Car hesistates at idle when putting the trottle down

[BigKriss]

Symptoms are, at the the traffic lights in drive, normal idle is 700 - 800 rpms. when i put throttle down, it goes to about 500rpms almost stalls and then pulls away fine again. it only happens at idle and not elsewhere. what can i check / do to fix this problem. i think it's afm (air flow meter) related, but i'm not sure and i've never mucked around / cleaned the afm before.

I searched through he old posts here http://www.bimmernut.com/forum/search.php?searchid=531698&pp=25 but couldn't find much.

cheers :)

[tim eh?]

Symptoms are, at the the traffic lights in drive, normal idle is 700 - 800 rpms. when i put throttle down, it goes to about 500rpms almost stalls and then pulls away fine again. it only happens at idle and not elsewhere. what can i check / do to fix this problem. i think it's afm (air flow meter) related, but i'm not sure and i've never mucked around / cleaned the afm before.

I searched through he old posts here http://www.bimmernut.com/forum/search.php?searchid=531698&pp=25 but couldn't find much.

cheers :)

i have this but only when it's just been started.
i wonder about the fuel pressure regulator sometimes but i really have no idea.

[BMW4LIFE]

mine does the same thing if i dont drive the car for a couple days right at start up...after 10 sec...its totally fine

[Jeff N.]

Maybe try cleaning the carbon wiper on the AFM. Wouldn't recommend mucking with the spring tension. Seems to make things worse overall.

[BigKriss]

I very much think it's afm related. around 30% throttle from idle, the car almost stalls. what product do you I use to clean the carbon wiper on the afm Jeff?

[632 Regal]

Wrong Jeff but you can use spray carburetor cleaner, very safe.
I very much think it's afm related. around 30% throttle from idle, the car almost stalls. what product do you I use to clean the carbon wiper on the afm Jeff?

[BigKriss]

okay, thanks for that.

[Bill R.]

mixture across the entire range, if its a little too lean it will do what yours is doing, look for cracks or leaks in the hose between your afm and the throttle body, if none are found i would readjust the idle mixture with the idle air bypass screw. a little bit makes a big difference so its better if you hook it up to an exhaust gas analyzer while doing it since you can see small changes there easily. A poor connection at the coolant temp sensor will also do this. Check and clean connections

[Jeff N.]

Hello Bill!

Yes - those are better suggestions and simpler than cleaning the AFM wiper area. I can second that if the car has a slight vacuum leak and is running a bit lean, it will definitely have problems on throttle tip in. Funky tracks in the AFM could cause a similar response but these would be more likely.

[Rus]

I had the same symptom of hesitation off the line before I replaced the crankshaft position sensor. Upon replacement, the symptoms completely disappeared. The CPS was the only change....because it failed on me. My guess is that an old CPS doesn't provide a clean enough signal and the ECM doesn't respond properly to the throttle opening. Worth a look if the other suggestions don't work.

[BigKriss]

Okay, I had a crack at it today. I checked the intake rubber boots, no visible cracks there. I opened the maf on the bottom and cleaned the carbon wipe with carb cleaner and tissues. I adjusted the air idle mixture about 1/2 - 3/4 of a turn in (but I didn't put the car on a exhaust gas analyser like Bill R mentioned), which means I'm richening the fuel mixture. I guess the car is better now, well I warmed it up and took it for a spin and the problem wasn't apparent. The problem seems if I jab the accelerator really quickly when the car is stationary the car still stutters a bit, the same problem as I mentioned in the original post but now not nearly as bad. I clean the idle control valve with carb cleaner also, that I do about 5 months ago. My idle is really improved now, no "hunting" idle issues at all.


Looking through old posts I found a member with a similar problem here;

http://www.bimmernut.com/forum/showthread.php?t=32380&highlight=afm+tuning

And found these useful websites on the matter, well afm realted anyway;

http://mysite.verizon.net/vzerdcib/e34a/id6.html

http://paulaxford.com/bmw730i/afm_service.htm

http://forums.roadfly.com/forums/bmw/bmw-3-series-e30/3147035-1.html

[alex 1993 525i auto]

If it was not already solved I would say it might be worn intake manifold gaskets or vacuum hoses causing a vacuum leak.

[BigKriss]

I changed the headgasket just over one year ago and these items were replaced then.


If it was not already solved I would say it might be worn intake manifold gaskets or vacuum hoses causing a vacuum leak.

[Rus]

How new is your CPS?

[BigKriss]

It's never been replaced. I will take it out today (or try to) and test the impedance with a multimeter.


How new is your CPS?

[Jeff N.]

I think a bit of the stumble is due to the nature of the AFM. I recall my car doing this before I swapped out the AFM for the MAF. (The MAF swapout created a whole new set of problems). My thinking is that when you stab the throttle, you have a momentary lean condition until the change in airflow can pull open the spring on the barn door and get the fuel to respond. It was never a problem while driving, only when lightly blipping the throttle - always would stumble. If you have something more than that, then that's obviously a problem.

A down and dirty way to check your A/F ratio is to drive the car and watch the MPG gauge. If the gauge surges back and forth on about 1/2 second cyles, the mixture is off relative to 14.7:1. You are seeing the injector cycling as the ECU attempts to correct the mixture. No surge means that the mixture is reporting back to the ECU as being correct and there is no need to adjust the ratio. Only works in closed loop mode (you go open loop when you depress the pedal more than about 2/3 of the way down or IIR when you exceed about 5k RPM).

Cause of this could be a bunch of things - old O2 sensor, vacuum leaks, out of tune, etc. Mess with your AFM spring and watch yourself really mess up the mixture. :D





Okay, I had a crack at it today. I checked the intake rubber boots, no visible cracks there. I opened the maf on the bottom and cleaned the carbon wipe with carb cleaner and tissues. I adjusted the air idle mixture about 1/2 - 3/4 of a turn in (but I didn't put the car on a exhaust gas analyser like Bill R mentioned), which means I'm richening the fuel mixture. I guess the car is better now, well I warmed it up and took it for a spin and the problem wasn't apparent. The problem seems if I jab the accelerator really quickly when the car is stationary the car still stutters a bit, the same problem as I mentioned in the original post but now not nearly as bad. I clean the idle control valve with carb cleaner also, that I do about 5 months ago. My idle is really improved now, no "hunting" idle issues at all.


Looking through old posts I found a member with a similar problem here;

http://www.bimmernut.com/forum/showthread.php?t=32380&highlight=afm+tuning

And found these useful websites on the matter, well afm realted anyway;

http://mysite.verizon.net/vzerdcib/e34a/id6.html

http://paulaxford.com/bmw730i/afm_service.htm

http://forums.roadfly.com/forums/bmw/bmw-3-series-e30/3147035-1.html

[Paul in NZ]

FWIW kriss my car never likes a blip of the throttle(never has) from idle,but its fine from an engine speed just above idle.....the second blip is fine but the first always bogs slightly then revs...

[Rus]

It's never been replaced. I will take it out today (or try to) and test the impedance with a multimeter.

No need to take it out. Pull the connector off up by the fuel injector rail harness and test at the pins as per the Bentley manual. Maybe check the airgap between the sensor and the timing wheel as well. It needs to be pretty small (but not small enough to hit the wheel ;) ). HTH

[BigKriss]

cheers, i'll get back to you soon Rus.


No need to take it out. Pull the connector off up by the fuel injector rail harness and test at the pins as per the Bentley manual. Maybe check the airgap between the sensor and the timing wheel as well. It needs to be pretty small (but not small enough to hit the wheel ;) ). HTH

[Jeff N.]

1 mm is the CPS spacing. Use a feeler gauge to set.