Hard Cold Starts in 380sl Caused By Idle Speed Control?
My 1981 380sl has always started right up hot or cold. Recently, however, when it’s cold, it starts to fire, but then reverts to cranking. So, after reading some posts on hard cold starts, I now turn the ignition on off 2-3 times until I hear the fuel pump buzz briefly, and I then pump the accelerator 2-3 times to make sure there’s enough fuel in the system.

Even after doing that, however, the car’s still often hard to start when its cold. But, once it does start, and the warm up regulator (WUR) takes over, the idle speed increases normally until the Idle Control Unit (ICU) takes over, and then the idle speed drops to the correct level. Once the car is running, if I turn it off, it will then start right back up, even after it has sat for the rest of the day. So I don’t think that the accumulator could be the problem.

I removed the cold start valve (CSV) and put it into a jar to see if it was putting out fuel, and it was, and the frequency valve buzzes once the car starts. In addition, the 16C oil temp switch is relatively new, and the thermo-time switch seems to be working OK, because its resistance increases as the car warms up.

Curiously, though, the car will start right up when it’s cold if I remove either of the two ICU’s I tried that with: one of those is an older one which otherwise works fine, and the other is a relatively new (rebuilt) Programa unit, which also works fine otherwise. So I’m assuming that something that signals the ICU must be creating the problem?

I should note that when I recently replaced the fuel filter, I forgot to take the clamp off the line to the filter when I started the car and ran it for a few minutes, which caused the pump to buzz, and may have put a strain on it even though the car ran OK. Nevertheless, since the CSV is still putting out fuel, and the car runs great otherwise, I’m assuming the pump is good. In addition, I recently replaced the O2 sensor, but I can’t see how or why that could be causing the problem.

As the problem is obviously way too complicated for me to solve at this point, I was hoping that someone who knows a lot more about the CIS system than I do can help me. So I don't have to take it to an Indy or — even worse — the dealer.

Thanks very much in advance.

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1981 380SL
Chassis: R107.045

without testing anything, the shot gun approach would be replace the over voltage relay and cross your fingers. if it's older than 8 years, it could be the whole problem. it powers the idle control, fuel control, abs, etc. good luck, chuck.