very unusual heater issue, only works for a while.

gstanfield · #1 (**permalink**) 10-28-2009, 07:09 PM

My truck is a '79 F150 with a 385 series engine swapped in from God only knows what (I'm told it's a 460, but...) The truck came with everything, fully loaded. The A/C works, but needs charged and to be honest I couldn't care less if it ever worked again. I live in WY and only use the truck when the weather is crappy or when I'm hauling off the trash.

Enough talk, here is my issue. When I leave the house in the mornings I have hot air blowing by time I leave my little town (450 people, so not too far) The heat works good and all is happy in the world. By time I get 15-20 miles down the road the heat has turned to cold. The temp gauge is still in the normal zone, the t-stat seems to be working fine - it just looses heat. The previous whoever has unhooked the vacuum lines from the trouble prone temp sensor, but the heater bypass valve is still in place in the heater hose. I'm not very familiar with these particular trucks so I was wondering if anyone had an idea?

Could it be the flapper under the dash "blending" it's way over to cool as I drive? Why would it do this and how should I stop it? I was also going to bypass the heater bypass valve in the hose since I care not about A/C and only care about HEAT!!

The truck runs awsome, barely turns half a revolution and fires right up even in below freezing and subzero temps. All I need is some heat that works to keep my windshield clear and ym fingers unfrozen.

thanks for any suggestions,
George

Blown 331 · #2 (**permalink**) 10-28-2009, 07:25 PM

I would say your heater core in clogged internally. I'm guessing that there is a lime or similar build up inside the heater core that is insulating the warm antifreeze from the heater core itself. It can get warmed up initially but once you turn the fan on the heat is quickly removed from the heater core and it can't warm back up fast enough. I bet if you shut the fan off for awhile then turned it back on it would be warm again.

gstanfield · #3 (**permalink**) 10-28-2009, 08:22 PM

Good idea, and it was one I had thought about myself based on experience with my '65 Impala SS having that exact problem. I just finished pulling the heater assembly apart to change the fan (already knew it needed replacing cause it rattled) The core flows nicely and is in suprisingly good shape.

Since I am already in there I can just inspect everything as I go. I'll report any findings I may have, but in the meantime please throw any more suggesitons out there in case I forget to check something while changing out the fan.

Thanks,
George

Jermafenser · #4 (**permalink**) 10-28-2009, 09:03 PM

Core plugged up.

Insufficient water level.

When you lose the heat, pull over and feel the heater hoses. Cold or hot? Both or one hose(s)?

TriumphTune · #5 (**permalink**) 10-29-2009, 02:50 PM

This happened to me. Turns out that the thermostat was sticking open. When I would first leave out in the morning when it was cold it would warm up and be hot. But when I stopped somewhere and then got back in the truck, it was cold and could never heat back up. So I figured that when the truck was on its first drive the cold air blowing over the engine kept it cool enough for the slow thermostat to stay closed. But when i stopped somewhere, the saturation of hot coolant and no airflow finally made the thermostat open. Then it would take it sitting overnight to close again. Changed the thermostat and solved the problem. Of course this worked for me but it may not be your problem.

Josh

Capitlj · #6 (**permalink**) 10-29-2009, 03:19 PM

It could just be some air trapped in the system too. My impala did the same thing till I fixed the o-ring that had let the air in and bled the system.

gstanfield · #7 (**permalink**) 10-29-2009, 06:04 PM

I'll burp the system good when I put it back together. I'm fairly certain the T-stat isn't sticking as the temp guage climbs just as it should and sits in the normal range while going down the road. A bad T-stat should cause the enging temps to vary.

At this point I have pulled everything apart and tomorrow it will go back together. I guess then I'll see if it works fine. I'm inclined to think the coolant may be a little low as everything else is getting ruled out little at a time.

thanks for the suggesitons, I'll burp the system real good and let you know if this fixed it.

George

Tees77f150 · #8 (**permalink**) 10-29-2009, 11:30 PM

Low on fluid is my thought, this happened to me, to the point when it got real cold, I was loosing ALL heat, and was getting ice build up on the inside windows. Sucks when you got to use the ice scraper on the inside!!