unusual heating problem
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unusual heating problem
i have a 78 f150 with a 302 in it.i bought it in the spring no problems with overheating po put a 195 thermostat in it.when temp is below 50 degree outside it will only warm up to 185.i changed the thermostat to a 180 in hopes of getting heat to work in the cab but now the temp wont go above 170.and i have half of the radiator blocked off.i unhooked heater hoses and ran water thru them and no blockage in core.any help would be nice.
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Installing the colder thermostat was a step backwards; the truck will only run cooler as you saw. In general you should stick with a 192-195 degree thermostat; this is the optimal temperature for which these engines are designed. Running cooler than that can accelerate wear.
Have you been able to verify that the coolant level in the radiator is sufficient? It should be a couple inches below the radiator cap. If the fluid level is too high, the radiator cap will dump the excess once the pressure is high enough. It self-regulates the fluid level if it's too high, but can't do anything if it's too low.
Have you been able to verify that the coolant level in the radiator is sufficient? It should be a couple inches below the radiator cap. If the fluid level is too high, the radiator cap will dump the excess once the pressure is high enough. It self-regulates the fluid level if it's too high, but can't do anything if it's too low.
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when i changed the thermostat i filled it to 1 inch from cap.what i dont understand is why it will not get warm enough to open the thermostat to get heat.i thought that is what its supposed to do.get to the temp rating of thermostat then open then close once it cools to a set point.it should atleast warm up to 180 and open then close back at a cooler temp then open at 180 again or am i wrong.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
It's tough to correlate the thermostat temperature with what your temperature gauge tells you. The 180 degree thermostat won't open up at exactly 180 degrees, and your coolant isn't exactly 180 degrees when the gauge says it's 180 degrees. Both could be off a few degrees in either direction.
What's interesting is that with a 195 degree thermostat, your gauge reads 185. With a 180 degree thermostat, your gauge reads 170. So you moved down 10 degrees and your gauge changes 10 degrees, but the "starting point" is wrong. Maybe your temperature gauge is off. The fact that your engine is warming up to some given temperature and staying there, and the fact that this temperature scales with the thermostat you're using, would seem to suggest the thermostat is opening and doing its job. But the number you're seeing could just be off. That would be my guess. Is this a mechanical gauge, or an electrical gauge? Is the sending unit mounted in the same location as the factory sending unit?
What was the original problem - the fan is pushing air into the cab, but it's just not warm enough? That could be a number of reasons, not all of which are related to the cooling system itself.
What's interesting is that with a 195 degree thermostat, your gauge reads 185. With a 180 degree thermostat, your gauge reads 170. So you moved down 10 degrees and your gauge changes 10 degrees, but the "starting point" is wrong. Maybe your temperature gauge is off. The fact that your engine is warming up to some given temperature and staying there, and the fact that this temperature scales with the thermostat you're using, would seem to suggest the thermostat is opening and doing its job. But the number you're seeing could just be off. That would be my guess. Is this a mechanical gauge, or an electrical gauge? Is the sending unit mounted in the same location as the factory sending unit?
What was the original problem - the fan is pushing air into the cab, but it's just not warm enough? That could be a number of reasons, not all of which are related to the cooling system itself.
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i installed a new mechanical gauge at the factory location.the problem is unless its warm outside the truck wont warm up enough to get heat.i turn the blower on and for just 1 or 2 seconds i get very little heat then it blows cold.when it starts blowing cold you can see temp gauge drop.the po put a new radiator in it and new water pump also.could it be the new radiator is to big?
Senior User
Joined: Dec 2002
Posts: 473
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From: SW Florida
If the temp gauge is showing 170 or whatever then the heater should be working fine. Are you sure the valve is opening up to allow the hot water to circulate thru the heater core? It's vacuum operated, so take a look at the valve while the truck is running, and have someone switch the heater on and see if the arm on the valve is moving.
You could bypass the valve with a piece of pipe just to see if that makes it work; the heater should be on all the time without the valve but with it being as cold as it is right now you probably wouldn't mind.
You could bypass the valve with a piece of pipe just to see if that makes it work; the heater should be on all the time without the valve but with it being as cold as it is right now you probably wouldn't mind.
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the heater hoses are run straight to the core no valves.if you turn the blower off for a minute then turn back on you'll get 1 to 2 seconds of somewhat warm air then goes cold again.
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Tuned
Joined: Nov 2010
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From: Caledon ON
Could the heater core be plugged with dirt and dust between the fins? Maybe the PO got the truck burried and clogged the core and now the air wont flow through it. The core might be hot but the air can't flow through it to bring warm air into the cab.
Also, on my first 78 I had little heat after changing the heater hose's. Someone told me to reverse the hose's on the inlets of the heater core. It worked, I had heat again. Not sure why, I didn't think it mattered but it worked, who was I to argue, I had heat.
Also, on my first 78 I had little heat after changing the heater hose's. Someone told me to reverse the hose's on the inlets of the heater core. It worked, I had heat again. Not sure why, I didn't think it mattered but it worked, who was I to argue, I had heat.
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i haven't tried that.but i still need the engine to fully warm up.with the 195 thermostat it would only heatup to 185 so i went to a 180 and now it wont get above 170
shouldnt the engine stay around 185
shouldnt the engine stay around 185
Elder User
Joined: Jun 2010
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From: High Point NC
regaurdless of how hot your engine gets you should be getting SOME warm air out. as little trouble as it is to pull the heater box id start there, just check it to be clear, mine was partialy cloged when i bought it and it had good heat, now after i cleaned it out i cant leave it running for long periods of time or it will roast me out.
Senior User
Joined: Feb 2009
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From: Southern NM
You lost a fifteen degree advantage when you changed to the 180 thermostat.Are the heater hoses hot?Are they in good shape and not blocked?If the core is blocked internally or packed with dirt/debris externally,it won't flow water in the core,and it won't flow air through the core.Is the blend door operational?
Senior User
Joined: Mar 2006
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This sounds more and more like the air flow diverter in the heater box isn't working properly. Even though you switched it to "warm", it's still pulling in cold air. Check those linkages under the dash and listen for the diverter movement when you switch between cold and warm. It's rather unlikely because that whole mechanism is pretty dependable but you never know. You may have a wire that rusted through or something.
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i switched the heater hoses around puts out good heat now.dont understand why unless my gauge is all of a sudden reading wrong.with it warm out and the 195 in it it would stay around 190 to 195 when cold 185.put in the 180 wont get warmer than 170.and thats with half of the radiator blocked off.will be putting a 195 back in since it is supposed to run warmer.thanks for all the help guys.i knew if i asked a bunch of fellow ford gearheads 1 of us would figure it out lol,again thanks guys
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I remember reading a similar thread a few weeks ago about heating problems. I was having a similar problem with my F100. It turns out for me that my heater core was working fine but the insulation around the core had degraded over time and the air was blowing around the core instead of through it. I would get a burst of hot air because the heater core would heat up the air inside the box and then nothing but barely warm air would blow. In the summer time it feels like the heater core works great but I'm in az and everything is hot then. Might you be having a similar issue?