So at this point I have replaced upper and lower radiator hoses, the radiator, and the thermostat (pre-tested motorcraft). I flushed it myself and again at a Jiffy Lube. I don't see coolant in the oil or vice versa. I'm sure that there's no air in the system and there are no leaks that I can see. I don't see anything leaking off of the water pump and its not making any strange noises. The fan blades feel viscous and they seem to blow at the right speeds. I have hot air and no gurgling from the heater.

It overheats while idling or driving at low speeds. Not all the way, but to the last white line. Revving the engine makes the temperature drop back down. It will also drop down to normal for no reason that I can see. I have also caught the upper radiator hose collapsing.

Could it be a corroded water pump? I haven't seen impeller chunks anywhere... That is the last idea I have.

 

Have you pressure tested the radiator? i had the same problem years ago and I changed everything. All it was, $12 radiator cap...I still shake my head in anguish. If you haven't changed the rad cap, I would do so with a Motorcraft one. If that doesn't solve your problem, I would break down and replace the water pump. I hope for your sake it's just a radiator cap, since the install and the removal on a V8 is much more time-consuming than on I6. I mentioned both since you don't have your current vehicle in your signature.

I hope this helps!

Also, Hgh-five to installing a Motocraft thermostat. As my signature states, "Inferior parts lead to failure sooner than later!"

 

X2 on the radiator cap.

 

Turns out you need 25 posts for a sig. It is a 5.8 auto with AC. Also forgot to mention that yes, the radiator cap was replaced too. The old one didn't look too rough and using the new one (a cheapy) made no difference at all. Kicked myself when I go home from the Ford dealer with just the thermostat and I had forgotten the cap. But I don't think the cap is the culprit.

 

I've been having similar problems in my 93 f150 with a 302. It would rarely overheat when idling, but as soon as I went for a short drive (5-10 minutes around town) the overflow tank would fill up and shoot coolant all over.

Over the last couple of months I've replaced all hoses, water pump, rad, rad cap and flushed the system as well. At the beginning of this problem, I first tried a new Stant thermostat. Then I put in a Motorcraft, prior to the hoses and rad. Three weeks ago, I figured I'd try one more thermostat prior to pulling the heads. I bought the more expensive Stant this time. It didn't have a hole to burp air from the system, so I drilled one prior to installation. This seemed to work, though I'm not sure why. The Ford stat does open when I put it in a boiling pot of water shortly after my meat thermometers highest rating of 190 degrees.

Up until a couple of days ago I have been able to drive daily at various speeds with no problems. Yesterday and today, the gauge went up between the L and beyond in the word NORMAL, where before it had been residing around the R. I have not actually overheated though.

In my case, I'm thinking I will install a real gauge to see what it says. I didn't before because I knew the factory dummy gauge was at least somewhat accurate since when it pegged in the hot zone it overheated.

I'll be watching to see how you make out with your problem. Good luck.

 

I think I would pressure test the radiator and if good, try replacing the pump, if you're sure you are getting no air in the system.

 

After I replaced my rad I installed one of those flush T's in the heater hose to make it easy to remove the air. After performing the regular burping procedure and the system is pressurized the T makes it easy to ensure no air remains. It's also convenient that it's installed at the highest point in the system.

 

I saw those T's at the part store. I may try one of those.

When I first turned on the truck after getting the new radiator in I had left the cap off. Doing this with the old radiator was okay. With the new one though coolant began spraying everywhere immediately, with the truck being completely cool. So I capped it and turned it back on with the heater on full blast. Strangely, I never had to add any more coolant than what I first put in there cold. Of the 50 or so times I've drained this thing now, I've had to add each time.

 

I usually squeeze the upper rad hose and top up coolant in the rad prior to starting the truck. Then I start it up with the cap still removed and let it run for awhile and add more as needed. Then after it's warmed up I replace the rad cap and after it's built up pressure I carefully slightly undo the fitting at the T to remove any additional air bubbles.

For the third day now my truck has been residing between the A and L in NORMAL on the gauge. After several weeks with no problems it is also losing coolant somewhere because I've had to add some twice in two days.

The odd thing is that there are no leaks, it holds up and maintains pressure when I put a pressure tester on the rad. It seems likely that there could be a small head gasket leak or crack, yet there's no contamination in the oil or tranny fluid. During one test it showed exhaust gas in the coolant, but on the next it didn't. It just seems funny that it hasn't been running hot for three weeks now and it started acting up again.

 

I should also mention that, like you, I thought that maybe my pump was bad although there were no bearing noises or leaks from the weep holes. My upper hose was not always getting really hot and pressurized. That was after I had the Motorcraft thermostat in, so I figured it wasn't the stat. My pump looked fine, but I changed it anyway since I had everything apart. The only thing that's made any noticeable difference was that third thermostat I tried several weeks ago.

I also coupled the heater hoses today to rule out the heater core, being the only component I haven't replaced. It made no difference unfortunately.