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Racing/pulsating cold start on 73 450SL

1524 Views 4 Replies 3 Participants Last post by  ScottinSoCal
I have a 1973 450SL that I have spent a significant amount of time (and $$) on trying to get to run smoothly. It is running much better than before but I still have a couple of nasty bugs I want to get ironed out.

The weirdest one has to do with a cold start issue. The car will start just fine but after about 45 seconds of running the idle begins racing in a pulsating manner; approximate RPM: 800->1500->800->1500->800->1500....this repeats until I put it in gear and then drive away. It does not happen again until the next cold start.

Other problems that may (or may not) be related:
- Hot start problem (there is a lot of info here on this problem; I am replacing the auxiliary fan electronics to ensure the compartment runs cool enough and am also replacing the fuel pressure regulator)
- Rougher idle than I would expect (not bad, but not clock-smooth), and the engine runs rough when first applying accelerator when pulling away from a stop (running it leaner appears to have helped this last issue).
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Other Thread said:
Watch that you don't get the cold idle above 1500. There's a cutoff built in to the ECU on the Bosch D Jetronic and if the idle speed goes to 1500 with the throttle pedal up it cuts off the fuel until the rpms drop down to around 800, then kicks in the fuel again. You wind up with an engine that races like someone is fluttering the pedal.
It's in there.
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