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1990 C3500 7.4L (454cu) Overheats/Loosing coolant

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I have an interesting overheating problem with my 1990 C3500 Silverado 7.4L. I recently bought the truck from an uncle who, I'm guessing, screwed me on the deal...


We use the truck to tow our travel trailer (22ft. roughly 5000lb). Prior to our first voyage out, we drove the truck around town to make sure everything worked well. After a few trips to our main hike spot I noticed the truck would run warm, 220-240, for a short time and then spike down to 210. I assumed it was the thermostat sticking. So I replaced the thermostat with a 180 degree unit. I topped off the coolant in the reservoir and it seemed to have fixed the problem.


We went on our first trip, now pulling the trailer, and at our halfway point, I got off the freeway to fill up the truck. about 20 seconds after we dropped our speed below 30 mph the truck spiked, redlining above 260. After starting it up, it quickly started to overheat again until we hit the freeway speeds and all was good.


This behavior continued up until we hit our destination. Thankfully, my mechanic brother-in-law was camping with us. He thought it was the fan clutch but after looking at the radiator, I was about 2.5 gallons low on coolant. Since we were out camping, I used the rest of the gallon I had and filled the rest up with water. Brother-in-law said the water pump looked like it was running good and it might have just been under-filled when my uncle replaced the radiator. Ran ok until we slowed down off the freeway. It looked like the coolant would get hot and boiling out of the reservoir. I just kept topping off the radiator and driving home (picked up coolant 50/50 on the way back)


Looked at it at home and noticed a leak above the drain valve. Tightened the valve about 1/4 turn, filled up the radiator and again the truck ran fine until our next trip. Same problems, had someone bring out a new radiator cap since it looked like the coolant was not being pulled back into the radiator. Coming back we had the same problem.


I since replaced the hose from the radiator to the reservoir and "burped" the system by running the truck and squeezing the upper radiator hose until the coolant level would no longer drop in the radiator.


The oil looks really clean (not milky or showing signs of water/bubbles). Coolant looks clean and I have topped of the radiator last with concentrate coolant to account for the water I added before. So I don't think it's the head gasket. I rotated the fan with the truck off and it stopped immediately after letting go. There wasn't any play when I tried moving the fan around so I don't think it's the fan/fan clutch. I hear an electric fan on when it's overheating/hot and the fan clutch kick in when its really hot.


Rather than purchasing a bunch of new parts I thought I would get on here and ask a couple questions and see if anyone might have some things to try out.


Would having enough air in the cooling system keep the coolant from being sucked back into the radiator?


Could the fan clutch not be engaging at a low enough temperature?


If the radiator is a 2 core, would that cause the coolant to boil enough to cause an air gap that would then keep coolant from being pulled back into the radiator?


This truck was originally purchased new by my grandpa in 1991. He was a mechanic and took amazing care of the thing (all receipts). My Uncle purchased it from him a couple years back and I believe it was just sitting on his property for most of the time.

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Could be something as simple as the radiator being plugged up.Either flush it out or install a new one if you're planning on having a while.Also could be leaves and debris built up between the condenser and radiator.Remove the cover that holds the radiator in place and have a look down between the condenser and radiator,get a garden hose and give it a blast of water.The fins could be plugged as well,blast the radiator and condenser with water,might help.

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  • 4 weeks later...

^ this. Check for debrees between condenser and radiator. If you still have issue maybe it's a partillay clogged radiator? Or water pump gong out and weeps coolant when hot and under load?

Of course post a pic or two of the truck :-)

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Checked for debris between radiator and fan clutch, nothing. After our last camp trip, got home and coolant was pouring out of the inlet to the heater core. This is new, hasn't done that before. All outside the firewall, thankfully. I'll have to look at her after I get back from a business trip. Could this have been a small leak that had gotten worse or a clog that hit a stoping point and burst a line? I'll update after I look into it. Just to update on the fan clutch, it seems to kick on when it needs too. Where is the temp sensor for the cluster gauge? I don't understand why she would overheat and not kick on the fan clutch at certain times.

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The temp sensor for the gauge in the driver's side head,just above #3 spark plug.The sensor that turns the aux.fan(I would assume you have one) on is in the opposite head next to #6.I tested the that fan sensor once and it turned the fan on somewhere around 220* IIRC.The fan clutch has no sensor,it's regulated by how hot the fluid inside the clutch gets(aka viscous,hope that's the right word).

Edited by grampadirt
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  • 3 years later...
  • 1 year later...
On 9/24/2020 at 3:58 PM, 454girl said:

Okay, people, I have a 1990 Chevy Cheyenne 3500 with a 454 in it, and SAME PROBLEM, new 4 core radiator, new water pump, new thermostat, new thermostat housing, new hoses, new lines, new fan clutch, NO LEAKS and STILL acts stupid and boils over when I stop! WTH?!?

Head gasket allowing exhaust gasses to enter the cooling system creating over pressure condition and blowing coolant out around the cap. Small enough gasket failure that only gasses can pass under pressure but not coolant so no contamination of oil or coolant. That’s my untested theory at this point anyway. Already r&r intake gaskets. Been down all the paths mentioned. Very nice truck with ~100K. Worth repairing IMO. Guess I’ll test my theory. 

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