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Old 01-20-2003, 01:13 PM
maheath maheath is offline
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Join Date: Sep 2001
Location: Austin, TX
Posts: 563
M103 hard start, rough idle, hesitation SOLVED

I didn't actually post any questions for my particular problem. I spent a lot of time reading the archives and following every M103 thread for the last 3 months. One thing I've noticed is that it isn't very often when someone comes back and says "Hey, this is what solved the problem." In light of that, here is what solved my problems.

Here's the background. I bought an '88 300SEL last October. At the time of purchase, it had absolutely no service records, was difficult to start, idled roughly and hesitated upon acceleration when the engine was cold. Was smooth as glass on the highway.

The first thing I did was to replace all of the fuses. That's a lesson that I learned with my 560SL. Changing the fuses didn't seem to change anything with the engine, but I wasn't expecting it to.

After reading a bunch of the threads on the M103, decided to attack the ignition system first. Having no records, I decided to change the maintenance items first. I replaced the spark plugs, wires, distributor cap, and rotor. When I took off the old distributor cap, it was busted up pretty good. I was surprised that it worked at all. This greatly improved the starting aspect. It went from literally 5-10 minutes to start the car, to 15-30 seconds. I have since discovered that the plug wires themselves are not a maintenance item, but the connectors are. I know that the connectors on the old set of wires were in bad shape. Every one of the plugs that I pulled out were a light tan on the ignition end. I took that as a good sign. Oh, and I also dropped a bottle of Techron into the gas tank, the size for 21 gallons.

My next plan of attack was the idle control valve. Not costing anything but time to take it out and clean it seemed like excellent bang for the buck. The first thing that I noticed after taking the air cleaner off was that there was a connector unplugged. It was the cold start valve. I plugged it back in and proceeded to remove the idle control valve. While doing that, I noticed that one of the breather hoses going into it had electrical tape around it. I couldn't see very much of the other breather hose connected to it and the hose going from the air cleaner into the valve cover was split where the vacuum line attached, so I decided to order 3 of the 4 breather hoses. Anyway, I removed the idle control valve, cleaned it out with air intake cleaner, but it wasn't really dirty at all. Checked it's operation by applying 12V, seems to work just fine. Put it all back together (didn't have the new breather hoses yet). This seemed to help the starting condition, reducing starting time to about 10-15 seconds.

At this point, it takes me 1 or 2 cranks to start the car, usually applying gas as I crank it. It idles very rough at first, hesitates when accelerating from a stop, and idles roughly at stop signs and stop lights.

Since my ABS light was going on and off intermittently, I decided to replace the OVP. The unit I bought had two fuses in the top while the unit originally installed had only one fuse in the top. I haven't seen the ABS light since I replaced it, but it didn't seem to change anything with the engine.

My breather hoses also came in and I proceeded the change them. The hose from the idle control valve to the intake manifold was a real bit...bast... well, it was difficult to install the new one. I did post a new thread on this forum about this one and got some good advise. The trick for me was not reusing the metal clamp that was on my old hose. After I got it on I used a cable tie to keep it tight. Replacing the hoses seemed to improve things, but not significantly.

I then pulled the fuel pump relay and checked the solder connections. Nothing obvious. I also pulled the relay next to it and checked it's solder connections also. I've forgotten which relay that was, but it didn't appear to have any problems either.

Next on my list was the coolant temperature sensor. One of the wire connectors had electrical tape on it, so I figured there was probably a problem there. Anyway, before I got around to doing something to it, my tank was nearly empty again (the reserve light was on). On a whim, I decided to drop a bottle of Techron Pro-Guard into the tank. I did it in my driveway and then drove about 10 miles to the gas station that I wanted to use. By the time I got to the station, my idle problem had disappeared. Driving back from the gas station, I had no problems at all. I figured then that the real test would be this morning when I started the car. It started this morning on the first crank, without me having to give it extra gas. It's idle increased to about 1100 and then dropped back down to about 600. You can still feel a slight roughness to the idle when at a stop light, but I think new motor mounts will solve that. And no hesitation, ever. I had thought about pulling the injectors and checking them out, but I guess I put too much faith in the first bottle of Techron that I put in.

Anyway, I'm tickled to death that the engine is running so well now and I have this forum to thank for that. I'm still considering pulling the injectors and replacing the holders and seals. They look pretty old.
Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino
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