Join Date: Mar 2002
Location: Austin, TX, USA
Re: Hot engine - no start
Besides a heat soak issue, you may have a bad connection somewhere in your electrical system that is heating up and creating enough resistance that the starter won't engage. The old heat soak problem is a common diagnosis - heat from the headers heats up the solenoid, increases it's internal resistance and prevents the starter from engaging. So it seems obvious that the longer the car runs, the longer the headers cook the starter solenoid and the worse the no-start-when-hot issue becomes. But when you think about it, any bad connection in the electrical system's primary voltage circuit will act the same way. The longer the car runs, the longer current flows through the primary circuit (and the bad connection) and the more heat (and more resistance) develops.
Use a digital volt-ohm meter (DVM) to measure from the battery (+) to the starter solenoid (S) wire while a helper cranks it. If you see less than 0.05V drop on the meter everything between the battery and the starter 'S' terminal is good. Repeat the test on the ground measuring between the battery (-) and the starter / solenoid shell while having a helper crank it. Less than 0.05V tells you everything in both the hot and ground circuits is in good shape and you probably have a heat soak issue.
If either test shows significantly more than 0.05V, you've isolated a circuit branch with a bad connection. You can use the same voltage drop measuriing technique to isolate the exact component. For example, measuring between the incoming power connection and IGN terminal on an ignition switch with the ignition on or engine running will verify the IGN contacts inside the switch. Measuring the drop on a wire that passes through the firewall bulkhead connector will verify the connection between the outer and inner half of the connection.
But start big as I describe in the first paragraph measuring from the battery to the load point. That will tell you whether further voltage drop measurements are warranted or if it's time to spend some coin on a new starter / solenoid.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI