about this prob. I dont quite understand what the ECU does with the throttle position sensor info. I thought the fuel injection process was NON variable?? Can replacing this be the fix?

[Simon]

ordered a Haynes Rover 75 manual today. Hope it has a few hints about this prob. I dont quite understand what the ECU does with the throttle position sensor info. I thought the fuel injection process was NON variable?? Can replacing this be the fix?

The ECU in your car is very clever. It continually adjusts the fuel/air ratio according to numerous variables to give the best output. The throttle position is just one of these many variables it monitors. For example, when you lightly press the pedal it knows you are after more economical output whereas if you push the pedal to the floor it knows your after the best power output.

[Rover Roy]

Thanks Simon D for the explanation.
If the ECU is making decisions based on the input from several different sensors then the ECU output could be influenced by any mal-functioning sensor. Question is, which one? (As we asay in the IT dept, GI=GO, garbage in = garbage out) Is there any literture avaiable on the ECU control program?
Is the ECU made specifically for Rover?

[r44712]

Is the ECU made specifically for Rover?

Yep and the firmware is calibrated to the engine.

My old 1.8T did this (had about 10K on it) and never saw it as a problem - took it as being what it needed to do to keep it going!

[M47Rman]

ordered a Haynes Rover 75 manual today. Hope it has a few hints about this prob. I dont quite understand what the ECU does with the throttle position sensor info. I thought the fuel injection process was NON variable?? Can replacing this be the fix?

The idle switch tells the EC that it should be running the engine on the idle map (tune), or on the part/full load map. If the switch is faulty and is not working at all , or is cycling between on and off, the ECU will be cycling from map to map, which will tend to give different levels of fueling, and cause surging, and or high idle speed.
Andy

[Rover Roy]

Recieved the Haynes manual tonight. A quick read through left me disapointed in the lack of explanitry detail on Engine management but page 4A-2 (1.8) does mention several sensors roles.

I think M47Rman has hit the nail on the head with the indecision of ECU mapping. I have already swapped the idle air valve ( with a used unit) without a fix which tends to suggest a sensor problem influencing the Part/full map. Ive checked engine response with rear window demist, and A/C and idle responds OK. As I see it that leaves the Power Steering sensor. Any ideas how to check it????

[Simon]

Not yet got my copy of Haynes manual (its an xmas prezzie, ya see) so this explanation is a bit of a stab in the dark...

If the power-steering sensor is malfunctioning then I would expect there to be a drop in engine revs at idle speed. The reason being the servos for the power steering require an awful lot of power (and I mean, an awful lot). If the ECU cannot compensate this power increase because the sensor is not telling it that you are turning the wheels then I would expect there to be a drop in idle speed when you turn the wheels.

As a way to check this, I say park, put car in neutral and leave all other loads off (ie, HRW, ATC etc all off) then listen to engine/look at rev counter while you turn the wheels (dry turning). If the engine speed noticiable drops then I would expect the power steering sensor could be faulty.

Of course, if this information is proved incorrect by a later post, then I do appologise :o

[Rover Roy]

Checked the P/S sensor using the method Simon D suggested. Idle response was as it should be so all OK. This looks like a process of elimination, but how many sensors are there left to check???? I have almost convinced myself that the throttle position sensor could be the culprit as the surging seems to be only at light throttle use. Took a look under the bonnet this morning and quickly realized that doing any work on removing the throttle body was beyond the ability of me & my tiny tool kit. Decided to check what i thought was the MAS sensor(Mass Air sensor, i.e. Density) Haynes describe it at a MAP (Manifold Absolute pressure) !!!
Guess what, the sensor that is located downstream of the rocker cover vent tube was soaked in oil!!! A quick clean with meths (not from the drinks cabinet) was all it needed. A testdrive of about five miles at low throttle use just now showed no signs of the symptoms experienced all week.

I shall post again on this thread later in the week as an update.
Thanks to all rovers who contributed, amaising what can be achieved with group effort and alittle lateral thinking. (cheap too)

[Simon]

Glad to hear eveything's ok now.

The throttle body can become soaked in oil residue from the blow-by from the pistons. Nothing to worry about unless oil is pouring out of the throttle body when you remove the upper part of the filter box.

As you say, cleaning the sensor with meth's or carb' cleaner shifts the muck and helps the engine out.

Let us know how things go!