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Old 06-10-2001, 02:14 AM
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My 91 420SEL (98K miles) performs well except at idle. Especially during the "warm-up" period, the idle surges. You cannot really see it on the RPM gage but can see the oil pressure gage bobbing. Also, at same time I get this rough idle as well. It is not a constant rough idle but more of a twitching idle that I feel in my seat. Sometimes the idle twitch/surging gets so bad the if I am at a stop for an extended period (ie long red light), when it is time to accelerate, the engine "chugs" for a second or two and then seems to "catch" and then all is well.

I just recently had the plugs, wires, and filters replaced. The distributor was check and I was told it was fine. The put it on the scope/diagnostic machine and was told that nothing unusual showed up. I was told by one mechanic that this behavior is normal for this engine/model.
Is this true?

If anyone can give me some clues as to what to look for I would be greatly appreciated. As the least, maybe I can educate my mechanic. I would go to another mechanic but, except for this issue, he has been right on with his diagnostics/repairs.

Oh, the car is running slightly hotter than it usually does (approximately 10 deg Celsius or so).

Any assistance/advise would be greatly appreciated.
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Old 06-10-2001, 11:03 AM
BENZ-LGB's Avatar
Strong, silent type
Join Date: Apr 2001
Location: Southern California
Posts: 1,663
420SEL idle...

Sorry to read that you are still having problems with the idle in your 420 SEL...Rough idle is not normal for these engines, at least not that I know of. Since Norman fixed the sticky exhaust valve problem in my 1991 420 SEL it runs just fine. If you tell him that your car is still not running as expected he'll keep looking for the problem, till he gets it fixed...

P.S. Maybe we can meet some day at Norman's and run our engines side by side. Then you could tell whether the situation that you describe is normal to all 420's.

P.S.S. What brand of gas do you use? I find that my car runs "happiest" when imbibing Chevron Supreme. It does OK with Union 76, but it doesn't like any other brands. One time I stopped at a "El Cheapo Discount Gas" and put in about a 1/4 of a tank. My car was definitely not happy with the cheaper gas and it let me know about it. After running a full tank of the usual Chevron Supreme, the car went back to its usual self.

[Edited by BENZ-LGB on 06-10-2001 at 10:10 AM]
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Old 06-12-2001, 02:52 PM
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My 560 started idling ab it rough about 6 months ago. Dealer said it was a leaking injector. Once you shut her down the injector drips gas into the cyl and floods it a bit. After it runs a few seconds it clears out. Once in a great while it will shake at a long light but if I nail it it clears out. Do you have similar problems or does it run like ***** all the time?
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Old 06-12-2001, 11:57 PM
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The idle problem is and on/off issue but and when it happens it is pretty erratic at idle (stopped in gear). I notice no hesitation/missing problems once the car gets rolling. Does sometime have a "studder" issue when taking off from prolonged stoppes (ie long stop light).
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Old 06-13-2001, 04:08 AM
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My 560SEL had a rough idle while cold but a good long run over the weekend seemed to clean it out. It ran fine for a few days and now is slowly getting rougher again. I do mostly stop start type driving which is probably causing the plugs to foul.
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Old 06-13-2001, 11:10 AM
Tim Robson
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Ah yes, the old "rough idle is normal for this model" reply. Well, it is very common (not quite as common as being told that the condition is normal), but you'd certainly hope that on a car like that it wouldn't be "normal".

I had a hesitation problem accelerating off idle with a 380SL (not identical but similar injection system). The car was also running with an extremely rich mixture. The warm-up regulator was replaced with no effect. It turned out to be the oxygen sensor. When they get plugged up or fail, they incorrectly sense a weak mixture send a signal to the fuel injection system which causes the fuel system to compensate for a weak mixture. This can cause a number of problems, including lousy fuel economy, hesitation off the mark, and so on. No guarantee that that's your problem, but you could see how the car runs with the sensor disconnected (when the fuel system apparently defaults to a safe setting).
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Old 06-13-2001, 11:38 AM
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Join Date: Apr 2001
Location: Dallas/Fort-Worth
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I am getting ready to try the oxygen sensor test this week as well. My 300E does the EXACT same thing as yours. Idle surges (you can see the oil pressure gauge needle fluctuating as well). It started out doing this intermittently, and now it's doing it at idle all of the time. I'll bet when you kick it into reverse, you can almost get it to freak out consistently.

Although some techs in this forum will scream at me for doing this, I had disconnected the air flow meter by the air filter. The problem is still there, by with the absence of the air flow meter information, the surge cycles are longer, more like relaxed breathing. My 7-yr old daughter even recognizes that it's a problem, because she mimics the surges!

I drive this car daily, and I got sick of sitting at stoplights with the car bouncing from the surges. So the disconnection is just to live with the problem until I can do some more diagnosis and figure out the fix.

I had already had a new temperature sensor replaced, and changed out the air flow meter to no avail. OVP was replaced for another problem a few months ago, so now I am down to the O2 sensor. Hope that works for you too!

And NONE of my MBs surge like this normally! And the 300E didn't until recently, so don't let anyone tell you that it's normal!
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Old 06-13-2001, 02:14 PM
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Join Date: Jun 1999
Posts: 74
Sounds familiar. I bet there are a lot more cars out there with this problem, some dont notice it and some learn to live with for a lack of exact inexpensive cure.
I was also told by my mechanic about the oxygen sensor. Also another culprit would be the mixture control unit. Also older cars with the mileage are not perfect in terms of sensors, compression, vaccum etc and the electronics tries to compensate for it.
As many expensive items are involved I just have learnt to live with it. But for some people who like their cars to be perfect it still bothers me. One thing I did do was clean the air flow valve with a throtle body cleaner and it did help but not completely. Also my oservation is the brand of gas does make some difference.
Sure would like some of the experts on the forum to comment on the problem.
If you do chase down the problem make sure to post the results.
89 300SEL
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