Mercedes-Benz Forum - View Single Post - Temporary rough idle warm & cold after starting
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post #5 of (permalink) Old 05-23-2019, 11:33 PM
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Date registered: Mar 2010
Vehicle: 1989 560SEC, 1989 560SEL, 1995 E420
Location: NorCal
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Originally Posted by JR420 View Post
Picked up a mirror, but don't think it will be necessary after all. Pushed plenum down and I can hear fuel dripping inside and can also certainly smell it. What does this mean? I am pretty certain fuel isn't supposed to be dripping inside here.
It likely means the control piston seal has failed. It's a straightforward replacement but follow the manual exactly so you don't introduce variations in the function of the fuel distributor. When you get your manual it's section 073-207.

I would do this first: With the car cold, prime the pumps two or three times by turning the key to run but not starting the engine. Then push down lightly on the air flow meter flap and see if you've got about 2mm of play before you hit resistance. Mind you, I am NOT an expert by any means. This is just stuff I've read in the manual. I advise people to do their own reading.

Anyway, I'd do the same check with the engine warm. In both cases, you might find you have to use a magnet to lift the flap up to the top of its travel. The main thing is there needs to be a little free play before the flap mechanism touches and starts moving the control piston.

If this all checks out, I would then check the car in the morning after it had sat all night. Push down on the flap and look for fuel in there. You won't get any resistance this time because the fuel pressure has bled down overnight. If the control piston seal is leaking overnight, you'll see fuel in there.

You can still buy the seal at the dealer.

Man, I wish H.D. would chime in. I guess he will if I make any horribly way off recommendations.
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