Heater issues
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New User
Joined: Sep 2007
Posts: 12
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Heater issues
Hey guys,
I have a 1986 F-150 with an inline six, 4x4, 4spd and ever since I got it, it will only blow heat after being above 3-4k rpms for a decent amount of time. I've also noticed the the temp gauge fluctuates from the 'o' on the word normal to the letter 'a' or 'l' and even then it will still blow cold. I haven't noticed any leaks on the inside or outside of the truck. The coolant tank is missing it's cap, which is about the only weird thing that I can think is causing this. Any ideas?
I have a 1986 F-150 with an inline six, 4x4, 4spd and ever since I got it, it will only blow heat after being above 3-4k rpms for a decent amount of time. I've also noticed the the temp gauge fluctuates from the 'o' on the word normal to the letter 'a' or 'l' and even then it will still blow cold. I haven't noticed any leaks on the inside or outside of the truck. The coolant tank is missing it's cap, which is about the only weird thing that I can think is causing this. Any ideas?
Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
First off, you should not rev a 300 Inline six past 3500 RPMs, it is not good for it.
I never rev mine past 3000 RPM.
I need to know if the truck has air conditioning or a standard or High Output heater, to give a more accurate diagnosis. The heater systems are different for each.
Things you can look for however...
1: check for a sticking thermostat.
2: Check for a plugged Heater Core.
3: Check that the cowl, and fresh air vents are not plugged up with leaves or debris.
4: Malfunctioning temperature control door and cable (standard heater) or vacuum assymbly (Air Conditioning).
5: Malfunctioning heater blower motor.
Frst thing I would do is flush the cooling system, replace the thermostat and radiator hoses (If needed), check for a plugged heater core, then clean out the cowl vents. You can clean the cowl vents by removing the heater blower motor, and removing the kick panels on the sides of the cab, under the dash, and removing the fresh air vents, (Standard heater), or inspection plates, (Air Conditioning) and removing all the debris that collects at the bottom.
Hope this helps.
I never rev mine past 3000 RPM.
I need to know if the truck has air conditioning or a standard or High Output heater, to give a more accurate diagnosis. The heater systems are different for each.
Things you can look for however...
1: check for a sticking thermostat.
2: Check for a plugged Heater Core.
3: Check that the cowl, and fresh air vents are not plugged up with leaves or debris.
4: Malfunctioning temperature control door and cable (standard heater) or vacuum assymbly (Air Conditioning).
5: Malfunctioning heater blower motor.
Frst thing I would do is flush the cooling system, replace the thermostat and radiator hoses (If needed), check for a plugged heater core, then clean out the cowl vents. You can clean the cowl vents by removing the heater blower motor, and removing the kick panels on the sides of the cab, under the dash, and removing the fresh air vents, (Standard heater), or inspection plates, (Air Conditioning) and removing all the debris that collects at the bottom.
Hope this helps.
Lead Driver
Joined: Dec 2002
Posts: 5,491
Likes: 16
From: Kuna ID
Thats what I was thinking, check your coolant level first, if it's low it may not get to your core to allow heat to be disappated from it. Revving the engine may be slinging some through it getting you the occasional heat. Your fluctuating temp is a sign it could be low as well. When the temp sensor gets in an air pocket it will read higher since the coolant isn't there to keep it down.
