The problem started when I replaced the water pump. Refilled antifreeze started truck idled it for about 30 min seemed fine. Drove home later that night and half way home the heater started blowing cold air, got home to find the antifreeze overflowing from the tank. I've driven it several more times and noticed the lower hose was cold when the engine is hot.

This is what I've replaced:

Water pump
thermostat
hoses
new temp gauge (aftermarket)

I even want as far as replacing the radiator thinking it was plugged up, but that did'nt change anything. The temp gauge reads a pretty consistent 240 degrees. Is it possible that the "new" water pump is defective? I even double checked the thermostat by boilng it in water found it to be a little questionable and replaced it with another new one, checked that one too. If anyone has any ideas please tell me I'm going crazy wondering whats going on.

Edit: This is on the 86 f150 302

 

Does it overheat just idling or does it overheat going down the road? Also, you said it reads "240". I assume you have a aftermarket guage?

 

It overheats when idling and driving. I forgot to say before that both the heater core hoses and the upper hose get hot, while the lower hose stays cold, or sometimes a little warm. I installed a aftermarket gauge after this started to get a accurate temp reading.

 

It sounds to me like there might be air in the system that needs to be burped out. You say
you let it idle for half an hour, was any of that time with the radiator cap off?

You also say "the lower hose was cold when the engine is hot." Are we talking about the
heater hose or radiator hose?

 

Can you take the radiator cap off (engine cold), start the motor, let it warm up 'til the therm
opens, and look down the neck of the radiator and see fluid being pumped through?

 

Yes the lower radiator hose. What amount of time would you say it would take to get the air out? This project started about 2 weeks ago andI've run the truck for probably 4-5 hours of idling/driving radiator cap on/off and air is still coming out.

 

Yeah, doesn't surprise me. The thing about this post that caught my eye was the mention
of the heater suddenly blowing cold air after your having just opened up the cooling
system. I just experienced something very similar on my daily driver car (1986 Ford
2.3L 4-banger) ... over the past few years, I'd replaced the heater core & water pump
cuz they leaked. Sometime during all of that, the heater started not working real well -
blowing lukewarm air. My daily drive to work is short so I didn't spend any time on it.

I removed the thermostat thinking it might be stuck open but it wasn't. Replaced it,
anyway, since they're cheap + it was already taken apart. It didn't change anything.

Just a few months ago I replaced the radiator (also leaked), and I *thought* I had
sufficiently backflushed & filled it to get all the air out. Ha!

I finally got tired of dealing with lukewarm air out of the heater and let the thing sit & run
with the cap off for at least 20 minutes. Revved the motor, watched the fluid in the
radiator, tried to get it to burp all its air. It burped a fair amount, and that procedure
helped. But, it wasn't as good as I remember it having been (wrt the heater blowing hot
air).

Ya know what *really* fixed it? I encountered this by accident... Just about 3 weeks ago,
I took it into the mountains (I live in Colorado) on one of those twisty, turny, two-lane
roads that requires downshifting to 3rd & 2nd (and sometimes 1st) gears to get up and
ascends about 3,000 feet.

I stopped & parked about 75% to my destination when I noticed my temperature gauge
being much warmer than it usually is, I thought I'd let it cool down. Kinda by chance I
heard what sounded like the coolant boiling! I could hear it from inside the car. All kinds
of horrible thoughts were going through my head.

Well, I opened the hood and found... the overflow tank was dang near full, and the
cooling system was burping its air into it - hence, the boiling sound.

So, I continued on and everything was fine. It has a brand new cap (replaced with the
radiator) I figured it'd blow if the pressure got too high.

And, ya know, after that experience, the heater blows nice, warm air again. )))

So, what *really* fixed it was to really WORK the engine for about an hour. I can't say
I remember the readings on the temp gauge and I'm sure they're not accurate, anyway,
cuze the probe is a good 5 or so inches from the coolant flow.

 

Yaeh I thought about just driving it for awhile to see if it would work all the air out once and for all, but the high temp reading on the gauge (240 deg) had me a little worried. I was thinking about taking my new radiator out and putting the old one back in, its about 6 years old, or should I just leave the new one in. It would be nice to get the $$ for the new one. It was about $200

EDIT: I am getting hot air about 95% of the time, and then every so often it goes cold, and either stays or goes back to hot again.

 

I'd prolly just keep the new one, and that's partially to be fair to the supplier the thing was
purchased from... this new radiator isn't defective, it turned out you merely didn't need it.
IMHO the parts retailers aren't in business to help people diagnose their car/truck so why
should they eat the cost.

Back on topic of your problems, I'd verify that coolant is flowing across the radiator by
getting it hot with the cap off and looking down the radiator neck, you should be able to see
a healthy flow of coolant coming from the upper cross-tube thingers (don't know the
technical term)

 

The coolant level stays full most of the time until some air bubbles come out, then I fill it up again. I do know that the radaitor gets hot on the passenger side, but is warm on the drivers side top, cold on the bottom. I'll probably leave the new radiator in if I don't end up taking anything else apart again, I've already wasted enough money on antifreeze.

 

Do you have a flex fan or a clutch fan? I had to replace my clutch fan when I rebuilt the motor , because of a similar problem.

 

I have a clutch fan.

 

This is a bit of a remote possibility, but, I wonder if you somehow ended up with a water pump for a serpentine belt application. They run reverse rotation, and, on a v-belt truck, due to the shape of the impeller, it would just cavitate and not move any water around, and could very well cause your problem. Just a guess out in left field, but, it might well be worth looking into.

 

You might have a bad head gasket.WHen you start it up cold, see how long it takes to build pressure in the cooling system. If the motor is pumping air into the coolant it will build pressure faster than nomal

 

If you let it sit and idle till the thermostat opens, the top radiator hose gets hot, and you see coolant flowing in the top of the radiator, then that should get all the air out. If it's cool outside, the thermostat will open, then shut, then open, then shut several times till it evens itself out.