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Racing/pulsating cold start on 73 450SL

1525 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?
I answered in the other thread, which I will delete. This is what I said:

x046866 - The original poster was right, I believe - there is a cut-off that causes the "loping" at 1500rpm. What you want to do, is adjust the rpm when cold to below 1500rpm - say 1400rpm - that should stop the loping. Then drive the car until warm and see what the rpms are at operating temperature. If still high, adjust them back to about 700rpm in Park, using the adjustment screw Then put in drive and see what they are. If it is about 500-600 rpm, then you are fine. If lower the car might not idle evenly and may cut out. Key is to stay below 1500rpm when cold.

If you can't get rid of the problem by adjustment (for example high rpm even with screw all the way in) , then you might want to check for proper operation of the Aux Air valve - it might be sticking (you will have to remove it, soak it in a solvent and cycle it through operating temperatures by putting it in hot water). Also check the resistance of the two temperature sensors per the manual. If everything is OK, then look for other vacuum leaks. There are only a few vacuum connections on a '73.
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