Surging Idle?!

[Frankalope]

Hello!

I recently bought a 1992 525i, the car has a surging idle I have been attempting to diagnose, but to no success so far..

Here some details:

When I start the car, it runs fine. It's when it starts to warm up, the car starts idling between about 1.5k and 2k rpms. I do not have a check engine light of any kind, but was originally throwing a TPS code on a stomp test which lead me to believe it was my TPS. I have a 1995 525i car that I have since decided to for parts as needed, so I swapped out the TPS from that car.( which was in known good working condition) This did not fix the issue at all.

Next I tried unplugging the MAF sensor, and this caused the Surging idle to stop. The car would idle at just under 1k, ( so I'm assuming anywhere between 500 and 750 rpms) but would idle a bit rough ( as I would assume any car running without a MAF). This lead me to believe that it was the MAF sensor that was bad. I found out that they changed the MAF sensor after 92' to the 4 pin version, so I went out a purchased a brand new 6 pin MAF sensor for 432 dollars. (which i will be now returning) This did not help the issue in the slightest. I have check the intake boot for leaks, and I have removed the Throttle body, but at this point im starting to run out of ideas as to what could be causing the issue.

Any help is much appreciated, I can also supply pictures.

Thanks,

Frankalope

[dennoz]

Hello!

I recently bought a 1992 525i, the car has a surging idle I have been attempting to diagnose, but to no success so far..

Here some details:

When I start the car, it runs fine. It's when it starts to warm up, the car starts idling between about 1.5k and 2k rpms. I do not have a check engine light of any kind, but was originally throwing a TPS code on a stomp test which lead me to believe it was my TPS. I have a 1995 525i car that I have since decided to for parts as needed, so I swapped out the TPS from that car.( which was in known good working condition) This did not fix the issue at all.

Next I tried unplugging the MAF sensor, and this caused the Surging idle to stop. The car would idle at just under 1k, ( so I'm assuming anywhere between 500 and 750 rpms) but would idle a bit rough ( as I would assume any car running without a MAF). This lead me to believe that it was the MAF sensor that was bad. I found out that they changed the MAF sensor after 92' to the 4 pin version, so I went out a purchased a brand new 6 pin MAF sensor for 432 dollars. (which i will be now returning) This did not help the issue in the slightest. I have check the intake boot for leaks, and I have removed the Throttle body, but at this point im starting to run out of ideas as to what could be causing the issue.

Any help is much appreciated, I can also supply pictures.

Thanks,

Frankalope

Following as well. Have the same exact issue in my 1992 520i. Replaced the TPS, changed the MAF, cleaned the ICV but still my rpm are stuck at 1500rpm.

I wonder why

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[The Chauffeur]

Ditto. Haven't had a chance to look more into it. Mine has the exact symptoms, but also a CEL, and the diagnostic port is not working; also, haven't been able to run a stomp test (tried until my ankle got sore...).

From what I've been gathering from the web, there's a couple of other possible causes, aside from the ones you've tried. Don't quote me on this, as I haven't been able to check them myself. Just to throw some suggestions: ICV, coolant and engine temp. sensors, purge valve, faulty wirings, DME and main relay.

Hope you can sort it out; I'll post any update on mine, also, if that helps.

[Frankalope]

Thanks for the suggestions! My next step is to remove the intake manifold and clean the Icv and check for holes in hoses! I wonder how many of those things I can try swapping over from my parts car as well. The strange thing for me is that when I unplug the maf, the car runs quite smoothly I almost wonder if it's the maf plug, mine is missing a shim

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[dennoz]

Thanks for the suggestions! My next step is to remove the intake manifold and clean the Icv and check for holes in hoses! I wonder how many of those things I can try swapping over from my parts car as well. The strange thing for me is that when I unplug the maf, the car runs quite smoothly I almost wonder if it's the maf plug, mine is missing a shim

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That looks like the Siemens version of the MAF. Bosch MAF has 6 pins and the plug is a 6 hole female plug

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[Frankalope]

Wait, there is a 3rd maf type?

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[dennoz]

Wait, there is a 3rd maf type?

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I only know of Bosch and Siemens MAF for the M50.

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[Frankalope]

I only know of Bosch and Siemens MAF for the M50.

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Well my 95 vanos m50 has a 4 pin connector, this one is the 6 pin.

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[Frankalope]

I just got some info on my Facebook group to try replacing the crank sensor. Don't know if either of you have tried that yet, but this will be my next course of action most likely!

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[The Chauffeur]

Great info, thanks! I'll look into that, also.

[mossyoaks]

My 95 had these symptoms plus would not rev past 1500rpms or so.

Stomp test code was "faulty MAF". (Check out some Youtube vids on how to do the stomp test. It's so important to know what engine component is faulty. Just takes practice!)

Once I umplugged the MAF, engine idle evened out right away.

A new Bosch MAF corrected the problem.

[dennoz]

I just got some info on my Facebook group to try replacing the crank sensor. Don't know if either of you have tried that yet, but this will be my next course of action most likely!

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Well I haven't replaced that, but I can try that as well

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[Frankalope]

My 95 had these symptoms plus would not rev past 1500rpms or so.

Stomp test code was "faulty MAF". (Check out some Youtube vids on how to do the stomp test. It's so important to know what engine component is faulty. Just takes practice!)

Once I umplugged the MAF, engine idle evened out right away.

A new Bosch MAF corrected the problem.

I am aware of the stomp test procedure. First got a tps error, then maf error. Input in a brand new Bosch maf, and nothing changed. That's why I'm here.

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[summitp]

Is the throttle valve closing all the way? Is there a vacuum leak? Leaking oil from the valve cover?
Check fuel pressure. Is it it high or where it should be?
Tested your coolant temp sensor? As the engine warms up, resistance should drop, and fuel should be reduced.

[dennoz]

Any progress on the surging idle?

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[dennoz]

Well my 95 vanos m50 has a 4 pin connector, this one is the 6 pin.

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6 pin is Bosch same as mine. 4 pin is Siemens.

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[Frankalope]

No progress yet. I'm in the process of getting ready to move into a lake house with an oversized 2 car garage, so I've taken a bit of a break to get my parts car ready to get towed over there and work on the suspension a bit. Will be starting a YouTube channel for the build here soon.

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