91 F150 overheating problems

[rickpilgrim]

Today I got a 91 F-150 4x2 w/4.9 and AOD. She has 107,473 mi and has a strange overheating problem.
You can start it up and leave it idle, no problems. Pressure test cooling system and holds 26 psi. Replaced thermostat and no change. Start driving around and soon she gets hotter and hotter until you shut it off. Fan seems okay only thing I noticed was a wet stain along the pass side inner fender that may be coming from the aluminum/plastic radiator.
Could that radiator hold pressure cold but when it gets to operating temp start to leak and then seal up when cool? I thought head gasket at first but there are no bubbles in the cooling system at idle w/cap off.

 

[5.0Flareside]

are you sure your radiator is full? that can cause similar issues.

 

[flareside_thunder]

I suggest getting it up to operating temperature then popping the hood and checking things out..... I suppose it's possible the end tank is splitting a little bit and shooting some coolant out.

 

[rickpilgrim]

We fill it but it seams to be losing it when warmed up but it must be spraying out where you can't see it, and even when you stop it is low again but only so far(about 6" down)

 

[rickpilgrim]

We ran it in the shop for 45 min and it didn't happen. 5 miles later it was HOT

 

[flareside_thunder]

Well drive it around and when it starts to get hot pull over and check it out...make sure you got a flashlight with ya.

 

[john112deere]

I found a coolant leak in my Escort by waiting until after dark, then going over it with a flashlight. Couldn't see the wet spot in sunlight, but by flashlight, it was shiny where it shouldn't have been. Worth a try, anyway...'cause it's free.

 

[surewhynot]

Is it possible that somebody put the wrong fan on it? I was working on a truck that would only overheat when moving. It turned out to be that it had a fan for a reverse rotation water pump, on a standard rotation pump. It was pushing instead of pulling. Sitting still, air would push through the radiator, but when you got moving, the wind would block all airflow.

 

[rickpilgrim]

First off, I'd like to thank everyone for their thoughts and ideas.

Second, today I found the problem. After replacing the cracked tank radiator I went for a ride around the block a few times and noticed the heater wasn't working anymore. When I pulled back in the shop coolant was going ouy the overflow bottle and my point & shoot thermometer said coolant bottle and radiator were at 192 degrees. Got the cap off and when I hit 20 psi the engine rotated a bit-blown head gasket.
I have never seen a blown head gasket on a 300 before but there's a first time for everything.