What are the possible sysmptoms of a old and worn out distributor cap and rotor. My '88 300CE idles rough, however it idles fine with the a/c on!

Why don't u take the cap off and find out ?



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The extra load of the a/c is helping with C/O control...lazy O2 will do this, as will air leaks. Blow by is another problem...open the oil cap while rough idling....puffing ?



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"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

Caps and rotors get dirty, especially with carbon worn off the contact point in the cap. This carbon eventually gets widespread enough to create a current path to short out the spark and send it to ground through the distributor body rather than the spark plugs. It can happen inside the cap or on the rotor; in the former case, the carbon tracks are often visible and mistaken for "cracks" in the cap. Sometimes only one cylinder is shorted out in the cap, but more often the carbon track runs from the center electrode out to one of the plug wire electrodes and causes all sparks to go to that one cylinder, making the car impossible to start, but producing spectacular backfires when the wrong cylinder fires with the exhaust valve open and the exhaust system full of unburned gas.

With the AC on the stationairy rpm is reased, wich explains the smoother idle.
Between the injection system and the thermostat you can find the idle control valve (round aluminium tube with electrical connector on the front). On the right side (seen from the front) there is a small screw. With this screw you can ajust the idle speed. It may be hard or impossible te turn, iron screws in aliminium is generally not a good idea. I drilled mine out and replaced it with a copper screw (much longer for easy adjustment).

Air leaks or worn airflow meter pot. Engine breathing demand will increase with the compressor on, so greater airflow will reduce the effect of an air leak. Also, the metering vane will move further, turning the pot shaft to a point where contacts are not worn and output voltage is stable. This latter would be particularly likely if idle is very uneven w/o air and very stable with.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

Very well possible that the airflow potmeter is worn. If you increasing the idle by 50 rpm it will idle smoothly again. I did this with my car and after 150K it still has a smooth idle. If you replace everything that is worn on a 17 year old car you have to spend much more than the car is worth.

Symptoms include an engine that will not start, or stops and will not restart... ask me how I know!

Best Regards,
Jim

My car must not have a cap and rotor. because i looked everywhere. I have a 89 300e. I found a coil. and a wire going to the front of my engine where all the other plugs are plugged into the front of the engine itsself. i cant figure there is a cap and rotor on the front of the engine. My car is to new to have Points Right?

Renton, that's the distributor cap in the vertical position on the front of the engine attached to the overhead camshaft. The rotor is inside the distributor cap, as usual. No, your 1989 300E does not have points.

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Fred Hoelzle

Ok thanks much.

ouch!

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Timothy Evans
TimEvans35@gmail.com
CELL 310 990 6780


1985 380 SL
2010 CL 6.3 AMG
*2009 CL 550 Sport 4matic
New addition-1984 380 SE
Newest Edition 1986 300E AMG