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Post Info TOPIC: 1990 Mercedes Benz W126 V8 560 / 420 HIGH IDLE WHEN WARM.


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1990 Mercedes Benz W126 V8 560 / 420 HIGH IDLE WHEN WARM.


Recently picked up a 1990 Mercedes Benz 420 SEL that had a high idle every now and again when warm, then eventually it just kept on getting higher until I did something about it. At the start it idled up around 1000 rpm, when warm, which went back down to normal when you put in in drive, but under a fair amount of load. Then when you put it back in Park it would race at about 1000 rpm or even 1400 rpm at times. During cold starts it would almost seem normal but it would idle about 800-900, but as it got warmer the idle picked up.

What I found was a bad Idle Air Control motor... I was seeing just under 2 volts with the key off to the IAC, which seemed a bit low, but it is what it is. Then I measured the ohm's of the IAC itself and found no resistance, which made me swap out a known good one. Odd thing is, when you would apply 12 volts to the IAC when the car was running it would stall the vehicle out like it should, so this got me to wondering... So I guess these things, even if they stall the vehicle with 12 volts direct to the IAC can still have issues. I should also note, that during the week it seemed to get higher and higher in RPM's due to the faulty IAC, the heat would fall off and all sorts of other issues due to the car's increased unmetered air supply, obviously messing with the cars vacuum. So what I'm saying is if you're idle is not right, it's going to effect other things as well. Some people claim you can clean them with carb cleaner but I doubt this would be a good fix because if it can seal up enough to stall the car, it's plain to see it's got to be something worn in the partially open position rather than fully closed.

With the new IAC motor installed it idles nice at 650 rpm as it should, both in Drive and in Park, both warm and cold. Hope this helps, I'll add some photos here in awhile.

Just some food for thought.

 



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Here is a photo of the old faulty IAC unit, and the known good unit installed. I always mark the bad stuff so it doesn't end up on another vehicle, but if needed I can pillage it later for anything useful.



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Yo,

Those IAC units were prone to erratic performance, even when the cars were new. I'm not sure why they changed the original design from water temperature control to electric, but there you go ! At least you don't have to top-up the coolant when replacing a dodgy one.

I'd guess that the really cold temperatures you guys have to live with sees the oil temperature never rise high enough, to let oil-fumes lubricate the sliding valve enough from not corroding & sticking, letting the excess air in there.

At least you have the parts to repair !!! Do you have the Euro - logs fitted yet ?......

Ciao,

Rastus

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I didn't know the early ones used coolant, but to be honest, it was pretty easy to fix and diagnose. I can't complain.

No euro logs fitted yet... I let a 560 SEL that was fitted with a Euro Engine and logs slip thru my fingers a few years back from a buddy of mine Gaftdesign and I been kicking myself every since. I don't know what happened that day, as I usually don't spook, but I am pretty sure it had more to do with me not having much money at the time and the car requiring more work than I had seen it last, that and a small title defect. 

I also had a set on a 1985 500 SEL euro, but I sold that entire car to someone in Ohio over 12 years ago, but it had euro logs and really went like all hell on the expressway. 

I'm not really much into the performance on these cars, but I wouldn't mind some euro logs.. I'm sure I'll come across a set, maybe even get lucky and find a set of AMG Tri-y's 

You never know.



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Hey SELLC,

I'm only continually hinting at them ( Euro logs ) so that you get the performance that already exists, that's all. It would really be a 20+ HP improvement across the rev-range, & of course, some AMG tri-y's may even find a few more horses for you too, if the rest of the exhaust has been uncorked.

Keep your eyes & ears out for them !!! I'm sure that once fitted up, genuine performance comparisons with your Vette would be likely, though I'd guess the 560 would maybe still have the top-end-power award anyhow.

Cheers,

Rastus

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