Duramax losing power in heat

Have you checked to make sure your trans is not causing the heat and slugging down the engine could need an extra cooler maybe it's a thought my sons did something similar lmk

so nice to hear from someone.. I'm a pretty savvy individual around vehicles.. I keep any eye on my engine and trans temps.. I've always noticed that the trans temp goes up a fraction and I've really begun to wonder if I need to replace the trans cooler? I've gently cleaned it with my garden hose sprayer.. the fins are in good shape but.. it's probably factory. I've changed trans fluids and filter in the last year. .. thanks again for your thoughts.. I need insight

No problem they also have aftermarket trans coolers or secondary coolers try and blow the lines or just for the cooler could be clogged good luck

You say your pretty savvy can I throw one at you IMY wife's 04 pontiac grand am gr8 is leaking water inside the car every time it rains generally on passenger side seals are good I'm at a loss but I'm always under a good so I'm kinda blinded help....

Power loss in a diesel is almost certainly fuel related. Check fuel pressure and the fuel filter. My guess is the high pressure pump is crapping out. If it is the original it has a lot of miles on it.

thanks for offering this thought.. keep in mind as soon as I stop, turn off the truck and TAKE the key out for less than 5 minutes the truck runs strong again.. I've gone through all related replacements regarding historical fuel delivery problems and I'll bet, it's stronger with each repair, but it still has this condition related to heat. doesn't do it in cool temps.. if it was a pressure pump it'd be a regular problem

Scan for codes. Maybe something will show up.

Fuel can’t keep up with required fuel pressure when everything is hot and under a load. https://youtu.be/xrBqiqv9QZg