Car will not start when hot

I have an old Chev with a 350 motor. If I drive the car on hot days or get tied up in traffic and then turn off the car it will not start for about 20 minutes. Any ideas why? What can I do?
Thanks for your response
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How old is "old"?
Possible vapor lock?
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its in a 59 Chev, not the stock motor. What happens is the car will not turn over, I think the starter gets overheated. Mike Marlow wrote:

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I'd run an extra wire to the starter that comes into the vehicle and attach it to a voltmeter.
Run the car till it fails. Then look at the voltage on the meter as you try to start.
If it stays high, then the problem is in the starter. Just replace it. If it takes a dive to 0, then the problem is before the starter, somewhere back the wiring.
Follow the problem back with the voltmeter. You're going to find where it's high at one point on the 'battery side' and low on the 'starter side' IF the problem is in the high current circuit. Heck, it could be the battery terminal is dirty. (Or the motor block ground strap) If the problem is at the starter solenoid, before you blame it, make sure it's getting the start signal to kick on. I've seen Ford solenoids that click and appear to function but just won't work. I loosened up the nut holding the starter cable and tightened it back down and it worked fine. Turned out that the 'shaft' that you bolted to could 'twist' as you tightened the nut, and internally move to where it wouldn't make a good contact. I saw the same kind of thing happen with my Dodge Caravan with the Mitsubishi starter with the built in solenoid. I ended up taking the cover off the solenoid portion of the starter and found a brass bar that made contact when energized. Moving the nuts would cause it to shift the contact points slightly and it would work. For a while. Then fail again. The bar was pitted, so I just flipped it over to the other side which looked smooth and virtually new and it worked until the vehicle was 'retired' 100K miles later. By the way, that usually acted like a 'thermal when it was failing.
Mike

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Spike wrote:

Some GM cars of the late '50s were susceptible to "hot starter lock". Look on auto parts sites for a starter heat shield.
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I'm with you on this. Sounds like heat soak to me as well. Jeg's and others make a shield
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My 77 Chevy van did the same thing. Especially when stopping for gas after pulling the boat. Tapping the starter motor while someone turned the key started it. New starter motor and solenoid fixed for the next 10 years..

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