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Old 08-11-2003, 10:28 AM
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Join Date: Aug 2002
Location: Milwaukee, WI
Posts: 126
90 300SEL Very Rough Idle - Help!

Such a nice day yesterday, that I gave the car a hot bubble bath. While at it, I also rinsed the dirt and dust off the engine bay - just like many times before with a garden hose.

Started her up to put away into the garage, and it idled very rough. It almost died after putting into gear.

Wouldn't start at all by evening. Have good spark, but didn't smell any fuel. I poured a little fuel onto the 'air plate', but that didn't have any effect. This morning, it started right up, but still had the very rough idle. When put into gear, it wants to die. Pushing on the accel pedal will get the engine to very slowly begin to rev, and once it gets to about 2000 RPM, engine runs as normal and smoothe.

As already mentioned, I have strong spark. Checked for loose/wet fittings and connections but found nothing out of the ordinary. Although I didn't remove the distributor cap, I did not spray water in that area and it appeared dry, and plugs are firing. It seems more like fuel.

Where do I go from here?
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Old 08-11-2003, 11:33 AM
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I know you say you have a hot spark, but HOW hot. I had a similar problem that I tracked down to a faulty ignition coil.
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Old 08-11-2003, 01:57 PM
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Join Date: Aug 2002
Location: Milwaukee, WI
Posts: 126
Yes, I thought of that possibility. Coil looks fine (no cracks or bad connections). Actual spark at the connectors is nice, fat, and blue. As mentioned, once the revs get going, it runs normally.

I don't hear or see any arcing or other indication of spark troubles.
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Old 08-11-2003, 02:59 PM
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I was surprised to find mine was a coil problem. I had a rough idle and a stumble and/or stall on accel from a stop. Once she was going, she ran great. The condition worsened as the engine warmed up, and progressively was getting worse until I localized the problem.
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Old 08-12-2003, 11:46 AM
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Join Date: Aug 2002
Location: Milwaukee, WI
Posts: 126

After two days, wherever water had settled apparently dried out. Car running okay now.
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Old 08-12-2003, 10:29 PM
Senior Member
Join Date: Apr 2001
Location: Beech Island, S.C.
Posts: 468
Did you check to see if water had gotten into the space surrounding the spark plugs? That can cause trouble.
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Old 08-12-2003, 11:54 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,123
I've heard of engine washing causing problems with the AC system. Seems that water gets into the temp sensors and makes the ACC think the engine's too hot. Maybe something similar happened to your car making the ECU think the engine was already warmed up.

95 S420
91 300SE
87 300SDL
83 300SD
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