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Good Morning;
I have a 1941 GMC 3/4 ton Pick up with a 248 cu inch engine. It now has an aluminum radiator, Air Conditioning; but retains the original 4 speed transmission. The Gauge Cluster with the capillary temperature gauge shows the top temperature of 212 degrees F. The engine which was rebuilt and now has approximately 1000 miles runs around the 200 degree F range.

What is the proper temp range of the thermostat that I should be running?
What is the proper pressure range of the radiator cap that I should be using
Do I have a problem with the engine running in the 190-205 degree F range?

Seems as if the temp comes up to very close to 210 degrees F; then backs down to 180-190 as long as I keep moving (electric fan installed; stock fan removed)



thanks for the help.

Hy RogerB1941GMC, I believe the factory used 180 degree F thermostats. The truck probably came with a zero pressure cap when new, so any pressure would help stop the coolant boiling, you could contact the radiator company and ask if they have a suggestion. Years ago the factory started putting 190 degree thermostats in the engines, this made me nervous as the temperature was now ten degrees closer to the boiling point, but it doesn't seem to have created the problems I was fearing. Hope that helps.

If electric fans are so wonderful, why don't they use electric motors to turn airplane propellers? Is there some reason you don't want to run an engine-driven fan?
Jerry

The temp is controlled by the thermostat. Pull it out and see what temp it’s rated at to open. Anywhere from 180-200 is normal. Newer cars run warmer for emissions reasons.
The radiator cap determines system pressure. It also should have a value printed on it. It may be in psi, kpa, or even bar. Typical systems run 15psi / 100 kpa / 1 bar

Does it go up to 200 when your idling at a stop light?
Is this with or without the ac on?
With or without the electric fan running?
How many core aluminum radiator?

If you use a pressure radiator cap it will tell you very quickly where every weak seam, loose clamp, worn hose etc. in your heater system is.
The damage from high water jacket temperature is that the engine will ping under hard load because the water nearest the exhaust valves and spark plugs in the head will boil, and allow that area to become hot enough to detonate. A pressure cap increases the boiling point and reduces the danger proportionate to how much pressure.

First off, 200 would be OK if it was mine as long as it stayed there or went lower. My replacement aluminum radiator came with a 19 lb cap, I switched to a 14 lb, the 250 can handle that and the new heater core can handle it. Your heater core will be the determining factor on pressure.

Lack of air flow is why it's heating up at slower speeds, you need a bigger fan, more powerful fan, or a mechanical fan made for A/C cars or trucks. It takes a lot of air to cool the condenser and radiator. All A/C cars had different pulleys and fans to move the air and coolant faster.

They do now!
https://www.harbourair.com/harbour-...rlds-first-commercial-electric-airplane/

They do, but little range compared to IC engine.

Ed

Just wanted to let Hotrod know that his crack to show his dislike for electric fans is out a date. I'm sure he can come up with something new.

For me, proper engine running temp is just past the E in TEMP.

look at where the temperature bulb is located. My 292 goes into the head right beside an exhaust port. It runs 210 all day, goes up to 225 on long hills. But the coolant going through the thermostat never goes above 185. Find a cheap Infrared temp gun to check it. some thermostat housings have holes tapped to put the temp bulb in.

Jon

Jon, my 250 did the same thing, I always worried it was way to hot. I finally moved the sender up under the thermostat. Now the gauge reads steady and 20 degrees cooler!

Ok, let's try this- - - - -electric fans suck- - - -or blow- - - -depending on where they're mounted. Either way, nothing beats a good engine driven fan. As long as the coolant doesn't boil, temperatures below 250 degrees are pretty common on modern engines. That's why most temp gauges read Cold/Normal/Hot instead of showing actual temp in degrees to keep drivers from getting an attack of the vapors when the coolant approaches the "no pressure" boiling point. The heater core pressure tolerance is the limiting factor on a stovebolt with a modern pressurized radiator, along with the piston skirt clearance. A 15 pound radiator cap will raise the boiling point of the coolant to 247 degrees, 3 degrees per pound of pressure over the normal 212 at sea level. As long as the pistons don't expand enough to create too much drag for reliable hot starting, all the angst about coolant temp is just something for snowflakes with lace on their drawers to obsess about. The thermostat sets the MINIMUM coolant temperature- - - -the cooling capacity of the radiator and the volume of air the fan can move is the determining factor of how steady the temp stays once the thermostat opens.
Jerry

OK. I'll step in it. Electric fans work great in new vehicles. The engineers go to to great lengths to design the front of the vehicle, and use a fan specifically designed for the vehicle to make them work.
Just slapping one on a 70 year old truck may or may not work. It is very likely that the best case scenario will be less than optimal, but not detrimental.

"I have a 1941 GMC 3/4 ton Pick up with a 248 cu inch engine. It now has an aluminum radiator, Air Conditioning; but retains the original 4 speed transmission."

Roger,

You may find that there was a factory fan shroud for your PU. In a 1936 Chevy service bulletin I found that there was one for my '36 Chevy PU and it took a diligent search but I did find one. Here's what it looks like. The '36 never had an overheating problem in the 55 years I've owned it but the fan shroud was irresistible because of the cool factor (pun intended) its rarity gives the PU.

If no factory fan shroud exists they're easy to make. This one on my 350 sbc powered '32 Ford roadster I made from a cookie sheet. In conjunction with a single 16" electric fan controlled by a thermal switch installed in the water pump casting it keeps coolant temperature between 180 and 190 in city and highway driving and even in gridlocked traffic on a 100* day the coolant temperature has not exceeded about 210*, and that situation is rare. My wife's 2017 Tahoe has a 210* thermostat indicating that 210* is not excessive. The Tahoe has basic electric fans like just about all vehicles now. So my experience is that a factory or generic aftermarket electric fan works fine, even on the small (16" x 26") '32 Ford radiator core.

Ray W

Thanks for all the comments and suggestions for the 1941 GMC cooling issues. Here is what I have found:

Capillary type heat sensor IS located at the back left of the engine. Very close to the exhaust manifold. It reads 230-250 degrees F while the engine gauge is at 195
The thermostat is a 180 degree thermostat. I changed to a 160 degree and it does a LOT better with the normal engine gauge reading of 180 after 40 minutes of driving. With or with out the AC being on. Seems as if the engine operates (at the rear temp take off) 20 degrees hotter than the thermostat setting.
It climbs to 190 degrees by the capillary gauge when I stop and depend ONLY on the electric fan. Once moving again it comes down to 180...seems as if the electric fan is not enough air movement for all ranges of operation.
There is a shroud on the electric fan that came with it new. The radiator is a Champion brand which came with a 16 pound cap. I changed to 8 pound and have an overflow container.
The electric fan was installed as the pulley arrangement required moving the radiator forward to accommodate the Alternator (12V) and the AC compressor and there was not enough room available for the mechanically driven fan.
Very possibly the improvement may be largely from my burping the system and purging trapped air. After cooling, the radiator sucked up almost all of the 50/50 coolant blend from the tank......that was a new experience.
Thanks again......lot of help here!
rb

You are getting there and finding out what works as you go, thats what its all about. If you need more cooling, you could always add a second small radiator under the bed with an electric fan, it would add capacity and some cooling and could be plumbed off the heater hoses, it might need and separate pump to move the water along. Even a heater core down in the wind would help.

I know my 250 is hotter at the back of the engine and at the head compared to the block and radiator, so it may not even be a problem as long as it doesn't puke all over the driveway when hot.

Hey ASU, whats up? Im right down the hill from ya. Well Pgh. Maybe we'll cross paths soon. On the heat, sounds fine to me, but beers are flowing easily. . Dont be afraid of operating temps. Everyone wants 180, but 220 if fine. My 51 Chevy with a 62 261, 235 head, mechanical fan, 180 thermostat, runs in the middle of the gauge, whatever that is, lol. But i notice when its toasty out, and the heater core flow is shut down, it runs atad hotter. So what. It working as designed and i can see da slight change with the core open or closed. What im saying is that everything you do changes something. Play with it and find out what your comfortable with seeing on a gauge. If you move the sender away from the hottest spot on the engine and like it better, so be it. That don't mean its running cooler, your just seeing a reading from a cooler area. If it was overheating, youll know it , regardless of the gauge. Running, starting, power, etc issues. Id put the sender where ever it seems fit, then leave it there. Electric fans are preference to some, but like previous stated, have there place. I like mechanical myself. Drive the crap out of it. Hot, cold, hiway, city, long, short, whatever. Youll get a feel for it. Then only then change something like, thermostat, fan, rad, whatever. Then drive it all over again. At that point. Youll know if you are helping or hindering the cooling aspect of your vehicle. Keep us posted.

I'll bet you had a defective thermostat which did not allow enough coolant flow once operating temperature was reached.
Changing to a 160 degree thermostat would have ZERO effect on the final, fully warmed up engine temperature, (unless it is replacing a faulty thermostat).