W124 M103 Fluctuating Idle Woes

[hazzac181]

Hi all,
I'm trying to diagnose/fix a 'bouncing' or fluctuating warm idle issue on my '89 M103 260E. When started when hot (above or at 80c) the idle speed will bounce from 450/500RPM to 1000RPM, back and forth. Reading pin 3 of the X11 diagnoistic port (with pin 2 used as a ground), I obtained a reading of 50.6% - O2 sensor. Checking the voltage of the O2 sensor (which, as I understand, should fluctuate between 0.1 and 0.8v), I determined it was dead (readings were between 0 and 0.1v). However, after changing the O2 sensor today, I am still getting the same 50.6% reasding from the X11 port on my multimeter. That reading will remain there for a while before the figure creeps up to 82% and oscilates between 81.9% and 82.1%. Any ideas? I'm exhausting most online forums and searching at this point.

Thanks in advance
Harry

[Bartman4800]

It sounds like you have a vacuum leak. And most likely, around the injectors.

Here is Kent explaining what to do about it: https://www.youtube.com/watch?v=pOZRHt-GkHY

[hazzac181]

It sounds like you have a vacuum leak. And most likely, around the injectors.

Here is Kent explaining what to do about it: https://www.youtube.com/watch?v=pOZRHt-GkHY

Thanks for the reply Bart. I'll look into the injector situation. What is the best means for testing for vacuum leaks throughout the engine bay?

[John Green]

Hi all, I'm trying to diagnose/fix a 'bouncing' or fluctuating warm idle issue on my '89 M103 260E. Any ideas? I'm exhausting most online forums and searching at this point.

My money would be on the air flow meter sensor. The small black box on the side of the air flow meter. They get dead sections in them over time. Mark its position carefully and then move it around a little to see what happans.

[Bartman4800]

It sounds like you have a vacuum leak. And most likely, around the injectors.

Here is Kent explaining what to do about it: https://www.youtube.com/watch?v=pOZRHt-GkHY

Thanks for the reply Bart. I'll look into the injector situation. What is the best means for testing for vacuum leaks throughout the engine bay?

Spray a bit of brake cleaner around the injectors, if the revs rise you have a leak...

[hazzac181]

Reporting back after a little investigation. I built my own smoke tester and found, low and behold, the seals on injectors 1, 2, 3, 4 and 5 are bad. Added to the 'to-do' list. Are parts available from MB or is my best bet online?

I also saw quite a lot of smoke coming from this plug, underneath the air flow meter housing. See the attached images. Is anyone able to identify what this is, and what it does? Either my googling skills are poor, or I've completely missed what this little guy is.

Once again, I appreciate all help

[hazzac181]

Reporting back after a little investigation. I built my own smoke tester and found, low and behold, the seals on injectors 1, 2, 3, 4 and 5 are bad. Added to the 'to-do' list. Are parts available from MB or is my best bet online?

I also saw quite a lot of smoke coming from this plug, underneath the air flow meter housing. See the attached images. Is anyone able to identify what this is, and what it does? Either my googling skills are poor, or I've completely missed what this little guy is.

Once again, I appreciate all help

Hmm, images not uploading as anticipated. See links below:
https://imgur.com/dlV7qR8
https://imgur.com/mTwVfqH

[hazzac181]

Hi all, I'm trying to diagnose/fix a 'bouncing' or fluctuating warm idle issue on my '89 M103 260E. Any ideas? I'm exhausting most online forums and searching at this point.

My money would be on the air flow meter sensor. The small black box on the side of the air flow meter. They get dead sections in them over time. Mark its position carefully and then move it around a little to see what happans.

Upon re-reading your post, it appears the black box in question is indeed the air flow meter sensor. Would I be correct? If so, 100% success rate from both of you so far!

[John Green]

Upon re-reading your post, it appears the black box in question is indeed the air flow meter sensor. Would I be correct? If so, 100% success rate from both of you so far!

Nearly all vacuum leaks are a constant. So for example if you have unmetered air getting in, it is always getting in. So not the most likely cause of a fluctuating idle.



The plug in red is not a plug. It is the switch on the throttle body. It essentially has three positions. Shut, full open and in between these points. The smoke will be caused by wear in the throttle body shaft or the alloy housing, but once again this is a constant.

[hazzac181]

Upon re-reading your post, it appears the black box in question is indeed the air flow meter sensor. Would I be correct? If so, 100% success rate from both of you so far!

Nearly all vacuum leaks are a constant. So for example if you have unmetered air getting in, it is always getting in. So not the most likely cause of a fluctuating idle.



The plug in red is not a plug. It is the switch on the throttle body. It essentially has three positions. Shut, full open and in between these points. The smoke will be caused by wear in the throttle body shaft or the alloy housing, but once again this is a constant.

Understood John, thank you