I have a 98 S10 4wd that I plow with. Auto transmision, 4.3l, 145k. Up until recently it was fine, but then it overheated. It had a new thermostat, but I replaced it with a "fail safe" thermostat. Still overheated. Here's the strange part: I can plow about 10 driveways with no issue, temp reads about 195. Then, when I drive to the next neighborhood to plow, on the way there it goes up to around 240. When I start to plow, it cools back down.
Checked the radiator cap, it was bad, replaced it. But the same symptoms. Today it overheated, so I stopped driving, but didn't turn the engine off, and it cooled down.
No leaks at the pump. The last time it overheated, there was antifreeze in the reservoir. Today there wasn't.
It runs fine, idles at around 550, and the oil looks ok, so I doubt the head gasket. Am I being fooled? Any other ideas as to what it could be?

 

your water pump could be slipping or your fan clutch is going bad um when your not plowing do you block the radiator with the plow? what color is the coolent. you should rent a pump to add psi to the radiator to see if you hae any leaks chevy motors always have a intake problem leaking coolent so just look on top of the radiator cap and see whats the psi to you to add.

 

Sorry about the long delay. I couldn't log in (was "drifter") so now re-registered.
OK, coolant is now orange and before I started reading, had added green, but that was long before the forum. It doesn't matter whether the radiator is blocked or not -- same results either way. New rad cap. Also noticed today: Plowed about 10 drives, drove to the beginning of the neighborhood temp stayed at about 195. Took over 30 minutes, so truck was heated up. Drove 1 mile to the next neighborhood, got up to about 230. Did only 1 drive (5 minutes max) and temp was back to normal. Drove 3 miles, temp 140. Did 10 drives, back to normal. Drove 5 miles back to home, temp about 250, did touch-up on my drive, about 3 minutes, back to normal temp.
Meanwhile, after replacing the rad cap, tossed the old, so don't know psi.

 

wait, did you say you added green antifreeze to it? you have to flush out the dex-cool fluid before you add green, never add green to dax-cool (pink) immediately flush your rad system or it will kill all of your components!! i am very serious about this, it will mess everything up, always use dex-cool if it is already in there, if you want to run green, then you need to flush the whole system...and good too, then you add green, never mix

 

Yep, that's what I've read. Fortunately hitting 40's this week, will probably do it tomorrow. Thanks.
BUT, is that reason for the erratic behavior?

 

Flushed and filled. Slow to heat up, then fine. Drive for a while, goes up to 240. Stop and idle, dives down to 195 within about 2 minutes. Fan clutch seems good -- resistance, but smooth turning, no free play. Radiator and reserve tank are full. So I'm still stuck.
Anybody? Bueller? Bueller?

 

take out the thermostat and leave it out put everything back and see if your still over heating

 

So since thermostats have all checked out as ok -- open at 170 to 190, depending on thermostat, close at same -- I'm thinking water pump. I thought they leak when they go bad. Can they also "slip?" but why only when on the road, not when plowing?

 

you should just replace the pump its easy and cheap. if your truck is still having the problem then you might just going to need a new Radiator.

 

I finally figured it out. It was overheating when I stopped for gas, so I popped open the hood. I checked the fan, which was spinning. But suddenly there was a different sound, and I could see the fan running faster. Then the temp started to drop.
So I changed the fan clutch, even though it seemed good -- nice resistance. But the new fan clutch seems to have solved the problem. Apparently the old one was worn just enough to fail, but not enough to be consistent. That's what made it hard to diagnose.
So thanks to all who suggested stuff.