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Old 02-08-2003, 02:43 PM
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Question 89, 300E, Rough idle and hard start


My 89, 300E was running great. I came to a stop light, and the engine died. I got it started and drove to my mechanic. He changed plugs, fuel filter, oxygen sensor, coolant temp sensor. & coil wire. He says that he put 30 hours into it and still did not fix the problem. He says that he has checked everything he can think of and can't find the problem. The car starts hard but once it starts, it's fine when it's cold. As the car heats up, it starts idleing rough and severely hesitates when accellerating from a stop. My mech says that he though it was the air flow meter, but when replaced, the problem still existed. I tried dry gas, valve cleaner, problem still exists. please advise.

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Old 02-08-2003, 04:08 PM
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I would suggest

that you look into the fuel pumps. You need to check the pressure under load, not just while idling or with the engine in neutral, but rather with the engine in gear and revved (foot on the brake).

What kind of MPG are you getting?

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Old 02-08-2003, 05:18 PM
it leaks, its german
Join Date: Mar 2002
Location: raleigh nc
Posts: 1,111
I had this once, combination problem.

The idle switch behind the a/f sensor in the linkage was bent and with closed throttle the CIS-E control unit had no idea the car was closed throttle. The same car has a freeze plug in the inke that had rotted to lower section out and was sucking air along with a line under the a/f sensor housing that was busted.

Dunno if it'll help but I'll damn sure remeber that one. At least it was my car so it wasn't a issue.

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Old 02-08-2003, 05:38 PM
haasman's Avatar
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Join Date: Jun 2001
Location: San Francisco, CA
Posts: 3,096

These symptoms are also of clogged injectors. Try a double-dose of Techron Fuel Injector Cleaner in no more than 3/4 of a tank of fuel. Get it going and go drive it. Once warm make the engine go into higher RPMs.

'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)

Last edited by haasman; 02-08-2003 at 05:50 PM.
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