Yesterday on the way home from work the temp gauge was suddenly up at the "3/4" level, usually runs no more than the halfway point. It's a 98 4.0, has 111K on it, has not previously acted up at all. I started her up this morning and it stayed normal till I was about 3-4 miles away and then jumped right back up, there was a slight hill to climb, so it went all the way up this time and Check Engine light came on. I turned on the heater full blast and turned around for home, that made the light go off and the temp guage went down to about 3/4 again. Does this sound like a thermostat problem?

 

Sounds more like a water pump problem or a bad fan clutch.

 

How Hot is it where you live? As a truck gets older its just not going to run like it did new. Going up hills any old truck will get hot, but it really depends on how hot it is out and if your pulling a load at like 3000 RPM and its like a 2 or 3 Miles Grade your climbing, if its a small hill and your empty with Not too much on the Motor you should not get all that hot..A New Complete Cooling System isn't really that expensive and its easy to do and with your miles its a good time...Over heating is the primary main cause of engine failure and the end of an engines life. Keeping it Cool and well Oiled is the Key...

I just did my Water Pump, Fan Clutch, Fan, T-Stat, Sending Unit, changed the Coolant, Replaced the Intake Manifold Gasket and it was around 200 Bucks, but I got the Coolant for free and wanted to flush the radiator but did not....Also if you have Mechanic Skills maybe look at an electric fan, it doesn't bog down the motor, but I just replaced the fan on mine cause I didn't want to deal with the wiring and all that fun stuff....Before I could not hear the fan, it had some cracks but now the New Blades I can hear humming and really pulling in some air!!!

If it were me, I would do all the above, but its sounds like a T-Stat that is sticking Shut....Going down the road you have enough wind coming through your radiator to keep it somewhat Cool if everything is working but your Fan Clutch but if the T-Stat sticks it will get hot regardless and sitting at an idle at 3/4 on the temp tells me its probable the T-Stat, but dude, just do the whole 9 Yards and keep her nice a cool for a long time too come......

 

Old thread, but I'll add my 2 cents anyways.

This sounds much more like a T-stat. Sticking or just spastic. My understanding is that when pumps go bad, there is usually a weep hole that starts leaking fluid. Not much to go wrong with a fan itself. The clutch, maybe. I'm chicken to do this, but I'm told that if the clutch is bad, you can hold onto the blade with your hand & stop it from spinning.

With that many miles, if your replacing the stat.... just do the pump too. No additional work, tiny cost, more reliability. USE NEW BOLTS.

Side note #1: I had a '67 F100 with a weeping pump. Replaced it easy pee-zee. Last bolt installed broke. Tried to remove it with an easy-out bolt extractor. It broke too. Paid to have it towed to a shop & have the old bolt melted out with a torch. The bolts looked ~okay~.... but apparently the coolant essentially rots them over time. Must have cost me $100. New bolts cost how much?


Side note #2: F100 with a cracked fan blade. Tried to get a shop to tack weld it for me but the balked. They insisted the blade was balanced & the weld would throw it off, and then ruin the pump. I gave in, paid them to replace the blade. What BULL HOCKEY. The brand new factory blade had 5 blades that weren't even close to evenly spaced so it was far more unbalanced than an ENORMOUS tack weld would have been.

 

You seriously bumped a 4 year old thread for that? The OP hasn't logged on in since 2012.