My 1994 E420 starts smoothly and runs well for about 7–8 minutes and then begins to sporadically misfire. This continues to worsen until the engine stalls and will not restart. After an hour or so the engine fires up normally. I need some suggestions for diagnosing this. My experience says an electronic part such as a coil or ezl is heating up and failing. i just don’t want to throw rare and expensive parts at it. There once were scopes which visualized the ignition impulses which would be very useful. Unfortunately those seem to be a thing of the past. Ideas? ( I’ve tried new caps and rotors).
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Losing ignition gradually after warm up.
- Thread starter Phoenix127
- Start date
Classic symptoms of wet distributors. Pull your caps and rotors then pull the rotor mounts and check the back of the insulator discs underneath . They will be wet with a weird jell. Dry everything carefully as you will also find a light coating of moisture on the caps a rotors and then use heat to make sure everything is completely and absolutely dry including the cavity in the heads behind those discs. If the insulator discs behind the rotor brackets have not been replaced replace them now . I also coat my insulators with a thin coat of high dialectic grease. There are many threads about this exact issue here and it's a very persitant issue with the M119 if you live in some climates.
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I think you have a bad / weak coil in your pair. The EZL will either work or it won't (either four cylinders or more commonly no cylinders). Coils can go bad as they heat up. They are not super expensive parts to purchase new, and if original should probably be done as a matter of course.
But first, inspect your caps and rotors. And insulators behind your rotors.
Thanks for the quick reply. I’ve just ordered new coils. They were both original. Not bad for a 1994.
i had the exact same issue with my 500e,changed caps and rotors many times but the misfire continued.The issue was resolved after replacing cam seals.
Update: this morning I recieved a new pair of Mercedes cam seals. After disassembling the driver’s side cap and rotor, I discovered the cam seal dripping in oil. (Explains the leak I was experiencing which I thought was power steering oil). After extraction, I found it was a Corteco brand seal. The material was stiff and apparently it needed to be changed. I sprayed brake cleaner and electric contact cleaner to remove the oil and then carefully installed the proper Mercedes seal. Then I carefully sprayed and wiped down the insulator which was coated on the back side. Not sure how it ingressed, but there was a mist like oily coating inside the cap. Eventually all was clean and dry I reassembled the rotor mount, rotor, and cap. After this it hasn’t acted up again! Thank you very much for the patient helpful advice. To maintain these old cars one needs lots of experience. This is a great place to find it! Hopefully this helps others.
This is the expected outcome. Every new owner should inspect these cam seals and replace them if there is any evidence of blow by. Then proceed to service other consumables up the chain as needed. I have not had the issues on either the E500 or E420 since I installed new seals some years ago.
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