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Old 12-07-2001, 08:04 PM
psfred psfred is offline
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
I had a rough idle/stalling problem with my 88 300TE (actually bought it for my mother) -- after a long overdue tuneup (plugs, wires, cap, rotor) it was better, but not fixed.

Having grown up in the age of carburetors, I started looking for what (to me) was obvious -- vacuum leaks.

Evey piece of rubber hose in the engine compartment was shot -- some the typical MB "soft and shedding" and others rock hard and leaking. The worst ones were the connection for the vaccuum modulator and the one for the MAP sensor on the firewall. Replacing all the rubber ends on the hard vaccuum lines helpled, but didn't cure the idle vibration, nor the occasional stalling. Did fix the hard shift problem.

I then looked at the idle valve, and found the two large hoses to be so hard that they were actually loose on the fittings. Replaced them (inner one is a big pain!) and finally cured the problem.

Still have an occasional stall, but I think that is the idle valve itself -- I didn't clean it. Usually happens if one stops from speed rather abruptly.

It's worth the few minutes to check these hoses -- they are accessable with the air cleaner housing removed (and check the mounts while you are in there, two of mine were separated!) -- if the hoses aren't pliable, replace them. You will probably need to adjust the idle mixture afterwards.

The reason the OVP and bad battery contacts can cause idle problems and stall is that the mixture control system is powered off the same circuit, and goes off-line if the voltage drops too low or too high, causing an over-lean condition.

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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