Problem never solved.... - PeachParts Mercedes-Benz Forum

davestx · #1 06-29-2007, 04:46 PM

I had posted in March that my 1991 Mercedes 420 SEL with a 1988 560 SEL motor was acting up while I was on vacation. David Poole was kind enough to take my phone calls and offer his valuable advice and was very helpful. Thanks David. The symptoms were that the car would start and then idle erratically with the rpms going from 750 to 1700 over and over and then stall.

Nothing helped and then, out of nowhere, the car just started to run normally. I was able to drive the car home (3 hours). I immediately brought it in to my mechanic (familiar with Mercedes but not a specialist). The car at first ran well (of course it did... It was in the shop) but then started acting up again.

My mechanic has since swapped all of the usual suspect modules: over protection, kickdown, rpm control relay located under passenger side floor mat. He has cleaned the throttle body and butterfly.

I had to travel quite a bit and the mechanic had a loss in his family and the car just ended up sitting until now. I would like to get the car running now. The current status is,the car starts up and runs normally for 5 minutes then the idle goes up to 1500 rpm for a few minutes then stalls. The mechanic has been asking around for advice and was told that the gasket under the intake manifold is the possible culprit. Any other ideas before he goes down that road?

saumil · #2 06-29-2007, 05:09 PM

If this were to occur on my 87, 260E, I would check the idle control valve. It is an electronically controlled air valve. It has a coil which can be tested for resistance, across the valve you should get about 7 to 10 ohms. Also you can disconnect the connector and apply 12V to one pin on the valve and ground to another, you should hear a buzzing sound, if you dont, just replace that valve. I am not sure your car even has this component and where it would be located, but you can ask your mechanic about it.

saumil · #3 06-29-2007, 05:14 PM

Also the gasket theory, you mean there is unmetered air leaking thru, but this can be tested by spraying carburettor fluid around the gasket, if the RPMs go high when you are doing this, there is a leak in the intake gasket. Does the car idle fine in the first 5 minutes, and also the throttle response is normal ? Like you can get >5500 rpm without flooring the gas pedal ? If this is the case and you get high RPMs after 5 minutes, it is less likely to be the gaskets. And if I think more about it, if there is a leaking gasket, why should RPMs first go high and then die.

saumil · #4 06-29-2007, 05:19 PM

Forgot to ask this: after 5 minutes, after the car has stopped, does it start back up without a problem ? How long does it then stay on ?

davestx · #5 06-29-2007, 06:00 PM

Thanks for your response. The car idles normally for the 1st 5 minutes (approximately) with normal throttle response easily revs and then abbruptly jumps up to 1500 rpm (approximately) runs for a while then shuts off as though someone turned off the key. The car restarts immediately, runs for a minute more (approximately) then shuts off.

As I had mentioned the car returned to completely normal operation out of nowhere enabling me to get home from a few hundred miles away after it started to display these symptoms. Now it consistently acts up after a few minutes.

davestx · #6 06-29-2007, 06:01 PM

Also, I believe the mechanic checked the idle control valve.

MB-Dude · #7 07-13-2007, 07:45 PM

Quote:

Originally Posted by davestx

... The symptoms were that the car would start and then idle erratically with the rpms going from 750 to 1700 over and over and then stall.

I had this exact same problem in an '88 420SEL!!! Took mine to the dealer (Claridge's Mercedes, now Fletcher Jones in Fremont, CA) and they worked on the problem for a little over 3 weeks. In the end, they charged me for a computer and 2 hours labor. However, I'm not so sure that was the real problem.

My only reason for adding this post is:
A) I'm real interested in seeing what the fix is and,
B) Lend moral support to davestx

It can be real scary watching the tach jump spontaneously above 1500 RPM with your foot off the peddle. Thank God I didn’t hit anything when it happened while driving.

davestx · #8 07-24-2007, 02:21 PM

Guys I think that was it. Now she purrs like a kitten!
Thanks for all of your help. Now for the trans flare.......