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Old 01-20-2008, 01:10 PM
Registered User
Join Date: May 2007
Posts: 13
1989 129 500SL rough running

Hi all,

I have a 1989 129 500SL which runs ok from cold but once it warms up it starts to misfire and eventually stalls. It is usually very difficult or impossible to restart it and it backfires as the engine is cranked over. If I leave it for about half an hour it will restart fine.

I'm guessing its a problem with one of the engine temp sensors causing the car to overfuel.

Does this sound reasonable or should I be looking elsewhere?

Can someone tell me what sensors to check and where on the engine they are located?

Many thanks,

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Old 01-22-2008, 10:06 AM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Usually, backfiring means a lean engine, as in not enough fuel, or a vacuum leak.

You can test your theory by trying to start it for a minute or two, then pulling out a spark plug or two. If it's flooding, the plugs will be wet.

Timing can also cause backfiring. Can you put a timing light on it when you are trying a hot start to check spark and #1 firing position?
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Last edited by ctaylor738; 01-22-2008 at 10:16 AM.
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Old 01-22-2008, 11:31 AM
Join Date: Dec 2007
Location: chicago
Posts: 53
Have you changed the fuel filter. I don't own a 500, I have 380, but I do know that if you have a semi-blocked fuel filter, you will get this problem. You can also check for any possible bends in the fuel line. I'd also try running some quality fuel injector cleaning in the system. That might clean up any minor blockage. Try all the easy(non expensive stuff first) and progress from there.
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Old 03-13-2008, 05:02 AM
Registered User
Join Date: Jan 2008
Location: spain
Posts: 7
Thanks For All Help Guys. Found To Be TemP Sensor , Was Suggested Originally By A Friend But Wanted To Diagnose ,first. Think Moral Of Story Is If Suspect Part Is Cheap Enough Replace It. Sposse It Takes Away A Bit Of The Fun Though.

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