HELP Overheating diagnosis

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

Please clarify you did change the radiator? With a new one? Is there any debris in the front of the a/c condenser? Is the coolant system 100% bled?

Napa sells a coolant tester that testes the co in the coolant. It is about $60


If you live near me, you can borrow mine.

You do not say what year or make Land Rover this is.

I just replaced the radiator with a new one, I believe the system is 100% bled. I filled the top radiator hose when I installed it, and filled the radiator from the top hose port, and ran the truck for a while at temp with the expansion tank cap off. I don't know what else I can do to bleed the system on this truck.
I bought the test kit, and it does not show any exhaust gasses in the colling system, even after running the truck for some time at 2000 and 3500 rpm and running it under load then testing.

None of the testing I have done seems to conclusively point to a leaking head gasket or slipped cylinder sleeve. I am going ot replace the water pump and viscous coupling and see if that solves the problem as I am out of ideas at this point.

I have a 1999 P38 Range Rover with 140K miles

Does the temp guage needle go up when there is an electrical load added. For instance if you turn the headlights on, does the temp needle rise. If so, then it is a ground problem that is very common in these vehicles. Let me know if this happens, and I will tell you how to fix it.