What's happening?? Car runs rough after prolonged hi-speeds - Mercedes-Benz Forum

 
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post #1 of 5 (permalink) Old 03-13-2009, 05:52 AM Thread Starter
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Date registered: Nov 2008
Vehicle: 1988 260E
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What's happening?? Car runs rough after prolonged hi-speeds

I really need help from you experts on a problem with my 124. The car normally runs beautifully under all conditions; idles fine, accelerates without hesitation or stumbling, pulls strong, shifts cleanly, runs all day long at 80 mph. Cold starts sometimes crank longer, but that's the only weakness. 193K miles on the clock.

Now the problem: last weekend made a 6-hour high-speed trip on Interstates, cruising for several hours at a time at sustained 75-80 mph. Pulled off for gas stop, and as soon as I'm on surface streets at low speed, particularly stopped at traffic signals, the engine idles very rough (RPMs go below 500), to the point that I have to keep light pressure on accelerator to keep it running, and it stumbles badly trying to accelerate from a stop. Accelerating, car is very sluggish, totally under-powered. Shut the engine off, even just long enough to fuel-up, and it starts up and runs fine again in every respect. Run along at 75 mph for a few more hours, slow down in town and the rough idle/sluggish acceleration returns. All it seems to take to get rid of the problem is to turn the engine off for even just a few minutes. Problem never exists if I just drive around town, only after prolonged high-speed driving.

Recent (within past 2,500 miles) maintenance/service performed: new OVP, new battery, all new filters, all fresh fluids, new correct NGK non-resistor plugs. Routine maintenance has always been performed. No obvious evidence of vacuum leaks ("economy" gauge usually pegged left at idle, except with A/C running, when it rides about quarter way). No CEL. What do you make of this situation? Thanks guys.


1988 260E (Anthracite Grey over Palomino)
Owned since new; still strong and smooth @ 197K

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Last edited by Cal Learner; 03-16-2009 at 07:12 AM.
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post #2 of 5 (permalink) Old 03-13-2009, 07:51 AM
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This is just a wild guess, but sometimes computers reset themselves after you shut the vehicle off. That can work with anything from a "check engine light" to a number of sensors.
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post #3 of 5 (permalink) Old 03-13-2009, 02:41 PM
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I could be way off but with that many miles, do you need a valve adjustment? Bad gas? That could cause your symptoms.

Last edited by Softgenious; 03-13-2009 at 03:16 PM.
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post #4 of 5 (permalink) Old 03-13-2009, 02:46 PM
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Could it be the gas quality you got at the occasional place?
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post #5 of 5 (permalink) Old 03-16-2009, 07:21 AM Thread Starter
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Thanks for the ideas. I'm thinking something's failing in the idle control system, possibly bad OVP (that powers the IAC valve and fuel pump relay) or engine temp sensor (affects both ignition control and fuel management). Time to haul out the multimeter and run some tests. I'll post results when I know something.


1988 260E (Anthracite Grey over Palomino)
Owned since new; still strong and smooth @ 197K

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