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Won't crank warm

1994 Toyota 4runner. Won’t restart when warmed up, like after filling up or coming out of store. Behaves like a dead battery and has troubles cranking and then won’t crank. I use a jump box on the battery and it starts fine. Starts OK first thing in the morning or after been parked a while. Bad battery, starter, or something in between? I await the wisdom. Thanks.

Engines may require more cranking power start when they are warm than when they are cold. The fact that you can start it from a jump box suggests to me that your battery is weak. I had the same situation on a 1974 Chevrolet Monte Carlo. A new battery solved this problem.

How old is the battery? If it is 5+ years old then it is likely failing. The heat may cause the battery to swell internally and break a connection lowering the power of the battery. As pointed out some cars have more compression when warm and demand a bit more “juice” from the battery for the starter to crank over the engine.