Howdy! I'd appreciate any input. My 1980 450SEL, W116 has been running great for the last 5 years, but since this past summer, it has developed a hot restart problem. It would start great cold, but after driving to the store, and letting it sit for 5-30 minutes, it would SOMETIMES (BUT VERY RARELY)restart with a VERY slow rough idle, pushing the accelerator made it worse or made it stall, and revving or flooring it would produce knocking or "small explosions". Basically, you'd have to limp back home.

Then you let it sit for many hours, and it would start up just fine, drive just fine with lots of power. But last night, it started up cold with the slow rough idle, giving it gas wouldn't even make it move uphill. I was able to floor it to get it back into the driveway, and then today when I started it, it started slow, but came to idle within minutes and ran and drove beautifully, without any hot restart problem at the store a few minutes later.

This is so random and flukey, that I'm thinking maybe a sensor is giving a bad signal randomly? It seems so heat related. We had an abnormally warm day yesterday for this time of year, though hot days don't seem to have anything to do with cold starts, but we had driven it 4 hours earlier.

It never has any problem with loss of power when driving if it starts normally, but if it starts with a slow idle, it never runs right. The only symptom I've noticed during normal driving is a rough patch or hesitation in acceleration that smooths out with increased speed.

Also, when it starts with a rough idle, it seems to be running on just a few cylinders and the passenger side exhaust at the engine sounds really leaky and loud.

I keep thinking that a tuneup will cure this. It almost feels like it is flooding during these bad times.

Thanks for any help.

Tune-ups usually fix a constant problem, but if it's been a while, it definately won't hurt.

Have you checked to see what your plugs look like?

Your problem sounds very temperature related. Could be a poor connection, a bad ground, poor wiring, or bad engine temp sensor (among other things).

Next time it's running smoothly, wiggle wiring & tap on stuff - especially modules and sensors. See if you can get it to stumble.

You could also start it up, and if it's running smooth, turn it off and take a hair dryer to a control module or two, then start it back up. If it starts rough, try a little upside-down air duster on the same modules then start it back up to see if it smooths out again. This is basically the no-tools heat-related problem approach.

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Thank you for your reply! I'm leaning towards the engine temperature sensor, and I will check it. I was feeling that in the back of my mind before I posted. Nice to have somebody else mention it. I'll get my discs or manuals out and give it a check. Thanks!

CIS FI system...
Suspect the cold start valve being bad.
Start by disconnecting the electrical connector.

Buy a spare paper gasket for it and disconnect from the manifold. With the business-end of it pointing into something and also wrapped in a rag to contain any possible spill. Start the car. If ANY fluel escapes with the wires disconnected, you have your culprit.

Oh, and temporarily plug the vacuum leak caused by the disconnected valve.

Thx -CTH

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