As many trucks as there are like ours out there, I don't think any problem, no matter how odd, is unique. Maybe someone here has had this before, and hopefully, this will benefit someone in the future.

'96 F150, 302. 147K miles. Cap, stat and hoses all replaced at the last coolant flush and change 3-4 years ago. everything else in the cooling system has been on the truck since 25K miles (probably original, but I got it at 25K)

For 11 years, this truck has consistently run with the temp gauge never going above the "O" in "normal" on the gauge.

Monday, when taking my daughter to school, after 20 minutes of highway driving, the heater went cold. I moved the temp lever to cold to see if it was putting out anything warmer than outside air. It IS warmer, but not much. Temp gauge was at the leading edge of the "M" in normal. This is warmer than normal for my truck, but not close to hot. Kept an eye on the gauge as I drove home. It seemed to fluctuate between where it normally runs and this new slightly higher temp. When it runs normal, the heat comes back. When it goes to that new higher point, it goes chilly again.

I also suspected the blend door, but when you go to warm, you can hear the door closing. Actually hear it hitting the stop.

I Was petrified about a head gasket. Got home, and when it cooled down, checked the coolant level. About 1-2 cups low. Not enough to cause lack of heat. No sign of oil in the coolant. Checked oil, no sign of coolant there.

I Also noticed that the radiator cap will move with the safety lever locked down. Ok, I need a new cap, but that shouldn't cause a loss of heat.

Tuesday evening after a ten minute drive (after truck was fully cool), I noticed a small puddle of coolant. I found the radiator leaking at the seam between the aluminum and plastic. Minimal loss of coolant- two days driving empties the overflow, but the radiator is still full. This leak does not happen after longer drives, but is very consistent after ten minute drives. (I live ten minutes from work, so I do a lot of those) No sign of any coolant in the cab, either.

I'm a bit baffled. If it was a stat sticking open, it should take forever to heat up, but stay warm once it did. If it was sticking closed, the truck should overheat. Boiling it down to basics, I'm either not flowing water through the core, or the water flowing isn't hot. But not consistently.

If the core was clogged, it should be cold, and stay cold. It shouldn't be intermittently clogged. The temp fluctuation isn't enough to cause that drastic of a drop in the heater temperature. This is a Ford, it's suposed to have a butt-kicking heater (it has for the last 11 years)

I really don't think I have a blown head gasket. I doubt that the radiator leak could cause it, either. The leak started when I said it did, too. I'm consistent enough about where I park, and am also consistent about looking for stains and puddles in the places I park. But it would be odd for the two not to be related.

My plan of attack is this: Flush it out, new coolant, cap and stat. I'm gonna try to milk the radiator through until past the holidays, if I can.

If anyone has any thoughts, I'd be interested in hearing them. I'll post the results so anyone having the same problem can benefit.<!-- google_ad_section_end --><!-- / message -->

 

yeah i have that same problem my heater only blows like slightly warm air i think it is the heater core is clogged i am thinking of taking the hoses and flushing the heater core to the bone and seeing if anything comes out it could also be that mice got into my heater core housing and made a nice little nest for me to clean out (my truck sat for 5 years before i saved her from the junkyard)

 

for the cost of a new heater core id say just go ahead and replace it, its probably on its way out anyways. idk about the 96, but my 88 i can change it out in like 30 min

 

on mine i would have to take the front seats out to get the damn thing out it comes out through the cab

 

I might indeed end up changing the heater core, but I can't reconcile it yet. If it is clogged, why does the heat come and go?

I'm thinking I might have an air bubble somewhere in the system, possibly caused by the cap

 

i doubt it the cap not being tight would just let the cooland dump out... the thermostat could be stuck open letting the heater get warm when u are sitting still and when ur driving and its cold all the cold air cools the coolant below 180* or what ever the thermostat in urs is set at and it and not letting the engine maintain the 180* operating temp thus the heater gets cold.... the cap will let the coolant dump out when its running if the heater isnt blowing heat and the radiator hoses are hot then u have a clogged core or hose running to it.... drive it up the road and when its not blowing hot air go grab the top radiator hose and see if its hot.

 

^ that could easily be true, as even when it's blowing hot, it's not roasting me out of the cab as usual. I'm hoping you're right. I'll know by tomorrow.

 

Take a long hard look at your water pump. See any coolant around the weep hole? If your water pump was on the way out it might be flowing the coolant but not very well. I've had this problem with a different vehicle.

 

^ I second that. If the t-stat is stuck open believe it or not the truck would over heat because the coolant does not sit in the radiator long enough to remove enough heat from the coolant.

 

well as much as i hate to argue the thermostat being stuck open will not cause it to over heat because liquid can only run threw the radiator so fast. my race car was running 240 with the thermostat in and i took it out and it never at no point got above 180*f only thing it does is maintain the engine at what ever temp its rated at if it goes below that rated temp it closes raising the temp back to the set temp. removing it will just cause it or it being stuck open will just cause it to take longer to warm up and will run lots cooler when driving so if u can find a site or manual that says it will cause it to over heat because my experiance tells me otherwise

 

i agree with hitokori i had a buick that the thermostat stuck open and the temp never got above lukewarm i froze my a$$ off

 

Interesting- I've got three different suggestions from 5 different guys. This IS an interesting one. As I said, tomorrow it gets a flush, new stat and cap. We'll go from there.

 

heater hoses may be colapsing on the inside, restricting flow. try switching the hoses on the heater core to get the flow going the other way. if this improves your heat, put on new heater hoses.

 

VERY interesting thought, Charlie. When I said hoses had been changed, I meant upper, lower and bypass. Heater hoses are original. I don't want to do too much at once, as I'd like to know exactly what it is that fixes it. That one is going high on the list.

 

I might add that I've seen pumps with the impeller rotted to nothing and others where it is starting to spin on the shaft.
How is the flow in the radiator if you take the cap off, start it, and then allow the engine to warm up until the thermostat opens?
Did you notice any sludge when you last flushed your coolant?