I was under the impression that as long as you werent pulling a hill or load that you should all but be able to remove the fan at highway speeds due to all the air rushing through the radiatior naturally. Is that not true?
I have heard that myself many times, but no one ever offers any data to back it up. I always pretty much believed it though.
Seemed to make sense to me.
My first inkling that it might not be that simple was when I bought my F250 with the supercooling package.
Extra big radiator, other fluid coolers, and a more powerful fan with heavier duty clutch.
Batting down the four lane highway one hot afternoon, after I had had it a couple of days, I heard a car or truck coming up along side me.
Startled me a little, because I had not seen anything coming in my mirrors.
After a few seconds, it seemed to vanish. Never did see anything. After a few minutes, here it came again.
Finally turned the radio off, and started listening.
Everything would be fine, then I would hear this increasing roar which would hang in there for a few seconds and then fade away.
Turned out it was that big fan kicking in. I could see the temp gauge move up slightly, the roar would come on, and it would move back down.
This was at 70 mph. Of course, it was a hot day and I had the air con on. I soon got used to it.
Couple of years later, I damaged the clutch driving through some deep water. The truck would overheat a little at idle, but would also run hotter than normal at speed.
I asked about it when I was having the clutch replaced, and the mechanic made a good point.
Said that it might be fine at highway speeds with no fan, but with it in there blocking the air and not turning fast enough to get out of the air's way, it was going to run hot.
Having said all that, I like the idea of the hose sucking flat.
Also, I have seen two different water pump problems that might give you trouble. One, was a rebuilt that actually had the wrong impeller put on. Everything matched, bolted right up, but would not pump worth a darn.
The other one, was also a rebuilt, that the impeller came loose on the shaft. Idling it would pump enough to keep things cool.
But when called upon to deliver more water flow, it slipped.
As for the clutch, look for oil stains, and give it a spin when hot and cold. It should stop almost instantly when hot.
Heck, just put your hand behind it and rev the engine. If it doesn't blow strong, suspect the clutch.
Also, making even slight amounts of change of where some of the hoses hook up could cause problems.
As fast as it cools off, it might be that the area of the engine where the temp sensor mounts is getting starved for coolant when the flow gets high.
Then when you slow down, it gets coolant again and responds fast
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