1996 F-150 Standard Cab Special with 5.8L. I believe I have the heavy duty cooling as the vehicle was originally a school or company truck set up for towing. The radiator is aluminum and quite thick. All original. Always drove cool enough, consistently between the "R" and the "M" on the temp gauge. Water pump started to drip and mechanic told me that means the seals were starting to fail. Never did overheat. I replaced the water pump. The temp gauge started to register very hot, overheating, etc. Though when I checked the coolant there was no super hot pressure at the cap or the overflow reservoir. Changed the fan clutch, as was original. Same, temp gauge was jumping around the hot side, though would usually settle down to cover the "A" on the temp gauge. Changed the thermostat. Same. Changed the temperature sensor. Checked the action of the the temp gauge both ungrounded at all and direct ground. Temp gauge working per that test. Still every time I drive it it shows very hot but then settles down to the "A" or "A" to "L".

Now the heater doesn't blow near as hot as it used to.

Clog? I believe that even if the thermostat is malfunctioning, the heater is hoses bypass the thermostat? Adequate coolant.

Could be reversed paddles in the water pump? Are their water pump versions with differing paddle direction?

If the thermostat was installed backwards, would that explain the sending unit getting very hot from the block and sending faulty signal strength, eventually opening though against the flow of coolant?

I had worked quite a bit to get all the air out, plus the engine has 2k miles on it since this issue came up. I figure air bubble at the temp gauge sending unit is unlikely now.

Is there a trick to removing the air system of which I am unaware?

Thank you

 

Your heater core is probably clogged. I’ve been told this, get a aftermarket gauge and put it on, if you can put it on the t-stat housing usually they have a screw in the top to put it in if it’s the 45 degree angle one like mine. I haven’t been able to get a gauge yet so can’t tell you the results of doing it, but I do know the coolant gauge on these can be inaccurate. To bleed it just run it with the cap open top it off as needed, do so until it’s ran with the thermostat open for a little while and you’ll know it’s bled once the level stays the same. Shut it off, put the cap on and it should suck coolant out of the reservoir when the fluid gets cool. And then just top off the reservoir. And radiator if needed. You may have to top it off after driving it as well. But if your engines like mine a reverse flow water pump is normal I think. It’s been a few months since I did mine so I don’t remember 100% which direction it is
Mine is a 88 5.0 so things may have changed with the cooling systems over the gap in age. Someone with a 9th gen will have to tell you.

 

Thank you very much!

 

You might have gotten the older pump with spins the wrong way. I have a hard time believing your heater core suddenly plugged with a new water pump. Your radiator does not need the heater core to stay cool.

 

Also check the coolant for "rainbow" colors and/or grayish water with fine bubbles....... Blown headgasket with give you combustion gas into the coolant and may be a one-way leak for a while.