SL/R129: 91 500sl rough idle, running rich. Any ideas?
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Joined: Mar 2007
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From: VA
1991 500sl
91 500sl rough idle, running rich. Any ideas?
Sounds like a loaded question I know but here goes. The car runs great at speed but is running rough at idle and you can smell unburned gas. Not much power at all from a stop. Once it gets above 2k or so it takes off like nothing is wrong. No check engine light. It barely ran when I got it but after a lot of work I am now in the final stages of getting this car through emissions. Here is what I have replaced that was definately bad: Caps and rotors, plugs and wires, fuel injectors, LOTS of vacuum hoses. Maybe wasn't bad but replaced anyway: Fuel distributor and today I had a mechanic change the o2 sensor (hard to do an o2 sensor on the driveway). The mechanic that changed the o2 sensor says its the catalytic converter causing the problem. It's definately bad as a heat test showed the same temp at the front and back of the converter. I'm just not convinced this is the real problem though. Replacing it might get it through emissions but I suspect there is something else causing it to run rough and rich at idle. Before you say get the DVD manual from MB I ordered it a month ago and its still on backorder. The other problem with troubleshooting this is that I can't legally drive the car. In this state you cannot get temp tags until it goes through emissions. I had to drive illegally to the emissions station just try and get it to go through so I can get tags. Any thoughts? Or should I just replace the cat and see what happens? Thanks! 
PS no codes either and that might be because I'm not able to drive the car long enough for it to log any
PS no codes either and that might be because I'm not able to drive the car long enough for it to log any
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Joined: Jan 2006
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From: Atlanta, unfortunately
'09 S600, (2) R129 300SLs, '03 SL500, '03 SL55
The basic mixture adjustment of your airflow meter may be incorrect. I would check the signal from the oxygen sensor and the current to the EHA on the fuel distributor as described here: http://www.mercedesshop.com/Wikka/EngineControls
Senior Member
Joined: Mar 2004
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toss in some denatured alcohol in the tank before you test it. old muscle car trick. search the net for it, lots of info. I'll be honest though, I'm too much of a wuss to do it in the Benz. Says it won't affect it none, but I'm kinda pessimistic =P
Thread Starter
Newbie
Joined: Mar 2007
Posts: 15
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From: VA
1991 500sl
The basic mixture adjustment of your airflow meter may be incorrect. I would check the signal from the oxygen sensor and the current to the EHA on the fuel distributor as described here: http://www.mercedesshop.com/Wikka/EngineControls
.
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From: Atlanta, unfortunately
'09 S600, (2) R129 300SLs, '03 SL500, '03 SL55
J, the OVP is in the engine compartment with the electronic control modules (picture). The relay in the trunk that you mention is for the convertible top hydraulic pump.
If the OVP or its fuse is bad, then the fuel injection control is "open loop" and has no mixture feedback correction from the oxygen sensor.
If the OVP or its fuse is bad, then the fuel injection control is "open loop" and has no mixture feedback correction from the oxygen sensor.
Thread Starter
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From: VA
1991 500sl
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From: VA
1991 500sl
Well good news. The cat got it through emissions but it still is running rough at idle and stumbles from a dead stop pretty bad. I think the brand new cat just worked so well for the emissions test right after it was installed that it passed. The better news is now I have tags on it and I can actually test drive it without fear of being busted. I'll take a look at the ovp relay tomorrow and maybe if they aren't too much I'll just replace it. I need to go to the dealer to pickup a new top relay anyway. Still waiting on the dvd manual .
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Thread Starter
Newbie
Joined: Mar 2007
Posts: 15
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From: VA
1991 500sl
Did it already.
Replaced OVP relay and no change. Running a little better and I saw the check engine light flash earlier today so maybe I finally have a code to work with. Still trying to fix the top.
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Joined: Sep 2011
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1953 300 Adenauer
Fuel distributor
Your problem sounds like a fuel distributor issue. There is an adjustment to enrich or lean - it takes a long hex key and it may or may not have a spring detent to engage. Only turn in small increments. Sounds like it is over enriching; I seem to remember you would turn counter-clockwise to lean out. Try the other direction if the black smoke gets more noticeable. If these adjustments do not make any improvement, you may have a faulty fuel distributor. There are companies that rebuild them for substantial savings over purchasing a new replacement.
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Joined: Apr 2019
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From: Houston
001 s600, 94 sl600
Have you checked the insulation on the wires at the mass flow sensors and ETA'a, The year is right at the beginning of biodegradable wiring in the cars and every one disintegrates and short out etc. Cut open the wire casing and see. New harnesses are NLA. There is a fellow on Youtube (restoreyour mercedes.com) that can rebuild the ETA's and wiring harnesses.
