1985 300SEL Rough Idle
12-02-2008, 04:05 PM
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1986 Mercedes 300SEL
1985 300SEL Rough Idle
A bit of history:
When I first got this car, I took it to the service department of the local Benz dealer for a thorough service. Among other things, they told me that the fuel pump relay was close to shot and should be replaced soon. I would know when it gave out when the car just stalls for no reason. Well, it did, so I put in a new relay.
Well, darn thing stalled again out of the blue. Had it towed back to the dealer who deduced it could be one of two things: a bad ignition control unit or a bad air flow sensor. He demonstrated on machines that the two things gave bad readings. He also showed me how a new ICU dramatically improved acceleration. Oh, and just by the way, he says, they found a loose wire from the ICU, which, I am now 100% convinced was the REAL culprit to the stalling. I got the ICU anyway, because of the amazing improvement in acceleration.
Then, on the dealer's assurance that it must be the bad air flow sensor causing the rough idle, I replaced that too. Well, it still idles rough. In neutral, the engine shakes. Stopped, but in gear, it almost sounds like a cylinder or cylinders are misfiring. The condition worsens as the engine gets warmer. It also is more pronounced if the A/C is on, IE more strain on the engine. The problem is only during idling. It accelerates and drives smoothly.
I scoured the forums and found several posts that suggest it could have something to do with the fuel delivery system. Perhaps a filter somewhere is clogged. When I had all the above work done, though, they had checked the fuel filter and injectors but found no issues. However, since then I used a couple cans of the MB gas additive that I wonder could have loosened something that is now clogging up something. I checked with my uncle who runs a Benz dealership in the US and his shop foreman suggested replacing the fuel injectors.
Thoughts anyone?
When I first got this car, I took it to the service department of the local Benz dealer for a thorough service. Among other things, they told me that the fuel pump relay was close to shot and should be replaced soon. I would know when it gave out when the car just stalls for no reason. Well, it did, so I put in a new relay.
Well, darn thing stalled again out of the blue. Had it towed back to the dealer who deduced it could be one of two things: a bad ignition control unit or a bad air flow sensor. He demonstrated on machines that the two things gave bad readings. He also showed me how a new ICU dramatically improved acceleration. Oh, and just by the way, he says, they found a loose wire from the ICU, which, I am now 100% convinced was the REAL culprit to the stalling. I got the ICU anyway, because of the amazing improvement in acceleration.
Then, on the dealer's assurance that it must be the bad air flow sensor causing the rough idle, I replaced that too. Well, it still idles rough. In neutral, the engine shakes. Stopped, but in gear, it almost sounds like a cylinder or cylinders are misfiring. The condition worsens as the engine gets warmer. It also is more pronounced if the A/C is on, IE more strain on the engine. The problem is only during idling. It accelerates and drives smoothly.
I scoured the forums and found several posts that suggest it could have something to do with the fuel delivery system. Perhaps a filter somewhere is clogged. When I had all the above work done, though, they had checked the fuel filter and injectors but found no issues. However, since then I used a couple cans of the MB gas additive that I wonder could have loosened something that is now clogging up something. I checked with my uncle who runs a Benz dealership in the US and his shop foreman suggested replacing the fuel injectors.
Thoughts anyone?
12-04-2008, 05:06 AM
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190D 2.5 (x2), 190E 2.6, W202 C240,W202 C43 (C55), W210 E55, W212 E250CDI
Have a look at the distributor cap and rotor button first, many people forget these items have a lifespan.
If it looks really burnt/corroded then replace it before carrying on with other checks
If it looks really burnt/corroded then replace it before carrying on with other checks
12-22-2008, 03:56 AM
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1986 Mercedes 300SEL
Problem solved!
It was the fuel injectors. They were cleaned a year ago when I first got the car, which is why the dealer here dismissed them as the likely cause of the rough idle. After he had cleaned them before, he gave me some fuel additive, which I think loosened some gunk in the system, which then clogged the injectors again. I asked him to check them again, and sure enough, they were gunked up. He cleaned them again and the engine purrs like a kitten.
It was the fuel injectors. They were cleaned a year ago when I first got the car, which is why the dealer here dismissed them as the likely cause of the rough idle. After he had cleaned them before, he gave me some fuel additive, which I think loosened some gunk in the system, which then clogged the injectors again. I asked him to check them again, and sure enough, they were gunked up. He cleaned them again and the engine purrs like a kitten.
