Hi All,
Ive recently bought an 1989 f150 which is an awesome truck but unfortunately I am having some overheating issues with it. Originally the temp gauge wasnt even working in it but I made some repairs to the cluster and now it works.

I have replaced the top and bottom rad hoses as well as heater hoses, thermostat, thermostat housing and had the radiator cleaned out. The fan clutch as also been replaced.

The issue I am having with the truck is that when I have the heater on the temp gauge sits at about half way ( where I would expect it to sit ) but once I turn the heater off the temp will begin to rise, it went to about the 3/4 mark before I realised and turned the heater back on, at which point the needle went back to sitting on the half way mark.

I have checked a couple of times to ensure that I dont have any air trapped in the system and most recently jacked the front of the truck up whilst burping the cooling system but the problem persists.

One observation I did make was that the bottom radiator hose was cool to the touch despite the top hose and the heater hoses being hot. I also noticed that one one occasion when the engine was reved the top rad hose sucked itself in.

The weep hole on the top of the water pump is not leaking any coolant and the after removing the belt there does not seem to be any free play in the pump.

The truck was parked up for about 12 years.

I am starting to run out of ideas with this truck, the only thing I was thinking was that due to being parked up for so long the impellor in the water pump would have all but rusted away as when I drained the coolant a lot of crud came out.

Have any of you guys experienced anything similar to this fault and how did you go about fixing it? or anyone got any ideas?

Cheers in advance for any help....

 

forgot to mention I also replaced the radiator cap..

 

It acts like a bad thermostat. I would try either removing the thermostat or letting the temperature gauge get to the end of the normal line to see if the thermostat might open.

The water pump is atleast pumping enough coolant to get it through the heater core.

 

What brand of t-stat? If it's not Motorcraft, I consider it junk. That goes for Stant, any fail-safe or other inferior t-stat.

Did you place the t-stat in boiling water to see whether or not it opened or closed?

Have you replaced the radiator cap? A non-functioning radiator cap will also cause overheating. I think that is ruled out as you said the upper radiator hose sucked flat.
I change radiator caps anytime a water pump, radiator, or t-stat is replaced.
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The t stat did cross my mind, but now you have mentioned about using only motor craft ill go out the shed right now and pull the one that is in there, pretty sure it is stant from memory. I didnt put it in boiling water.

I did replace the rad cap, it is a stant one i put on there, should I chuck it in favour of a motorcraft one?

 

Is your belt routed the right way or did someone put on a reverse rotation water pump

It shouldn't suck the top hose flat bottom is low pressure or suction

 

I did check belt direction when i put the belt on but will double check before i go any further. As far as I am aware it is the original pump on it or has at least been on there for a long time. Truck has only done 108, 000 km.

 

So it was the pos stant thermostat, it has gone in the bin and ill be ordering a motorcraft one like you suggested timbersteel. Thanks all for the help!