Trouble starting when Hot!

Two things I'd consider are deficient battery cables (they get resistive as they get hotter) or a bad starter (esp if you happen to have LT's).

+1 on the bad starter. Do you have one of the old style starters or the newer mini starter? The old style will do that when they get hot, especially if you have headers. The heat causes more resistance in the starter (As Hisson50 said about the cables) which causes it to turn slowly. The newer style mini starters are not affected by heat as much because they use permanent magnets. I had this problem on a previous car, and upgrading starters completely solved it.

Before you waste time trouble shooting or replacing anything, take your battery into a parts store and have them test it. I chased hot start issues for a LONG time, replaced the starter, replaced the cables, re-did all the grounds, even wrapped the headers with thermal wrap. ALL of this, because my battery seemed fine because it was reading 12 volts with the engine off, ~13V engine on. Finally checked the cranking voltage, which was like 8-9 volts or something. Somehow it was managing just enough power to start the car when it was cold, but not hot. Took the battery in, and sure enough, it was dead as could be. Replaced it with a new one and the car hasn't had a single issue since.

Definitely have the battery checked out first. If it checks good, be prepared to do some voltage drop testing to find any bad connections or bad cables.

1.) Will the car start if it is jumped? Then clean battery terminals and check battery for low charge and dead cells. A good battery will measure 12-13 volts at full charge with the ignition switch in the Run position but without the engine running.
A voltmeter placed across the battery terminals should show a minimum of 9.5-10 volts when the ignition switch is turned to the Start position and the starter engages or tries to engage. Less than this will result in a clicking solenoid, or slow cranking (if it cranks at all) or a starter solenoid that sticks and welds the contacts together.

Most auto parts stores will check your battery for free. It does not have to be installed in the car to have it checked; you can carry it with you to the auto parts store.

The battery posts and inside of the battery post terminals should be scraped clean with a knife or battery post cleaner tool. This little trick will fix a surprising number of no start problems.

The clamp on with 2 bolts battery terminal ends are a know problem causer. Any place you see green on a copper wire is corrosion. Corrosion gets in the clamped joint and works its way up the wire under the insulation. Corroded connections do not conduct electricity well. Avoid them like the plague...

A voltmeter is handy if you are familiar with how to use it to find bad connections. Measure the voltage drop across a connection while trying to start the car: more than .5 volts across a connection indicates a problem.

See Automotive Test Tools for help for help troubleshooting voltage drops across connections and components. .



2.) Check the battery to engine block ground down near the oil filter, and the ground behind the engine to the firewall. All grounds should be clean and shiny. Use some sandpaper to clean them up.

3.) Jump the big terminals on the starter solenoid next to the battery with a screwdriver - watch out for the sparks! If the engine cranks, the starter and power wiring is good. The starter relay is also known as a starter solenoid.