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I am working on my wife's 1974 F100 2wd 360 or 390 (not quite sure) with a C6 trans plagued with a heating issue. When I got it somewhere along the line the previous owners hacked it up when they removed the AC (probably when they put the bigger engine in).
Problem:
Has weak/inconsistent cab heating. When driving down the road the vent blows cold/lukewarm air, when idling gets warmer but not hot.
What's been done:
Found the cold/heat selector cable locked up. Replaced it with new.
Pulled apart the box to verify the heater core wasn't damaged and to verify the fan motor was working correctly.
Found a 180-degree thermostat. Replaced with a 195-degree thermostat.
Added a mechanical temp gauge while testing. The truck runs around 190 at cruise and 200 at idle in ~30 degrees outside temp.
Did a coolant flush to ensure the heater core was free and clear and I didn't know the status of its last replacement I burped it to remove the air in the system.
Reconnected the vacuum system. Since everything was hacked off it's a known issue to tackle to make it 100% correct.
Hacked a dryer vent flex pipe to the passenger vent until I can get a proper replacement. Mainly did this to prevent dumping right on the floor when in that setting for testing.
Anything jumps out at you for me to look at? Vacuum lines on drivers side firewall Blower motor/heater core box Heater hose connections Looks like a hole here (just noticed this for photos) Looking in glove box Climate controls Front Grill/Radiator
Well, not seeing any of your pictures, let me ask ... is the heater valve in the heater hose hooked to a vacuum line from the AC controls in the cab or what? If hooked to the carburetor or intake, vacuum is shutting the valve and cutting off coolant flow to the heater core. If the AC is gone, they may have plugged the hose to the intake to intentionally shut off coolant flow to maybe get cooler vent air through the dash vents in the summer?
It looks like the heater valve has been omitted and I can confirm both hoses are hot when running. I didn't get a chance to test the theory today but I am wondering if the culprit is the hole left by what I think was the AC evap coil
The hole for the a/c evap is what I noticed. Could be part of the problem. Another thought is the fresh air vent on passenger side going through kick panel. Should be a vacuum control behind glove box.
Look in the top left of this picture, the evaporator is in place but the tubes not sealed where they go into engine compartment. That's the hole that's open on your truck if the evaporator is gone. The tubes cross the rectangular tunnel coming from the heater motor to the interior. Below, sans evaporator. See how big of a cold hole you have then.
Here are some more pics, Thank 77&79F250 here, I think almost certain was he whom I robbed them from. I study them from time to time, good set to keep in a file. I put mine in a non AC truck after robbing two carcasses .... but it was in the '80s and I never thought to photograph the big box for future use.
Maybe they'll help you ... or someone else too. And just some more while I'm here, charts for you.
