Hey guys, I'm new here, but I'm an ASE Master Tech, so lets assume I'm sound mechanically. My Daily driver is a 1975 F-250 supercab, 2WD, 360, c6. Nice truck. It has dealer a/c, which works mediocre, 50deg air is about as good as I get. All stock.
I drive it 15k/ year, no problems. However, we take some long camping trips with my slide-in camper, and on these, in the summer heat, it will run hot on the highway. You have to watch it to keep it under 215deg at 70mph. Truck had a 2 core rad, then a 3 core rad, I had that recored to 4 core. No help. Flows coolant well, I know, because I once put a head gasket on backwards, which didn't help, so now I'm real aware of coolant flow. Stock shroud, but I've tried the factory fixed fan and a new flex fan. For a while I also had a puller inside the shroud, which helped cool it at idle, but I took it off to see if it was interfering on the highway. I used to keep a bike rack on the front, and took that off which helped a bit, but its still real hot. If it rains, its fine. 80deg day, no problem. But if we are in 90deg weather or up, its scary. I'm pretty sure its an air flow problem. I'm all ears on suggestions though. Its tough to chase, because it only overheats with my family in it on road trips with a trailer or a camper. My next thought is to take out the condenser and see if there is some way to improve air through the radiator and not lose the A/C. Never loses any coolant, and the coolant is clean.

 

Without a load, what temperature does it get to on the same 90 deg day?
Does the temp drop from 215F @ 70mph if you were cruising instead at 60 mph?
Do you have a transmission temp gauge on it?

 

Going to go out on a limb here, what thermostat are you using? The original was the RobertShaw thermostat and they prevented the majority of overheating caused by the inferior single disc thermostat. My suggestion is to run the RS modified style.

Here is the already modified RS T stat by Stewart

https://www.summitracing.com/parts/e...RoCKFYQAvD_BwE

Or here is the plain version which needs to be modified by the user and drilled for three holes.


https://flowkoolerwaterpumps.com/products/robertshaw-330-180-degree-thermostat

 

I don't have a trans temp gauge, but I should. I do have an aux. trans cooler, off to the side, probably not efficient but not in front of the condenser. 60mph will take out a couple degrees, but in the mountains it ain't much help, probably because i'm pretty deep in the throttle anyway. Without the load, it will stay around 200, maybe 205 if you wring it out. I like the high flow stat idea. That's helpful. I'm also thinking I might take off the engine fan, and put on a set of fancy derales with a shroud. Can't see how it would hurt, and I've been wanting an excuse for a 3G alt for a while anyway.

 

Stay with the stock fan, or a stock clutch fan. The electric fans don't pull near enough volume. The RS thermostat isn't necessarily high flow but its balanced flow. Thats critical because cheap thermostats only adjust to the pressure side and cause hot spots and erratic opening and closing. I see the RS thermostats are on back order, I have a brand new 330-180 here in the shop. Send me a PM with your address and I will send it out to you.

 

apparently I haven't been a member long enough to do that. complicated site rules.

 

Make a few more posts and you should be good.

 

Should just be a 25 post count then PM capabilities. The rule helps keep the riff raff trolls out here.

X2 on ck for correct thermostat and that it is functioning properly. How long has it been since you flushed the system, block, heater core. And how old the water pump?

 

Less speed means less airflow across the rad but your temp's go down.

Due to the above, plus aggravated by load, the issue is with coolant flow/capacity.

I would start with: flush the block, high flow thermostat, plumb the trans cooler before the radiator in case you are picking up too much heat from the trans. (or maybe you already have it plumbed this way?) add a trans temp gauge out of the cooler before the radiator

The above is based on the assumption you already have proper fuel and ignition curves. If not confirmed, start there.

 

Dump the flex fan they inhibit airflow at speed. Install a clutch fan any AC application should have a clutch fan. I would also install a shut-off valve on the heater core feed this will stop hot engine coolant from being recirculated back into the engine from the heater core.

Also, it would be worthwhile checking your fuel-air ratio. With modern fuels on original carbs, they run quite a bit leaner so you may be on the lean side for fuel-air mix and the engine could be running hotter than it would have originally when pushed hard.

 

Not to do with your heat issue, but I can tell you the 3G alt is the single handiest upgrade you can make to any old Ford. I’ve done it many times and it’s well worth it. You could source an alt from a 98 mustang v6 for example and pair it with the alt harness from a 97 f150 - that pair on your dent side makes it look like it belongs there. Just the right length.

 

Ok,

I've tried adding fuel, its only a 2bbl so its pretty straitforward. No change. I also have tried more or less ignition timing, no change. It has a pertronix, so I think that's pretty reliable too. I haven't put in a timing chain, but it doesn't have much slop, so I think that's a reach. I'm going to start with a RS stat, and I think I might put the puller back on it that I already have. That won't give me room for a clutch fan though, so either back to the OE fan (my first choice) or nix the puller and go to a clutch fan? Votes? To the reply above, I've often wondered about the flex fan at speed, because I've been suspicious that it created turbulence and hurt air flow.

For the records: Coolant is very clean, I've had the heads off a few times, and I can vouch for the block and the flush. I've added surfactin as a crazy try also. I will add a trans temp guage, and I think this might be part of my problem, but I'm nervous that the trans temp will be pretty high with only an air cooler. Might try it and see.

I've got a rollback made out of a 64 f-350, with a healthy 312, 4spd, 3.73 gears. Been through the mountains over 20,000lbs with that a bunch, and Ive got nearly 300k on it now with only a 4core rad, shroud, and flex fan, (which I've never replaced) and it will barely pass 200deg.

I appreciate all the help here. I've been getting to the 3G conversion, but almost every trip my 65amper Napa rebuild fails, so I keep getting free ones. No excuse. Sometimes bearings, sometimes no charge, but I'm on like the 4th one in 50k miles. Shame about the quality of that stuff these days.

 

About the water pump, I've owned the truck for 20 years, and about 60k miles. I've never replaced it, but it don't leak, flows well (visually in radiator) and seems happy. I've been hesitant to touch it, see the comment above about the alternator. Its hard to beat a known good part. If I DID have to replace it, any one use a high flow on these?

And is other than fuel mileage, what is the benefit of a flex fan over the fixed OE? I dislike the noise of the OE at road speed, but I got a bitchin radio and 2 backseat teenagers, so that might be less of a problem than its worth to fix.

 

Provided there is no blockage of the radiator, overheating at speed is due to coolant flow/capacity issues - not airflow. Any time/money spent on the airflow side is in vein.

With a radiator opening of 19"x25" at 60mph there is 17424 scfm flowing across the radiator. Considering most HD puller fans will only do about 1/3 of that number, you need to look elsewhere IMO.

Also, I would not suggest only using the air/air trans cooler, but your loop would be trans out to air/air then (gauge) air/water (rad) then back to trans.

 

Trans cooling circuit is routed as follows and is the way its done by all oem manufacturers.

Trans to radiator
radiator to aux cooler
aux cooler to trans


This allows the hot output from the trans to drop the most heat to the radiator and also always the coolest return to the trans. If you go as prescribed above you lose the initial high temp cooling ability. You actually heat the fluid to coolant temp before returning to the transmission which isn't good. The gauge doesn't belong in the cooling circuit unless you have two gauges and are measuring cooling deltas. The gauge needs to be in the pan or some install in the test port. Remember the C6 has no lock up so torque converter output temp can be up to 70 degrees higher than sump temperature.