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Old 07-02-2010, 06:18 PM
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Default 1986 300e idling problem

I have an 86 300e, I have replaced the fuel pump and filter, but the idle goes up and down now, did not before the new pump, it was very noisy. can you tell me what to check next? thanks
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Old 07-03-2010, 12:56 PM
Join Date: May 2009
Posts: 30

I'm guessing this is fuel injected, correct? I would consider the fact that your fuel pump may actually have been bad, but besides that, you may want to look at:

Idle Controller (if you have one--I'm not familiar with this year)
Possible fuel leak somewhere (gaskets, lines, etc etc)
Check fuel pressure while running and see if it correlates with your rpm jumps.

Good luck!
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Old 07-03-2010, 09:43 PM
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I have checked the fuel pressure, the original problem was stalling at take off, replaced the filter, pump, screen, hoses and the overload relay. Cars runs smooth down the road but the idle goes up and down, I am wondering about the idle control sensor? Such a pain it has no computer connection, cant read codes...
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Old 07-04-2010, 08:34 AM
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can someone tell me what an idle control valve is and how to tell it is bad?thanks
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Old 07-07-2010, 12:07 PM
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I believe idle control valve on these was screwed into side of the block under the intake manufold, sensing coolant temperature and opening/closing air supply. It would not make the RPM fluctuate because it is working in very slow motion and it is closed when engine is warmed up. Check all the clamps on fuel hoses to pump and tank, you may be sucking air into fuel while pumping it out of tank. You can check your new pump by taking off supply hose from fuel distributor, volume should be one quart of fuel minimum per 30 seconds. There is also screen in the fuel distributor inlet where the supply hose fitting is screwed into and also in warm up regulator under supply hose fitting. Follow the fuel line from pump to engine and you will find them.
1999 ML320 230K miles
2003 Lexus GX470 285K miles
2001 ML320 162K miles
1980 280SE EURO W126 156K miles 142mph top speed SOLD
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Old 07-14-2010, 04:56 PM
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Either your idle valve is bad or you have numerous vacuum leaks.

Under the air filter assembly you will see a silver tube like object, say 4" long with two 1" hoses running to it, Its about 1 1/2" diameter, shaped like a cylinder.

Also all the vacuum lines need to checked for leaks.

As for how to check it? Tough. Get one off ebay for $30 & see if it helps.

I may have one, I'll check. After 20 odd years my money is on the vacuum lines being shot.
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'92 400E
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Old 11-23-2010, 01:18 PM
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thanks for the help, sorry it took so long to get back..it was the idle control, and the fuel pressure accumulator...
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1986, 300e, 86, benz, control, forum, fuel, idle, idling, lines, located, mercedes, problems, pump, vacuum, valve

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