V12 idling problem

[KLP 92]

I'm having problems with my V12. The problem is when the car is running at optimum temperature and is put into park and neutral then the revs rise to 1500 revs and dop to 1000 revs and continue going up and down. While driving or when the car is cold there are no problems at all.

Well sounded like a common MAS problem, or so i thought. I have 2 MAS sensors, 1 on each bank. To figure out which one is faulty i disconnected them 1 at a time to see which one was faulty. When the passenger side MAS sensor was disconnected the car idled fine, no lumpy idle! Bought a new MAS sensor plugged it in and errr..... same problem!!! So i disconnected it and put the old one back on. Same thing, when i disconnect it the problem goes away. So with the old MAS sensor in place i disconnected it and connected the new MAS sensor (so the new sensor was sitting in the egine bay, loose) the idle proble had gone! But as soon as i plugged it into the aiflow pipe then the problems back!

Went down to GF and had the car put on the diagnostics. There was a fault but for some reason the diagnostic machine couldn't access it and tell us what it was for. So a quick reset of the system followed by another check shoed there are no fault codes. So even though there is a lumpy idle there is no fault stored on the ECU and the short test comes back with nothing.

It has been suggested that the wiring loom to the MAS sensor may be faulty and may need changing. The only way to tell is to strip down the loom and check.

Could it be a loom problem? Or any other ideas? The main problem i have is that it drives fine with the MAS sensor disconnected on that side and when the car is cold!

Thanks

 

[saorbust]

Sounds like you might have a misfire. My V8 AMG did the same thing. Had to replace the HT leads, and the plugs.

 

[KLP 92]

Definately not a misfire. Car drives and revs perfectly I've had it checked and the plugs are firing fine.

 

[stats007]

Unfortunately this could be a whole host of things from injector pulse width due to faulty sensors or a vacuum leak and the ECU is shutting things down at higher rpm. Does yours have an auxiliary air valve from the intake? Try squeezing this to see if it makes a difference as it's used as a bypass on some cars.

You also have two idle speed controls - one in each ECU. N3/11 & N3/12.

 

[Vlad]

I would try posting the problem on Mercedesshop.com. They have a huge number of Techs visiting the forums and with the V12 probably being more common in the US than they are here, I would imagine that they may be able to point you quickly in the right direction.

 

[Dieselman]

It could be an air leak confusing the MAF reading.

 

[jgevers]

I can't remember properly, but I think the engine breather system has a valve in it somewhere that can let in unmetered air as well as the brake servo vacuum pipe.

regards,

Job

 

[KLP 92]

Just picked the car up, not sure what was wrong with it as GF was extremely busy. I heard the mechanic say something about an actuator or something but they've fixed it and it drives perfect! Thanks for the replies.