I know overheating and running hot is a regular topic here...but just in case this is different enough to be relavent...i will ask.


Years ago on the 400 (77 2wd) my grandfather and i got tired of it running warm. I wanted to get an elec fan or flex fan...he was too cheap - so we welded the factory fan up - so it 100% locked up 100% of the time. Now that spring is here and we are touching the 80's - the temp start creeping up on any extended drive - highway is worse than in town. Could the stock fan be turning fast enough to "stall" or block air thru the RAD at highway speed? Disclaimer - i have not yet check to see if it has the correct thermostat. But it runs fine - cool in fact in cool weather - to be expected I guess.

Thoughts?

 

At highway speed there should be enough air flowing through the radiator to cool the motor without the fan. That's why they used thermostatic fans. They save fuel and make more power available and they are not needed at highway speeds.

How warm is 'warm'? These motors came with 192 degree tstats.

You say it was running warm before you welded the fan. So it's not the fan, although it is putting extra load on the motor that way. Load equals heat. You may have a point about blocking air flow, if the fan can't keep up with the natural flow.

I would look at the water flow. Have you ever flushed the cooling system? A restriction in the water flow could cause overheating under load. You can't get enough air to cool it, if water doesn't flow properly. In cool weather the temperature drop in the radiator is greater than in warm weather, and you are putting cooler water back into the motor. The cooling efficiency is the temp drop and the amount of water circulated.

 

One more thng to look at is the Fan Belt. A glazed fan belt can slip without being detected. The reduced water flow that results can produce overheating on the highway.

 

I'm not sure about physics on these big fans, but I did a lot of researh in the past on cooling computer hardware.. if you put two 120mm fans next to/on top of each other blowing in the same direction, it would reduce cooling ability. It worked better just having one 120mm fan.. I could definitely see the 'wind' from highway speeds causing reduced cooling against a locked fan.
with computer fans it would put load on the fan being blown onto and lower the RPMs, it aso would almost create a vacuum in between the two fans because of the air velocity, causing the lower fan (one blowing onto the heatsink) to have less air to move through to the heatsink.

to sum it up, it would drop the fan from 2400 to 2000-2200RPMs, but the fan would only blow as much air as if it were running at 1200-1600RPMs, depending on the motor, blade design, etc.

So you're not only increasing the load on your engine but you're also reducing the amount of air that moves through your radiator. In other words, I think your theory isn't a wild one and that might be your problem.
Get a clutch fan, mine made a hell of a difference, my truck hits 195F quickly and stays at that point regardless of how I'm driving.

Of course I'd look into the obvious first, test the thermostat, make sure the choke opens up all the way and the truck isn't running too rich/lean, timing is right, radiator, inside and out and all water channels are nice and clean with new coolant that stays green after 100 miles. I'd also get an aftermarket temp gauge on the thermostat housing to find out exactly what temp the water is at.

At operating temps I can feel the heat of the engine in the cab of the vehicle, you get little insulation between you and the engine and these engines run warm all the way around. There are no waterchannels in the intake and a lot of the heat rises through it, it acts like a blanket on the entire setup.

 

Thanks for the input everyone...we are pretty sure the fan clutch never really locked up..its the original and was always pretty sloppy - even when hot. I am going to pull the Tstat and have a look. I feel like there is a 50/50 chance it has a W tstat.