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.

Arthur Dalton · #7 06-29-2007, 06:24 PM

Do a fuel pressure/volume test right after the stall...

saumil · #8 06-29-2007, 07:29 PM

Fuel pressure test is a good idea. Sticky idle control valve can give you such symptoms, double check that the mechanic tested this. After the first 5 minutes, the only way again for it to stay that long is if it cools down ?

Arthur Dalton · #9 06-29-2007, 08:15 PM

If you find low fuel flow after test, this is the last place everyone looks , but it should be the First, specially with an older car...

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davestx · #10 06-30-2007, 10:54 AM

Thanks for your responses.

Yes the only way to get the car to idle correctly is to let it sit. I assumed that this had to do with temperature.

I will ask the mechanic to recheck the idle control valve and do a fuel pressure/volume test right after the stall.

Arthur Dalton · #11 06-30-2007, 11:37 AM

High idle rev intermittant problem can also be caused by OVP on that model
Do you have an ABS lamp at same time??

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david s poole · #12 06-30-2007, 11:50 AM

check the female pin socket that the ovp plugs into for corrosion and for the small pin being moved downwards in the plug so that it makes intermittent contact.next,when the car stalls check for spark and see if it's present.you said you have to let it sit to start up again and it could be the flywheel pickup that feeds info on top dead centre to the ignition unit.

saumil · #13 07-02-2007, 01:13 AM

Does this car have a coolant temperature sensor (also called fuel injection temperature sensor) ? If it does check this one also. This will be a resistance check, 2.5 to 3.5K when cold and 250-350 ohms when engine at 80 deg C. On my 87 260E, this is the last sensor on the engine, towards the windshield. This sensor informs the computer about the temperature and the computer adjusts the air/fuel ratio accordingly. If the resistance does not drop by a factor of 8 to 10 when temp changes from 20 degC to 80 degC, replace the sensor.

davestx · #14 07-02-2007, 11:52 PM

Sorry for the slow reply. I was away for a few days without access. Thanks once again for your replies. I have swapped in 2 other OVPs without a change. I will have the mechanic check the plugs where the module plugs in. I seem to remember that when the problem first occurred the ABS and traction control seemed to be acting up (I was on ice). I will also have the mechanic check the sensor and let you guys know how he makes out.

David, the car will restart immediately but will idle high and cut out until it has a chance to fully cool down.

david s poole · #15 07-03-2007, 09:47 AM

the high idle suggests that the ovp relay socket may be the issue because the ovp also powers the idle speed relay.