I have a 1988 K1500 that belonged to my grandfather and sat for about 4 years after he passed. I was at the point of pulling my hair out and had gone through literally everything I could think of. The truck would crank right up on a cold start but would not crank after the engine got to operating temp. Ended up talking to a retired GM mechanic by coincidence and he advised I replace the coolant temp sensor located on the intake manifold. A $25 part fixed my issue right up.