Ok here's the deal. I have gotten a 2001 chevy express van with the 6.5 TD. i have put 2 brand new batteries and cables on the truck within the last 2 weeks as when i got it there was no batteries in the van. when it's cold it starts right up no problems but the warmer it gets the slower the engine turns over.. iv checked the batteries and the cables and all the engine grounds but im either not seeing anything or im missing something... thanks guys and look forward to some answers and or ideas

 

NMake sure that the connection point between the 2 batteries posative cable is secure and if using stock cables and bolts taht the lead spacer is between them.

Symptoms sound more like a bad starter though-vans seem to eat starters from the heat trapped in that area.

 

I had that problem with my 95, replaced the starter and both batteries and now it cranks fine. I work for Sears Auto Center, if you have one near by stop in and ask for a full die hard service, they'll test your batteries, alternator and starter.