Z3 M IDLE SPEED PROBLEM

[Glen]

I have spent several months trying to overcome an idle problem.car will only idle at 750rpm when stationary.rolling up to a stop it will always sit at around 1100 rpm as though it has a sticky throttle cable its just odd that as soon as you stop it drops immediately to 750 RPM.
Action so far to try and overcome the problem include new TPS ,MAF clean ,breather hoses checked and renewed where required , cold start bypass valve removed and cleaned, oil temperature sensor renewal,vanos rebuild, throttle bodies checked and cleaned.The car has never shown a problem on my diagnostic tool. It runs fine other than this ,performance is great its just this annoying little problem that I have been unable to rectify ,perhaps I may just have to live with it .
Any ideas anybody ?

 

[t-tony]

Could it be connected to the speed sensor Glen, either on the Diff. (if it has one?) or wheel speed sensors?

Tony.

 

[Glen]

Hello Tony
I was beginning to lean towards the ecu ,
but mine is not an s54 which has dsc,electronic throttle etc
This being an early one it has no driver aids other than abs.
Thanks for the reply any suggestions are appreciated
Regards Glen

 

[handsomejackuk]

i think they all do that. my 1.9 does it too.. i think it must be due to when the car is moving as when the car stops it dips to lower revs to save fuel...

thats my take on it anyway...

 

[Althulas]

I had a similar problem and was down to the top connection to the vanos I must have caught the cable.

 

[Jack Ratt]

I had a similar problem with my Merc and it turned out to be a sticky micro switch on the throttle body. I only found it by accident.

 

[Pingu]

When you stop the car from high revs it does rev at around 1100rpm for a short while (less than half a second). It then drops to approx 800rpm. It is almost imperceptible. I think that the reason is that the VANOS is in the wrong position to allow the engine to idle smoothly. The delay is to allow time for the VANOS to return to the idle setting.

Also, the engine runs at around 1100rpm when the engine first starts and then returns to around 800rpm. When the engine is cold, the engine holds 1100rpm until it is warm enough to drop to 800rpm. When you come off high revs and go to idle, the ignition shuts off the spark to each cylinder individually. Sometimes all six cylinders are shut-off - in effect the engine is off. There is a possibility that when the ignition restarts, the ECU sees this as an engine start and goes through the 1100/800 algorithm. There will be a number of sensors that feed the ECU to tell the engine to return to 800rpm. A possible cause of your symptom could be that one of the sensors (or wire to it) is broken.

Because of the many possible causes of a symptom, the best way to diagnose ANY engine fault on an M engine is to use INPA. When you see the fault code, use TestO to see how the data is changing in real time.

 

[Glen]

When you stop the car from high revs it does rev at around 1100rpm for a short while (less than half a second). It then drops to approx 800rpm. It is almost imperceptible. I think that the reason is that the VANOS is in the wrong position to allow the engine to idle smoothly. The delay is to allow time for the VANOS to return to the idle setting.

Also, the engine runs at around 1100rpm when the engine first starts and then returns to around 800rpm. When the engine is cold, the engine holds 1100rpm until it is warm enough to drop to 800rpm. When you come off high revs and go to idle, the ignition shuts off the spark to each cylinder individually. Sometimes all six cylinders are shut-off - in effect the engine is off. There is a possibility that when the ignition restarts, the ECU sees this as an engine start and goes through the 1100/800 algorithm. There will be a number of sensors that feed the ECU to tell the engine to return to 800rpm. A possible cause of your symptom could be that one of the sensors (or wire to it) is broken.

Because of the many possible causes of a symptom, the best way to diagnose ANY engine fault on an M engine is to use INPA. When you see the fault code, use TestO to see how the data is changing in real time.

 

[Glen]

Hi All
Thanks for your replies.what I can't understand is why the rpm will hang at around 1100rpm as I coast to a stop in neutral and will drop to around 800rpm at exactly the point I become stationary every time without fail .I have put a diagnostic tool on the car and it is fault free as a point of reference TPS shows 0.46 v at idle .If I sit the car in the garage and increase RPM with the engine warmed up all is fine .The problem only occurs in real time driving.
Thanks again
Regards Glen

 

[Jack Ratt]

I'm not a techy and don't profess to understand all the modern systems on our cars, however I am an engineer, and may be as the car is still moving the ECU is telling the engine to run a little faster to build up pressure for the braking system.
As soon as the car stops, the ECU tells the engine to slow down because it no longer needs the higher pressure in the braking system.
The wheel speed sensors will send signals to the ECU saying if the car is stationary or moving.
There's nothing wrong with your car, it's just doing what it's supposed to do.

 

[the Nefyn cat]

Engine speed doesn't really affect brake pressure, the assistance is vacuum based off the inlet manifold. A fast idle surely means that the throttle body is open a bit and therefore not generating as much of a depression in there. Have you run anything on it (INPA etc) that'll show you the throttle position in real time?

 

[Jack Ratt]

The faster the engine runs the more fuel it draws therefore it must increase the suction in the inlet manifold. It has to affect the brake pressure, that's what servo assistance does.

 

[Glen]

Hello all
I have no wish to cause friction between members as lets face it we are all like minded enthusiasts .But I did have a 2.8 before I bought the M and really liked the car but I can honestly say it did not exhibit this problem that I have now with the M variant.I have run a diagnostic tool live and seen the TPS voltage rise and fall to 0.46V at idle.What I can't understand is why it drops to 800rpm as soon as I stop ? If it was an S54 perhaps the wheel sensors might come into the conclusion but these earlier ones are not fitted with the far more complex driver aids, something that I deliberately tried to avoid due to the fact its something else that could go wrong !
Thanks all again any suggestions are appreciated.

 

[handsomejackuk]

Hi All
Thanks for your replies.what I can't understand is why the rpm will hang at around 1100rpm as I coast to a stop in neutral and will drop to around 800rpm at exactly the point I become stationary every time without fail .I have put a diagnostic tool on the car and it is fault free as a point of reference TPS shows 0.46 v at idle .If I sit the car in the garage and increase RPM with the engine warmed up all is fine .The problem only occurs in real time driving.
Thanks again
Regards Glen

i not saying this is the reason why but maybe it helps with smother changes between gears too.. as if it always dropped to 800 rpm the gear change would be less smooth... i know we dont all do slow changes and let the revs drop back to high idle but theres probably a load of other reasons too i didnt realise unitl a few weeks ago there was a sort of clutch delay in the clutch master cylinder...

 

[handsomejackuk]

be interesting to see how many other owners can confirm that this happens or maybe they have never even noticed.. i spotted this behaviour as soon as i started driving the car and thought it was a fault but dont think it is a fault and is a feature of the cars.... maybe other cars could have been remapped or have different original ecu software that maps this feature out..

On my car m44 1.9 that only has manual throttle I think this is done with the idle control valve, maybe with cars with electronic throttle by wire i.e. a servo controlled throttle the higher idle is done with that...

food for thought maybe different countries may have different emission standards...and dictate a lower idle speed reduced emissions..

the way i see it works is that it senses no car movement possibly by abs sensors or differential speed sensor then it just drops the throttle by closing the idle control valve more as its all closed loop its continuoulsy monitoring idle speed and making minute adjustments to the fuel injector pulses and idle control valve value to achieve a lower idle...

hope this is allowed from sister site... similar topic

https://www.zroadster.net/forum/viewtopic.php?t=43418

 

[Fraser]

I think it's an intentional setting on most BMW's to reduce emissions at idle.

I've only had 2, and my current Z3 (M44 1.9) does it, and a previous E46 325Ci I had did exactly the same.

When I first came across it, it used to concern me that it might stall when I came to a stop, but it didn't and I soon got used to it.

If it's causing the car to stall, then I'd get it checked out, but if it's not then I think it's meant to do it.

 

[Redline]

I’m guessing, but I think it may be a mechanism to stop the engine stalling to make sure it’s available to provide power while the car is moving. Is the flywheel mass lower on these cars to reduce lag when power is called? If so, the ecu will do some of the work.

 

[the Nefyn cat]

The faster the engine runs the more fuel it draws therefore it must increase the suction in the inlet manifold. It has to affect the brake pressure, that's what servo assistance does.

Much though I hate to be pedanticbut there's very little depression in the manifold with the throttle open compared to when it's shut, which is when the greatest vacuum is felt in the servo. Luckily, when the throttle is shut is when you're most likely to be using the brakes. I know, there is such a thing as left-foot braking, used to do it myself when racing, but that's very rarely called for in normal driving. Hope so, anyway.

 

[Lee]

Remember you have individual throttle bodys on your M, In the past on mine I had to give them little service and if you have a slightly sticky throttle cable that wont help.

If the revs stay higher while coasting to a stop it's possible you have a vacuum leak after the maf. Food for thought.

 

[Glen]

Hello Lee,
I thought pretty much the same .I have been all over it took the air plenium off ,found and replaced a split breather hose checked all the throttle bodies even slackened off the throttle cable all to no avail.I think the problem may be linked to the ecu just have to live with it and admit defeat, its not that bad, just a bit annoying.
Z4 coupe in progress ?