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Racing/pulsating cold start on 73 450SL

1522 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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I will try this over the weekend. The thread you mention did not list a condition where the idle would race up and down in a pulsating manner though (high-low-high-low-high-low on maybe just less than a 1 second frequency); would this be a condition you would expect with a faulty AAV?
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