Hi, I am brand new to the forum but glad I found it. I have a reverse temperature problem! My 1989 F150 Lariat runs normal at lower speeds and at idle but the gauge climbs at interstate speeds. Running in town and 55 or below the gauge stays on the O but moves up to the R,M area at 65. When I climb to 70-75 the gauge goes up to the A,L area of the normal. It isn't a false reading or bad gauge because I have pulled over when she nears the L and I hear the crackling as she fights overheating. Even leaving the truck idleing she drops right back down to the O. I can simply drop down from 70 to 60 and the temp declines.
I have put a new clutch fan on and a new thermostat. I installed a flush kit and have flushed it several times. It has help (it takes a little longer to climb) but still climbs. My fan shroud is still in place but I don't know how to tell if I have a single or double core radiator. I just got the truck last fall and the temp outside Definately has a lot to do with it. It is still spring and summer is right around the corner. I can drive the speed limit but am afraid to go too far. All help or advice appreciated. Thanks

 

I have the exact same problem with my '87 4.9L 4spd.

I have done two chemical flushes and it has helped each time. I am taking the radiator to a shop on monday and having a full work over of it, flushed and flow checked. Here, it will cost me $25, and I take the radiator out (too simple not to).

Feel the radiator when it heats up, start at the top on each side and feel for spots that are much cooler that others, working your way down to the bottom.

On mine, the lower 1/4 of the radiator is cold when the inlet is too hot to touch for long. And your right, outside temp has a lot to do with how hot it runs. That is because the difference between the outside air and the coolant is less.

55F outside - 180F coolant = difference of 125F

85F outside - 180F coolant = difference of 95F

What really makes the difference is the amount of surface area of the radiator. If your radiator is clogged, the number of passages for the coolant to flow through is less. That translates to less surface area that is being cooled by the outside air.

I.E Runs hotter in warmer air.



HINT: put in a 160F thermo and keep using the the chemical flush mixed with water for a couple days. That will get you a while until you can have the radiator done right.

BTW, check that you did not cook your oil. When mine was overheating, I cooked 6qts of SAE30 to the point I could hear bearing noise. If you cannot get the temp down, at least run 20w50 until the problem is fixed. It will withstand more heat before breaking down.

Good Luck,

 

You either have a thermostat that is not opening all the way or you have a partially clogged radiator or the last alternative is the fan clutch. Start with the thermostat, then have the radiator recored or rodded (same thing), and then check the fan clutch. My bet is it is one of the first two.

Good luck and welcome to FTE.