The gauge goes to about the middle at highway speed while it drops to about 25% at idle or in traffic. This is the opposite of what I would expect. Truck also has an idiot light saying engine hot which is now blazing at highway speed. WTH? I know the 31 year old gauge and light are probably inaccurate. Does not seem to be running hot. no coolant loss...
Collapsing lower radiator hose?
Brakes dragging or gears (rear end or transfer case if it has one) binding up? That would put a bigger strain on the motor at speed.
-Rob
I think diesels are a little different than gas engines - they run really cool at low speeds, and as the throttle increases the heat output jumps up a lot.
I vote replace the radiator cap (if under pressure normally), then take a cheap IR gun and check temperatures on the radiator and hoses after a highway drive.
In reply to Brokeback: Has the water pump ever been replaced? What about the coolant? Old coolant gets corrosive. The pump impeller could be worn way down or spinning on the shaft. Sounds like what happens to the old VW 1.8T when the plastic impeller would get loose on the pump shaft. Cool at low speed, hot at high.
How old is the thermostat? Could be something as simple as a thermostat that's stuck partially open.
Diesel engine speeds are controller by controlling the amount of fuel that gets injects rather than the air that gets into the engine, so higher speeds = more fuel = more heat.
Check for a mattress of bugs and grass between the rad and the condenser. Sounds very much like the rad is not doing its job correctly.
Get a real temp gauge on it and find out what it's doing. 1/2 isn't necessarily warm, so it might actually be running too cold in traffic due to a stuck-open thermostat or something.
If it's running at normal temp in traffic and warm on the highway, it's either a radiator with compromised cooling capacity (either clogged, dirty on the outside, etc.) or an airflow problem that only shows up at higher road speeds. It's definitely not a fan issue because it stays cool in traffic.
That's actually a pretty common symptom of a bad radiator.
APEowner wrote: That's actually a pretty common symptom of a bad radiator.
That was my experience.
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