Overheating dilemma

I have a 1946 Chevy pickup on a late 70’s blazer frame with a 350 engine. It has always performed well as a daily driver. This summer it began to run hot (220 or higher) when it had not in the past. It has an aluminum two row radiator that has performed fine in the past. No heater or A/C is involved. Me and my fellow shade trees have replaced the water pump, replaced then removed the thermostat, Flushed the radiator on several occasions, tested the electric fan mounted in front of the radiator, replaced the radiator cap and have tried water and coolant as well as additives. When overheating, the radiator temp at inlet and outlet is almost identical. We are running out of options and because there was not problem last summer, we are severely puzzled. Anybody have further suggestions???

Does it overheat no matter whether you are rolling along on the highway or stopping and going in town? Is the weather a factor at all? Has anything at all changed with the cooling system since last year? You say the fan is in front of the radiator - was it always there? Is that fan somehow running in the wrong direction this year?

Depending on the ambient temperature, 220 degrees is normal.


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Is the thermostat back in? I’ve seen this kind of thing happen before where someone troubleshooting an issue introduces another unforeseen problem during diagnosis that continues to cause problems even after the original problem is rectified. Inlet and outlet temps being the same seems to indicate that no heat exchange is taking place. Removing the thermostat can cause this as the fluid doesn’t dwell in the rad long enough to dissipate much heat. If you did things in the order listed and the tstat was done early on, then you may have fixed the problem (replaced rad cap for example) but now because the tstat is out, it continues to run hot. Something to check/verify…

Fan is operating in the proper direction. Summer weather in Texas has not changed significantly. Air is moving through radiator. Stop and go does cause quicker heating, but highway driving takes it above 200 in the first ten miles. I do not lose any coolant in the recovery tank but I have not tested for a head gasket leak since there is no loss of coolant. In stop and go it will climb to 240 pretty quickly.

Thanks. I have run with and without a thermostat and best result has been without so I am running without one at present.

Kinda strange. Are you sure you haven’t changed anything in the engine compartment since last year (removed radiator shroud, added headers, anything?).

If you have access to an infrared thermometer, I would be looking at the radiator to see if it has hot(ter) spots than others indicating it is restricted in some way.

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No other changes since last year.

No hot spots on radiator. Infrared therm shows about a 7 degree differential from input flange to output flange.

Is the radiator plugged up with bugs?

A good pressure washing might be needed.

Do you have a large enough fan? If it one of those cheep low profile electric fans it may not be moving enough air.

Have you checked the timing? Does it have points? Dwell angle can affect timing.

It is an s blade fan but one of the lower rpm models. It seems to be moving reasonable air as you can feel the heat being pushed out.

Are you sure it is overheating and not just an issue with the sending unit? Did the IR thermostat confirm the overheating?

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I have tried timing from 0 degrees all the way to 11 degrees, runs best near the recomended 8 degrees. Ignition is electronic

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Driving in town it got up to 250 and I pulled over and cooled it down. It was definitely out there. The IRT shows the radiator temps at 220 or so when the heat guage reads 215. Block temp down on the cast iron shows about 10 higher.

If the truck has an automatic transmission, the transmission could be overheating causing the engine to run hot.

Have you checked the color of the transmission fluid if it’s automatic?


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ok Thats one that I havent followed up on yet. Transmission functions great, but I have done nothing with it such as filter or fluid recently. I will check

Have you checked if there could be a blown head gasket?

Personally, with your diagnosis, I am leaning towards the radiator or fan. You could try jumping the fan on (temporarily) and see if that helps. As said, it may not be moving enough air.

Mostly, I’m leaning towards a radiator with tubes that are partially plugged. You won’t necessarily see hot spots. You mention 2 row aluminum so depending on the brand, there could be corrosion. You might do better with more rows, as well. If it was the original, they’d simply rod out the old unit, but they might still be able to hot tank your radiator, assuming it doesn’t have a plastic tank.

If I was choosing a radiator for this truck, I’d build one similar to the Chevy 3/4 ton 4x4 w/ towing package and big V8. For a truck, too much is better. Oh, don’t forget a good tranny cooler, too. But that’s just me.