83 pickup 4.1L Straight 6 "Overheat?"

My good old 83' GMC Pickup has had a funny symptom for 2 years that is getting worse, and I hope you guys have a clue...
On a long high-speed highway uphill, the temperature gauge goes up a
little, maybe to 190 from about 180, then suddenly pins full scale hot. After running slower with less load, it suddenly pops down to 180 or so. The truck never actually overheats.
Engine runs well, doesn't burn oil at 175,000 !! Replaced water pump for a bad bearing recently, and the internal water jacket looked nice and clean.
I replaced the temperature sensor, and it didn't change the behavior. Again, the truck runs strong and doesn't actually overheat or afterboil..
What do you think is going on here?
Thanks!
Terry King ...In The Woods In Vermont
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On Fri, 01 Jun 2007 06:36:05 -0700, " snipped-for-privacy@terryking.us"

I suspect that your radiator core is becoming plugged up and is on the threshold of being able to properly cool your engine all the time. You might check clutch fan operation as they tend to get soft with age and not engage properly. One thing I want to stress here, that is not a engine you want to overheat at all because while it is a sturdy motor it can have a nasty habit of warping the head and blowing head gaskets when it is overheated enough. A trip ot two to the around the 240 or 250 mark can do it sometimes. You want to keep it cool. ----------------- TheSnoMan.com
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Thanks for the replies, guys!
Here's some more to help me think about:
- I discovered that this engine has an additional temperature switch located in the head near the back. It is just a switch so when it activates, the gauge goes to fullscale. I temporarily disconnected it so I can watch the coolant temperature. Also, I recently replaced the temperature sensor and the new one reads slightly higher than the old one; probably just a calibration issue.
Basically, the cooling seems to regulate OK, over a small range.
The gauge has major markings at 100, 210 (1/2 scale) and 260 (fullscale). My rough calculation is that a "needle width" is about 8 or 10 degrees F.
The indication goes up to 210 over 5 mins or so at startup, and drops to about 180 quickly, which I assume is the thermostat opening. I drove 250 miles back yesterday, watching the gauge. Hot (80+) day, 60 to 75 MPH highway. The indication stayed within 210 and a small +- 1 needle width range. Even with a long uphill, and heavy throttle, it didn't go any higher. (This is the condition where it previously went fullscale before I defeated the second switch).
Snoman, I want to make sure I don't damage this engine, per your advice! What do you think is happening here? This engine seems to have the intake manifold and the top of the exhaust manifold cast integral with the head, is this right? Could this be aging of that switch? Could it be less-good circulation in the rear of the head, with age?? What else??
I plan to run some serious cooling system cleaner soon; do you think that's a good idea?
I'm thinking of running a cooler thermostat; 210 (if that's accurate) seems too high. What do you think about that?
It seems the basic radiator capacity must be OK, but I see some deposits lower down in the cross-flow radiator; cleaning may dissolve some of that..
The fan clutch seems pretty loose; can it be adjusted??
Thanks for the suggestions, and I hope to keep this oldie but goodie going for some more years here!

I'm reading thru your great site. I'm on an unmaintained dirt road in rural Vermont, and THAT'S a subject for me!!
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On Sun, 03 Jun 2007 01:20:21 -0700, " snipped-for-privacy@terryking.us"

THese temps are a bit high but not in the range that can damage engine. Sounds like a weak clutch fan. Contray to popular beleif, ram aitr cooling is not always enough to do the job.

Intake is cast as part of the head that year engine but exhaust should not be as I recall

Never a bad idea but a raditor of that era used a brassed core that was soldered in and the lead in solder forms slats with time the can block the coling tubes/fins internally and flushing does not ususally remove this. Look inside the tanks and look for deposits on ends of tubes. If present, they use a process called rodding it out at a radiotr shop where they remove end tanks and run small rods through core passages to clean them out. Vinegar also can work well to remove these salts sometimes. Drain and flush system then drain it conpletely and fill it up with vinegar and start it up to circulate it and get air out of system and let it sit overnight then flush next day. If it still look "dirty" in tanks repeat. When done, flush vinegar out and replace with fresh water and about 1/3 cup of baking soda to neutrailize the acid (vinegar is a weak acid) then flush and refill with fresh coolant

Cars of that era usually used a 190 to 195 Tstat some even 205's. The problem with using a lower Tstat is that it can actually reduce cooling efficency because when you lower coolant temp, the tempature differentail between air flow and coolant is reduced and so it the rate of heat exchange.

You can flush as mentioned above

Yes, as long as it does not wobble on shaft. You will find more info on this in the link below.
http://forum.snoman.com/viewtopic.php?ty

----------------- TheSnoMan.com
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I'm confused as to what this second switch is? Do you have an aux electric fan on your truck? If so this switch could be to control that and if so should complete a ground when a certain temp is reached and should not be inline with the gauge. A new fan clutch made a big difference in running temp on my truck when I replaced, get a decent one though.
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I am not 100% sure if it applies here but they used to have a high temp switch on some models that would increase idle speed to increase when engine reached a certain tempature or some other function as the ECM's back then where very primative and limited on what they could do so they depended on switches and such. ----------------- TheSnoMan.com
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I got a wiring diagram finally, and it shows a separate switch in parallel with the temperature sender. So that would send the gauge fullscale if it actuated, which I think is happening here. I'm going to do the serious acid clean/flush and see if that makes a difference. The switch is labelled "Metal Temperature" and is in the metal of the head between cylinders 5 and 6, quite close to two exhaust ports. So this could be:
-- The switch has moved to be more sensitive in 175,000 miles, or -- The rear of the head has some deposits and is getting less cooling, or -- The exhaust temperature is higher than normal in heavy loading.. , -- OR??
Thanks for the info and suggestions! It's been a big help... ...and I'm checking the fan setting.. thanks SnoMan..
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On Fri, 01 Jun 2007 06:36:05 -0700, " snipped-for-privacy@terryking.us"

Had much the same thing happen with a farm tractor. The *piddle* valve hole (for want of a better name) in the thermostat was plugging up. Would seem to overheat right away but after shutting down the engine for a few minutes the thermostat would open and then all would be well until the next time. The piddle valve allows heated coolant to bypass the thermostat (really small hole) so that heat could get to the thermostat to cause it to open.
My not be the same thing here but a thermostat is cheap insurance. By the way, the dealer sells good ones. If you chose to purchase aftermarket you will likely get what you pay for. - Regards Gordie
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do a leak down test to eliminate head gasket, and change your thermostat.. "The Nolalu Barn Owl"
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