My truck has me good and stumped. Lately, it has been acting up in the worst sort of way - it is overheating when under load.

I replaced the thermostat first thinking it was that, and radiatiator (unrelated issue). It will not overheat when idling, but when it is driven the guage climbs to infinity, but as soon as I stop and let it idle, it will slowly climb back down over time. I know the guage is accurate, because I have measured the actual temperatures at vaious points using my infrared thermometer.
Today I ran a compression test and compression leakdown test, and the results do not point to a leaking head gasket, I get an average of 7% leak and 140psi compression. No 5 cylinder has the lowest compression and highest leak, but it seems to be from the intake valve not completely sealing - air was slightly leaking out of the intake no matter where we tested the cylinder in the 3 strokes it should be closed.
The viscous coupling appears to be working properly, but other than seeing how hard it is to turn when the truck is hot vs cold I don't know how to check it. (I replaced it about a year ago, 7k miles).

I really need to determine if I am getting exhaust gasses in the cooling system when under load so I can determine if it is internal engine or external issues. Anyone know what the best way to test for exhaust gasses in the cooling system when it's under load?

Jeremy
eisenbrj@msoe.edu