Heater only works sometimes?
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8th Gear
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Heater only works sometimes?
Hey, I have an 86 f250 non-ac and I can’t figure out what’s going on with my heater. When the truck is not working hard (idling or in town driving) the heat blows ice cold regardless of what the temp gauge says and it claims to always be overheating in the winter. But when it’s high idling or driving hard the heater will blow you out the truck, when I let it run at full throttle while going down the road after about 10-15 second I can feel the vents start to get hot.I live in the mountains of Canada so it can get good and cold this time of year. The issue also only seems to happen when it’s colder than about -10 degrees Celsius or colder, so about 4 months of the year. Any input would be appreciated
Last edited by Hayden Lanting; Dec 30, 2025 at 01:23 PM.
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Joined: Jun 2013
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From: Tolland, CT
Assuming you have good coolant level, the fan blower works on all setting for fan speed.
Do you know what temperature the T'stat is ?
Which engine do you have ?
Have you flushed the coolant system, including the heater core ?
Is the OEM clutch fan on the water pump ?
I'm just asking so the experts will not have too.
Non AC, how many rows or in the radiator ? is it a thicker core newer one ?
https://www.garysgaragemahal.com/1986-evtm.html That may help you...
Do you know what temperature the T'stat is ?
Which engine do you have ?
Have you flushed the coolant system, including the heater core ?
Is the OEM clutch fan on the water pump ?
I'm just asking so the experts will not have too.
Non AC, how many rows or in the radiator ? is it a thicker core newer one ?
https://www.garysgaragemahal.com/1986-evtm.html That may help you...
Last edited by Max Capacity; Dec 30, 2025 at 01:28 PM.
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8th Gear
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It is a 300-6, the coolant level is ok and the blower motor and blend door work good, my thermostat works fine in the summer but in the winter gets funky, the coolant was drained and replaced with 50-50 mix but was not flushed. Just drained. It is a clutch fan but the clutch is worn out so it’s always spinning and it is an oem replacement rad. No idea how old
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From: Angier, NC
All good questions When you check the coolant level when cold as you never want to open the cooling system when hot, let the motor run at idle keeping an eye on the coolant.
Also feel the top radiator hose and keep an eye on the temp gauge. When the gauge starts to go up the hose should get warm / hot shortly after.
As the hose gets hot and the gauge goes up you should see the coolant in the radiator start to move.
If the gauge is into the hot zone but the hose does not get hot or see movement in radiator you got a problem.
Try raising the RPM to say 2500 to 3000 RPM and see if the coolant moves.
If still not moving I would replace the stat as it should be easy and not much money.
If still not right replace the water pump or do pump & stat so both are known good.
The impeller may be loose from the shaft or rusted away.
Just my .02
Dave ----
Also feel the top radiator hose and keep an eye on the temp gauge. When the gauge starts to go up the hose should get warm / hot shortly after.
As the hose gets hot and the gauge goes up you should see the coolant in the radiator start to move.
If the gauge is into the hot zone but the hose does not get hot or see movement in radiator you got a problem.
Try raising the RPM to say 2500 to 3000 RPM and see if the coolant moves.
If still not moving I would replace the stat as it should be easy and not much money.
If still not right replace the water pump or do pump & stat so both are known good.
The impeller may be loose from the shaft or rusted away.
Just my .02
Dave ----
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A little recent history, please. Is this a new problem out of the blue? Or did.it only began after some other work, perhaps seemingly unrelated?
Maybe as simple as a slipping drive belt?
Was the water pump replaced recently? Did the 300-6 use a reverse rotation water pump on later models with a serpentine belt? Wondering if you've got the wrong style on there. Might work okay at higher RPM, but not at idle..
Maybe as simple as a slipping drive belt?
Was the water pump replaced recently? Did the 300-6 use a reverse rotation water pump on later models with a serpentine belt? Wondering if you've got the wrong style on there. Might work okay at higher RPM, but not at idle..
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From Troubleshooting 101, always take care of the known faults, even it seems they wouldn't matter. Just thinking out loud, but maybe improper airflow through the radiator is somehow affecting heat output from the other radiator, aka heater core.
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8th Gear
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To answer all the questions in one. It never had any coolant system work done to it. And I can feel the hoses get hotter as the truck warms up. The fans clutch is worms so it never locks up and the fan itself is always running. The belts are all brand new and there is no belt slipping. The issue of the heat only working sometimes started this winter as it began to get colder outside but the temp gauge deal was a noticed deal last winter as well but goes away as soon as the weather gets warmer. I did some research and I am suspecting that my heater core might be plugged from mice. How viable is that to be the cause?
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8th Gear
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I’m not worried about the temp gauge issue because I know the trucks not actually overheating that bad, and it’s getting a new stat and rad anyways when I swap in a 351w from the original 300-6 that’s in it in about 2 months. But working heat would be pretty nice
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From: Tolland, CT
While the engine is out, remove the heater box and clean it out. It's easy to remove, even with the engine in place, as I replaced my heater core about 10 years ago.
Bigger engine going in... is the fuel supply line big enough...? one of the experts may know.
FWIW, my temp gauge would suddenly jump to HOT from time to time years ago. Then suddenly drop to normal. I replaced the sensor above the T' stat housing and connector from NAPA.
Not sure if your 6 cylinder has that. After replacing the corroded coil wires... the gauge has worked perfectly. But that was about 12 + years ago, not sure what the corroded coil wires had to do with that.
Point is make sure the wire connections are not corroded.
Bigger engine going in... is the fuel supply line big enough...? one of the experts may know.
FWIW, my temp gauge would suddenly jump to HOT from time to time years ago. Then suddenly drop to normal. I replaced the sensor above the T' stat housing and connector from NAPA.
Not sure if your 6 cylinder has that. After replacing the corroded coil wires... the gauge has worked perfectly. But that was about 12 + years ago, not sure what the corroded coil wires had to do with that.
Point is make sure the wire connections are not corroded.
Last edited by Max Capacity; Jan 3, 2026 at 12:54 PM.
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From: Angier, NC
The 300 six motor coolant temp sender is on the right side of the block to the rear and down under the manifolds with 1 wire going to it.
If the motor has a sensor on the stat housing with 3 or 4 wires going to it that would be for the feed back computer system.
Dave ----
If the motor has a sensor on the stat housing with 3 or 4 wires going to it that would be for the feed back computer system.
Dave ----
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From: Virginia
To answer all the questions in one. It never had any coolant system work done to it. And I can feel the hoses get hotter as the truck warms up. The fans clutch is worms so it never locks up and the fan itself is always running. The belts are all brand new and there is no belt slipping. The issue of the heat only working sometimes started this winter as it began to get colder outside but the temp gauge deal was a noticed deal last winter as well but goes away as soon as the weather gets warmer. I did some research and I am suspecting that my heater core might be plugged from mice. How viable is that to be the cause?
The next time you do not have heat, get out and feel both small hoses going to the heater core. They both should be hot, hot enough to where you can't hold your hand on them for very long if the engine is fully warmed up. If neither one is very hot, but the upper radiator hose is very hot, then your heater core may be plugged internally. You may have a bunch of rust in the coolant system.
If this only happens when it's really cold outside, like in the 20's or lower, then you actually may be freezing up. That will cause the heater to quit working and the engine to run hot. have you checked the anti-freeze level with a gauge lately?
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