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'09 ProStar CM871.

This just started yesterday. My engine is scaring me, I thought it wasn't going to start. It was off for about 1 hour. I went to start it and it started to crank very slow over one cylinder, then faster over the next 3 or 4 then fired up. It has done this a few times since, and a few times started normal.

Is this possibly a hydro lock situation?

I had the starter replaced a couple months ago. Volts reads 12.4 after truck has been off for an hour. Engine sounds normal once running.

could possibly have an injector leaking down after shutdown. not sure if it would diagnose this exact situation but doing a bubble test might find it.

Almost sounds like a parasitic draw if the voltage drops that quick. Which will impeded the starter... How old are the batteries? And have you ever crawled under the truck to visually inspect the cabling running from the battery box to the motor and accessories? I am sure you read my experience with my factory wiring, exposed cable, no jacket?

I would also check the ground cables from the starter to the frame rail, and the one from the cab to the frame rail, just opposite the starter one, Rawze caught that one. Hell, I never knew it was there! And it was shot. That is where I would start, as it is fast and free... If all looks good, go on to more extensive diagnoses.