BigSwede
Roll Me Over
Reged: 06/12/00
Posts: 3702
Loc: Minnetonka, Minnesota
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Runs hotter at highway speeds than city driving - why?
06/10/05 04:42 PM
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On the way back across country, I had lots of time to watch the temp gauge. It was interesting, the faster I drove, the hotter it ran, almost regardless of rpm. At 70+ mph in hot ambient temps, the gauge would creep up near the red, while in stop-and-go driving in 90+ degrees it was below half on the gauge.
In fact, when my direction changed and I went from a headwind to tailwind, the temp would actually drop some! This seems counterintuitive, but as I said I had lots of time to observe and confirm this behavior.
So I came up with a theory...my old 99 Trooper had a thin metal extension from the radiator skid plate rearward, sort of bridging the gap between the radiator and engine, underneath. This extension is also under the remote oil filter location, and so is a little annoying when doing oil changes. I asked an Isuzu mechanic about why it is there and he said something vague about airflow. Well, my 93 does not have this skid plate extension...not sure if it ever did, but either way there is a gap there.
My theory is that at highway speed, air is cramming under the vehicle and forcing its way up between the engine and radiator, stalling out airflow through the radiator.
Does this make sense? This is the only scenario I can think of to explain what is happening...if the radiator was clogged it should have problems in low-speed more than high-speed, I would think. The water pump is new, and the symptoms were the same before and after it was replaced. The fan clutch couldn't cause these symptoms.
I guess the easiest thing to do is get the extension piece from a boneyard and see what happens, but if you guys have any other ideas, I'm all ears. 
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Steve Carlson - 95 Trooper LS expo rig
Serenity now!
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