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Old 01-23-2017, 06:25 PM
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Join Date: Jan 2017
Posts: 2
mercedes 1990 420 sel

Bought my son a 420 sel. Ran when we bought it this summer. Changed plugs, wires and 02 sensor, fuel filter. Ran great for 2 months and then would not crank. towed home. Found out it was fouling plugs. Got it started, black smoke boiled so I put another fuel distributor on it. Cranked up and ran without the black smoke. A little rough since I had not changed the last round of plugs that fouled. Ran for about 10 minutes. Turned off , would not crank. I changed plugs today putting in new bosch. Took about 30 seconds of turning over but got it cranked. Ran with a little more black exhaust. Ran 10 minutes. Would not crank. Pulled brand new plugs. fouled. Fuel is dumping. Why. to recount new filter, fuel pumps, distributor, wires, 02 sensor. Have checked pressure regulator for fuel coming out of vacumn tube on back side of regulator. non leaking. Any guidance on why plugs are fouling is appreciated. Thanks. Sam

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Old 01-23-2017, 10:21 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,800
What kind of plugs are you running?

83 300SD
98 E320 wagon
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Old 01-24-2017, 05:31 AM
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Join Date: Aug 2002
Location: Southeastern PA
Posts: 3,052
sam44, when you say "new distributor" I take it you mean you replaced the distributor cap and rotor. Hopefully, you used Bosch parts. Check the Owner's Manual for the proper spark plugs and gap. You should not be using platinum spark plugs in that engine. What brand ignition wires did you install?

It seems you are using the term "crank" when you mean "start". "Cranking the engine" and "the engine turning over" mean the same thing. When you say "Turned off, would not crank" ... I take it to mean the engine would not start.

It's possible you need a new coil. How many miles are on this vehicle? It seems that the valve stem seals are shot, and the engine is burning oil. Your best bet is to have a MB-trained tech evaluate the condition of the engine and overall condition of this 27-year old vehicle. Most buyers opt for a PPI (pre-purchase inspection) by a MB-trained tech so you have a professional evaluation and ballpark estimate of necessary repairs before the purchase.
Fred Hoelzle
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Old 01-24-2017, 08:03 AM
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Join Date: Jun 2013
Location: PNW
Posts: 242
Be sure you are using factory specification parts for anything spark related - plugs, wires, cap, rotor!

Then you need to do some TESTING using instruments to see what is working properly and what is not. Guessing and replacing parts will cost a fortune, not fix the problem, and you can introduce new problems if an incorrect part is installed.

-Test fuel pressure, fuel volume, and fuel internal leak test.
-Test vacuum.
-Test compression of each cylinder.
-Check electrical of spark coil and wires with multimeter.
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Old 01-25-2017, 07:35 PM
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Join Date: Jan 2017
Posts: 2
Hello to all from Sam44. Thanks for all your replies and suggestions. Still working on it. Will follow up in the future.

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