Stubborn overheating problem. 2004 3.5L HO
Running out of options. 2004/3.5L HO is overheating, but not when idling..it only approaches the high end of the temp gauge when you're driving over 60-70km/h for about 10 min. Running the heater at full blast or coasting at higher speeds is the only way to reduce the temperature when driving.
Mechanic's initial diagnosis was a blockage of some sort, so the rad was replaced to no avail. Mechanic replaced rad, thinking first was defective, but same result. Thermostat was replaced both times /w rad and is installed in the right direction. Latest tstat was checked using the "boiling water and thermometer" method and it opens at the specified temperature (180 F). Hoses all appear fine. If car is driven for until the gauge goes just over 1/2, the upper rad hose is hot, but the lower is only just warm.
No observable leaks. Engine oil shows no signs of coolant contamination. I've spoken to a couple of different mechanics and they keep suggesting there's compression escaping into the coolant.
So the questions I'm throwing out there:
1. Could it really be the head gasket, even if there's no coolant in my oil?
2. No one's ruled out the water pump, so how do I check if that's any good?
3. I've heard rumors of "anti-rust" compound coming loose in the engine and creating havoc with the internal coolant passages. Has anyone else heard of such a thing? Would a chemical flush remove it?
Appreciate any light anyone can shed on this. Love and still owe money on this car, so changing it for another is not really an option.