"Overheating" Issues, Cant figure out
Thread Starter
Intermediate
Joined: May 2025
Posts: 33
Likes: 6
"Overheating" Issues, Cant figure out
Hey everyone,
Looking for some input on what the issue could be for my 79, non-original motor, has a 350 in it from on older 72-75 c10/20, Cam, Headers, aluminum radiator, Electric fans. The issue I'm having right now is:
For context, I’ve already:
Curious if anyone has run into something similar where the gauge reads high under load, but actual measured temps don’t fully match up.
Appreciate any input, just trying to get this figured out so I can feel confident driving it more.
Thanks!
- Troy
Looking for some input on what the issue could be for my 79, non-original motor, has a 350 in it from on older 72-75 c10/20, Cam, Headers, aluminum radiator, Electric fans. The issue I'm having right now is:
- At idle, temps sit right around 180 and stay steady
- Once I start driving, the gauge climbs to 220–240, especially under load or going uphill
- As soon as I stop and let it idle, the gauge drops back down pretty quickly
- Upper radiator hose: usually around 195–215
- Thermostat housing: around 170–190
- Lower radiator hose: around 180–205
For context, I’ve already:
- Replaced the temp sensor
- Installed an aftermarket mechanical gauge (same readings)
- Burped the system
- Verified the lower hose has a spring
Curious if anyone has run into something similar where the gauge reads high under load, but actual measured temps don’t fully match up.
Appreciate any input, just trying to get this figured out so I can feel confident driving it more.
Thanks!
- Troy
Thread Starter
Intermediate
Joined: May 2025
Posts: 33
Likes: 6
Navigator
Joined: Dec 2025
Posts: 9
Likes: 0
From: North Carolina
Check timing first...But sounds like an air flow problem...Or, a coolant flow problem...Is the air properly ducted through the radiator as it should? Is the radiator of proper size? Meaning is the radiator the right one for the car?
According to what you are saying is, "It cools fine at an idle/or not moving".
According to what you are saying is, "It cools fine at an idle/or not moving".
Le Mans Master
Joined: Jan 2001
Posts: 6,678
Likes: 1,363
willymot
Sounds to me like your gauge is not reading correctly. Probably the wrong temp sensor.
Those temp sensor have a somewhat restricted operating range and yours sound like one of those. I've tested several on my own car. I'll find a link for you to read but I really think that is all it is since you checked temps with your temp gun.
Do you have the old temp sensor?
Sounds to me like your gauge is not reading correctly. Probably the wrong temp sensor.
Those temp sensor have a somewhat restricted operating range and yours sound like one of those. I've tested several on my own car. I'll find a link for you to read but I really think that is all it is since you checked temps with your temp gun.
Do you have the old temp sensor?
Le Mans Master
Joined: Jan 2001
Posts: 6,678
Likes: 1,363
willymot
You can follow my temp sender experiments here. When you read the thread you will see that they seem to be reading ok and then the real temp goes beyond their "accuracy" range and the readings start deviating quite far from real temp.
Do you have the original temp sender (hopefully?). If the original sender is good and if you can find a wilcox adjustable resistor (ebay??) then you can probably dial it in. Read the thread below for other somewhat adequate options --- that is if you can find the willcox adjustable resistor to dial the gauge in.
https://www.corvetteforum.com/forums...-question.html
You can follow my temp sender experiments here. When you read the thread you will see that they seem to be reading ok and then the real temp goes beyond their "accuracy" range and the readings start deviating quite far from real temp.
Do you have the original temp sender (hopefully?). If the original sender is good and if you can find a wilcox adjustable resistor (ebay??) then you can probably dial it in. Read the thread below for other somewhat adequate options --- that is if you can find the willcox adjustable resistor to dial the gauge in.
https://www.corvetteforum.com/forums...-question.html
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Thread Starter
Intermediate
Joined: May 2025
Posts: 33
Likes: 6
willymot
You can follow my temp sender experiments here. When you read the thread you will see that they seem to be reading ok and then the real temp goes beyond their "accuracy" range and the readings start deviating quite far from real temp.
Do you have the original temp sender (hopefully?). If the original sender is good and if you can find a wilcox adjustable resistor (ebay??) then you can probably dial it in. Read the thread below for other somewhat adequate options --- that is if you can find the willcox adjustable resistor to dial the gauge in.
https://www.corvetteforum.com/forums...-question.html
You can follow my temp sender experiments here. When you read the thread you will see that they seem to be reading ok and then the real temp goes beyond their "accuracy" range and the readings start deviating quite far from real temp.
Do you have the original temp sender (hopefully?). If the original sender is good and if you can find a wilcox adjustable resistor (ebay??) then you can probably dial it in. Read the thread below for other somewhat adequate options --- that is if you can find the willcox adjustable resistor to dial the gauge in.
https://www.corvetteforum.com/forums...-question.html
Safety Car
Joined: Dec 2012
Posts: 3,779
Likes: 1,684
From: Summerville SC
How old is the radiator? I had similar experience a long time ago in a 300ZX. I eventually figured out tht there was crud in the bottom of the radiator which was restricting the flow at the bottom hose. I put a new one in and problem solved. I would suggest flushing it out but again, my experience is when you flush it out, you get rid of al the crud that was keeping all the little holes from leaking. 
Le Mans Master
Joined: Jan 2001
Posts: 6,678
Likes: 1,363
I suspect you should stick with reading the temp on thermostat housing. Just for consistency.
I'll try to get out this week and take readings on my thermostat housing with the temp gun. Pretty sure I have but don't have pictures. Then I can report what the temp is in comparison to my temp readings.
I'll try to get out this week and take readings on my thermostat housing with the temp gun. Pretty sure I have but don't have pictures. Then I can report what the temp is in comparison to my temp readings.
I do, I bought an aftermarket sender with a gauge to test with and it was showing the same results as the in car sender. But I did verify timing today, everything is good, took the car on a 15 minute drive, again went up to 230, was still 230 when I got home, I immediately popped the hood and the gauge still said 230 , but the upper rad hose said 200 and the thermostat housing said 150. Another I found though is that the #1 cylinder exhaust header is reading 300 Degrees lower than the rest. Going to pull plug tomorrow
Race Director
Joined: Mar 2017
Posts: 11,880
Likes: 4,468
From: Virginia
I'd trust the IR gun before I'd trust the stock sensor. I have a 3-wire sensor hooked up to an Arduino when I really want to know what the temperature is when running.
Squeeze the lower radiator hose to make sure the anti-collapsing spring is still installed. Then go enjoy your car.
What is your timing set to, BTW?
Squeeze the lower radiator hose to make sure the anti-collapsing spring is still installed. Then go enjoy your car.
What is your timing set to, BTW?
Drifting
Joined: Jun 2025
Posts: 1,605
Likes: 564
when (year&model) did GM move the coolant temp sender For Gage from Gen I sbc driver head to manifold ?
Race Director
Joined: Mar 2017
Posts: 11,880
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From: Virginia
- In some 1980s GM trucks (e.g., certain 1985–1986+ square-body applications with TBI), the gauge sender could appear in or near the intake, especially if the vehicle used a combined or relocated setup for emissions/ECM reasons. However, even then, head-mounted gauge senders were common.
- Corvettes (C3/C4 transition) and passenger cars: The head location persisted through the end of C3 production (1982) and into early C4s. Relocations to the intake were more often owner/modifications (e.g., for clearance, aluminum heads without the port, or aftermarket intakes).
Race Director
Joined: Nov 2006
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From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
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Cruise-In VIII Veteran
Reading through all of this. And having had similar issues in the past.
one of two issues or perhaps both. Ignition timing off idle. Also check vacuum timing.
And electric fans mounted on a flat pan. ? Guessing here as you state electric fans. But don't proudly tell us what electric fans. Moving down the road your fans shouldn't even need to run if you have a spoiler and all your rad seals are in place. But some electric fans mounted on a flat pan will actually block air flow.
Gen. GM C5 Corvette fans work extremely well and don't block air flow. Mine never need to run moving. My temps stay at 180 on a 100 degree day.
one of two issues or perhaps both. Ignition timing off idle. Also check vacuum timing.
And electric fans mounted on a flat pan. ? Guessing here as you state electric fans. But don't proudly tell us what electric fans. Moving down the road your fans shouldn't even need to run if you have a spoiler and all your rad seals are in place. But some electric fans mounted on a flat pan will actually block air flow.
Gen. GM C5 Corvette fans work extremely well and don't block air flow. Mine never need to run moving. My temps stay at 180 on a 100 degree day.
Drifting
Joined: Jun 2025
Posts: 1,605
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*identical OE location as in all sbc C3; although senders' size & thread is reduced. Gage senders' signal is most reliable when sender is submerged within a continuously moving stream of liquid coolant; devoid of air pockets.
Team Owner
Joined: Feb 2003
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From: Sitting in his Nowhere land Hanover Pa
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why are you guys trying to fix something that is not broken. his IR gun is telling you his engine is not over heating
Melting Slicks
Joined: Aug 2001
Posts: 3,046
Likes: 124
From: Wichita KS
willymot
Carburetor? Quadrajet?
Is your vacuum advance connected to manifold or ported?
What is your ignition advance at 3000 rpm with vacuum advance plugged AND with VA connected? Use a dial-back strobe to measure.
Mapman
Carburetor? Quadrajet?
Is your vacuum advance connected to manifold or ported?
What is your ignition advance at 3000 rpm with vacuum advance plugged AND with VA connected? Use a dial-back strobe to measure.
Mapman