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Discussion Starter #1
Over the past week my truck has been bogged down when trying to start it, but not all the time. Starts up fine for a couple of days and then this AM, almost like the batteries were dead. This PM, started fine, no issues. Doesn’t matter how long it sits, hours or days it’s just been random. Batteries are both fine, tested. What could be a possible problem? Starter going out? Stationed on Guam, so trying to look for a little insight before I take it to a diesel mechanic here. TIA.
 

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First thing you need to know is, "are the batteries actually down?" Down could mean low voltage, or low amps from batteries, or both. For a few mornings or start attempts, disconnet the battery negative cables (make SURE it is the negative) and then measure the voltage of each battery. Record dates, times, voltages so that you have a record, and note if you have starting problems. There may be a parasitic draw, maybe it is happening some times, sometimes not, and you need to find and fix that. Only tool that is needed is a volt meter. If that all checks out OK, your voltage is reasonably steady about 12.6 or so after all night rest, then have the batteries load tested, one at a time, to make sure the voltage has enough amps to actually start the truck. Main culprits are batteries, circuits (cables, mega fuses and connections), starter. Lots of (easy) work on your part, but it all should be free. Once you have a good picture of the electrical system, you should be in a much better place to judge what to do about it, parts to buy, etc.

I know you said batteries were 'tested', but that could mean lots of things. What testing was done?? And were the batteries tested separately?
 

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Discussion Starter #3
First thing you need to know is, "are the batteries actually down?" Down could mean low voltage, or low amps from batteries, or both. For a few mornings or start attempts, disconnet the battery negative cables (make SURE it is the negative) and then measure the voltage of each battery. Record dates, times, voltages so that you have a record, and note if you have starting problems. There may be a parasitic draw, maybe it is happening some times, sometimes not, and you need to find and fix that. Only tool that is needed is a volt meter. If that all checks out OK, your voltage is reasonably steady about 12.6 or so after all night rest, then have the batteries load tested, one at a time, to make sure the voltage has enough amps to actually start the truck. Main culprits are batteries, circuits (cables, mega fuses and connections), starter. Lots of (easy) work on your part, but it all should be free. Once you have a good picture of the electrical system, you should be in a much better place to judge what to do about it, parts to buy, etc.

I know you said batteries were 'tested', but that could mean lots of things. What testing was done?? And were the batteries tested separately?
I tested as you stated above. CCA were around 900 for both batteries and 12.53V for both. I'm by no means a mechanic, but a friend of mine said something about compression chambers not releasing pressure could cause it to not turnover. He's a big gear head, but said he's not familiar with Duramax engines.
 

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I hope I am wrong, but you might have a head gasket leaking coolant into a cylinder causing hydrolock.
Are you adding coolant? Is the coolant over flow filling up?
 

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Discussion Starter #5
I hope I am wrong, but you might have a head gasket leaking coolant into a cylinder causing hydrolock.
Are you adding coolant? Is the coolant over flow filling up?
Checked it this morning. Nothing in overflow and coolant level is the same. Haven’t had to add any.
 

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Is the pressure side (drivers side) of the tank full to the top?
 
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