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Old 02-01-2000, 01:58 AM
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My '89 300E will go thru cycle of not idling when I decelerate at stop, so I use left foot on brake and right foot to keep slight pressure on gas pedal so RPMs are 600-700. Engine often idles rough at this time also. Goes thru cycles of doing this several time then will run well -holding idle above 500 without problem. Had MB mechanic drive it for week. Eventually happened 3-4 times and always restarts easily, but by the time he got back to shop it was OK. Could not find problem. MB dealer put it on scope, and gave estimate of $2,500 to tune-up, replace parts such as lamda tower, injectors,etc. but would not guarantee that it would be fix. Said it could be many components including climate controls, fuel pump, etc. Earlier problem w/engine dying at road speeds fixed w/fuel pump relay switch. This problem when engine should idle. Cleaned trottle linkage, added cleaners & gas-line antifreeze to tank.

Notice that dash lights dim when RPMs decline. Alternator rebuilt 1 yr ago. It would appear to be related to some electrical switch since it dies fast and one has to be quick to catch it or it dies. Big problem in traffic.

Any recommendations? before I have to take it out and shoot it.

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Old 02-01-2000, 11:51 AM
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Join Date: May 1999
Location: So. Burlington, VT
Posts: 78
I had the same apparent problem with my '89 300E a couple of years ago. It idled rough and occasionally stalled while driving and releasing accelerator. My dealer had no problem diagnosing the problem as UV Protection Relay and O2 sensor. He said the UV Relay was the major culprit. I elected to replace it and the stalling stopped. It still idled rough so I then replaced the O2 sensor and now it idles smoothly. Total cost was about $150.00. The relay is very easy to do and a nimble fingered weekend mechanic should have no problem doing the O2 sensor. The wires for that go thru the transmission tunnel and connect under the passenger side carpet. If you now the O2 sensor has been replaced in the last 60K miles, go for the relay.
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Old 02-01-2000, 12:21 PM
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Don't overlook the possibility of a vacuam leak. It's easy enough to check with a vacuam guage. Check all rubber hoses, connections, etc for one which has deteriated from the underhood heat. Also keep the edge of the flapper valve underneath the air filter. Simply wipe it off with a lint free cloth.

Good Luck,
Larry Bible
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Old 02-01-2000, 12:26 PM
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I had another thought. My '88 300E has a vacuam guage on the dash. It's called an economy guage. It is a manual transmission model, your's is probably an automatic. I don't know if this guage is on an automatic model. If it is, the red area indicates low vacuam. This may help you to check for a vacuam leak as I suggested in my previous reply.

Good Luck,
Larry Bible
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Old 02-01-2000, 08:47 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
It sounds as if you are having a common problem. I would have the overvoltage relay checked, and the fuel pump relay checked. Also I have seen the connector for the CIS-E control unit fill with water and cause a simmmilar problem. This happened when the plastic cover was missing from behind the battery.

1981 280GE SWB
1987 16V

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