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Old 09-21-2009, 10:42 PM
86560SEL's Avatar
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Occasional uneven idle and uneven acceleration... M103

I had posted a couple of things about this on the "spark plugs" thread or another one, not sure, but the issues there dont seem to match mine exactly.

The car is a 1988 Mercedes 300SEL with 158K miles. Dont leak or burn oil.

My problem is that I have this "spluttery" idle when in drive and the idle would occasionally go up and down... even when in gear as if stopped at a stop light and I have to depress the brake harder to keep it stopped. Recently I have noticed on acceleration that the cars power to the transmission is "uneven", kindof like the car is getting more power, then down a little, as if I am "working" the pedal, when I am actually holding it steady. Sometimes when in park the idle will go up and down some. The "check engine" light will come on about 30 seconds after the car is started and will go out about 90 seconds after that and never comes on again until turned off again. Someone said maybe o2 sensor is bad... would that cause this? Can a place like Advance Auto parts store check this cars CEL code?

I dont have a ton of money to check this and that and wondering if anyone here can recommend any specific thing that may be causing this? Perhaps the spark plugs need changing? Other than that, all I know is to keep driving it until it gets worse and dont run anymore.

Thanks in advance.

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Old 09-22-2009, 12:46 PM
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The symptom may have more than a single cause. The CEL is on because the O2 sensor voltages may be out of range. Until the 1988 California emissions version of CIS-E the CEL was merely an oxygen sensor malfunction indicator light. The fact that the light goes on and off makes me tend to agree that the o2 sensor may be defective or the mixture so very lean as to inhibit the voltage output of the sensor. Procure a 3mm allen wrench and muster up the courage to twist the 3 mm allen screw in the short little tower under the air cleaner. I would rotate the screw in a clockwise direction no more than 90 degrees at first. See if that doesn't help. It's cheap. "You can do eet"
Call us in the morning if that medicine does'nt help. You can always turn the adjustment back to where you started.
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Last edited by dpkreuze; 09-22-2009 at 05:38 PM. Reason: tweek
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Old 09-22-2009, 03:13 PM
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You can check the codes yourself. Search this forum for the code reader you can build.

Don't touch the A/F mixture yet as stated above. Fix the CEL first before moving onto other things.
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Old 09-22-2009, 04:12 PM
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TRY THIS (from my 300E log): << cleaned throttle position sensor and lubed with wd40 - hunting idle is gone. >>
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280
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Old 09-22-2009, 05:24 PM
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Originally Posted by gmercoleza View Post
TRY THIS (from my 300E log): << cleaned throttle position sensor and lubed with wd40 - hunting idle is gone. >>
A good suggestion, although I would recommend Caig MCL over WD-40 for this application. My biggest problem with WD-40 as an electrical contact cleaner in a moving part (potentiometer) is its affinity to collect dust and dirt.

Cleaning moving electrical moving parts can be a bit tricky, there is a thin line between ineffective cleaning, and washing away necessary lubrication. Often the lubrication in pots is a light grease, and WD-40, MCL, or any contact cleaner is going to tend to wash away that grease and replace it with a much thinner oil. Problems begin when the contact cleaner dries out, and now the metal wipers of the pot are in direct contact with the carbon traces, and now wear begins. Friction goes up after the oil starts to dry, and often the pot shaft will begin to stick. Additionally, the lack of lubrication between the wipers and traces begins to wear out the carbon traces, and this is when the electrical characteristics begin to degrade.

Ideally, one would prefer to first wash away the previous grease along with all the dirt accumulated over the years, and then replace the heavier grease after the solvent has dried. Some contact cleaners have silicones in them, and these are often enough to provide the necessary lubrication. But really, applying a grease afterwards is the way to go.

Just remember, if you spray out a pot and work the shaft several times and the action goes from smooth to crunchy, you know what has happened. You need to spray it out some more to remove the last traces of dirt, and then re-lubricate with something a bit thicker.

It is possible to destroy a pot by trying to clean it out, I have seen it happen. The cleaner washes all the grease away, the wiper hits a piece of dirt, except now without grease, the piece of dirt catches the wiper arm and bends or breaks it......

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Old 09-23-2009, 03:00 AM
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Oh wow... alot of data there.

Basically, I should replace the o2 sensor first and see what happens?

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