Rough smooth intermittent idle - PeachParts Mercedes-Benz Forum

keithed · #1 12-06-2002, 02:05 PM

Quote:

Rough smooth intermittent idle

In the past I've posted about a rough idle only at startup for 5--10 seconds and then smooth as a kitten.
Well this morning about the third time I started it up and it didn't get smooth. It idled rough like I'm not really sure. The question and problem is a W126 89' model V-8. Does or can the fuel relay and or the fuel filter cause this. When I had a timing chain job done a few months back I asked them to change the fuel filter. I was told the fuel lines looked bad so they didn't want to take the chance and mess with changing the fuel filter. For several times I've tryed something I've never done before over the last week or so. I would turn the key on but not crank it over, just enough to work the fuel pump. Turn the key back for just a second or so and start the engine. It didn't seem to idle as rough before becoming perfectly smooth. Today it would not go away so I got on the Interstate and drove about 10 miles and when I came to a stop at the exit, it was smooth for a few seconds and then rough again like a miss. New Bosch plugs and cap rotor so I'm about 95%sure its fuel delievery problem. By the way this happens even after the car is hot and not just at cold start up. Can a bad fuel rail etc cause this problem. I've read the other threads but none seem to apply to my problem. Thanks everyone for you help and info.

Benzmac · #2 12-06-2002, 07:29 PM

I would have a tech put the ignition system on a scope. If the tech is good, he can see whether it is a fuel or spark related issue.

keithed · #3 12-06-2002, 09:25 PM

Thanks Donnie I was leaning that direction anyway. Besides trying to guess this and that and parts changing can get very expensive.

Q · #4 12-06-2002, 09:55 PM

Trust me on this one. Same thing happened to me not too long ago. After it did the rough idle thing for a while, it started to cut out completely on me every once in a while. Finally, it stopped all together.

I did my homework and started diagnosing the problem. Turned out to be the fuel pump relay. I was able to resolder three broken solder joints on mine and the problem is now gone.

If the car ever shuts down on you and will not start, look around on the forum for info on it. If you have the manual, it explains how to diagnose it too. I found that by jumpering two pins on the relay socket, I could bypass it. When bypassed, the car started right up.

Good luck

keithed · #5 12-06-2002, 10:38 PM

Funny you say that because in the back of my mind I was thinking about that today and called and got a price on a new one. I've heard of others solder them and fixin the problem. Thanks for the info.

Keith D

Q · #6 12-07-2002, 11:04 AM

Soldering it was sooo easy. The pins you need to solder (the legs of the biggest relay) are so big that even Mr. Magoo could do the job. The hardest part for me was getting the cover back on.

keithed · #7 12-07-2002, 12:08 PM

Thanks alot. I guess that is straightforward enough. I take it that age and or vibration can cause these breaks and soldering them back is no biggie. Thats a lot cheaper than a new unit for sure. It is behind a black box type cover, behind the battery area in the firewall on the passenger side correct? Thanks much

Keith D

Q · #8 12-07-2002, 12:31 PM

On my 85 380SE, it is on the drivers side under the fuse cover. It was black and in the rear-most drivers-side corner position. Maybe yours is in a different location.