I had a '70 Maverick. The heater only had about 2/3 the normal amount of heat though. I found a thermostat that wouldnt close but when I replaced it, still only 2/3 the heat. I then found the heater core to be 1/3 plugged. When I replaced it, still no improvement! I found nothing else wrong, but there wasnt much flow through the heater hose. I concluded that the cooling/heating system was designed poorly in that first year for the Maverick. There was another odd thing-from time to time the oil pressure would drop to way low for a while. I had trouble catching the problem in the act though. Fortunately, it was easy to get at the oil pump. But finally I saw a piece of broken rubber valve stem oil seal caught in the high pressure relief valve. But how did it get past the pump’s sump screen? There was a hole in the center of the screen covered by a metal strap spanning the screen. It would seem that this was to provide a bypass inlet through the screen if someone used non detergent oil & the screen got clogged up. The clogged screen would then act as a diaphragm & open, allowing oil in through the hole. But my screen was clean! So the viscosity of the oil alone must have caused the screen to be caved in, opening the hole, & allowing the debris to get hung up in the bypass valve. My Dad & I soldered a piece of brass window screen over the sump screen with its hole & I never had any more oil pressure problems.
Maybe in super cold weather that 1970 oil, poorly refined by today’s standards, formed wax clumps and temporarily clogged the screen? Just a WAG.
Back then certain brands (old timers know which) had a reputation for being waxy or clumping for mysterious reasons.
Thermostat bad, fixed, heater core bad, fixed, and still 2/3 of the correct heat … lol … well at least you were trying. My guess, the heat control valve was clogged, so not enough coolant flow through the heater core. Cars are like those chain-reaction puzzles you see, one event causes the next. So often difficult to narrow down what’s exactly causing the problem. Just have to fix each part of the chain until eventually it all works I guess. The oil pressure problem could have been the result of a problem that occurred at the time the engine was assembled at the factory.
The Maverick had a flow control valve to the heater core that was vacuum operated, they had a bad habit of not working very well. I had to manually open the valve in winter to get any heat and close it in summer to keep it from overheating.
The oil pressure problem was a bit of hardened, broken rubber valve stem oil seal intermittently trapped in the high oil pressure bypass relief valve.
I looked for something like this but couldn’t find any kind of a flow valve. This Maverick had the 200 Cubic Inch engine.
It’s on the firewall where one of the heater hoses connects to the heater core. My 71 had it there, also 200 CI auto. When I returned from Japan in 76, I rented a new Maverick and had a miserable trip because this valve had stuck and it had no heat. That is when I learned about this, I bought the 71 a year later. I think all Fords of this era had that “feature”.
IIRC my 69 Mercury Montego also had it.
It’s certainly too late now, the car had to be sold for salvage in 1975 when i was home on leave from the Navy. But since hindsight is oh so much better than foresight, I wish I would have searched thoroughly for that heater water control valve until I found it. And if i thought that it would be like dealing with a chronic sticking automatic choke, i could have tried switching to a valve from a car that used a manual cable rather than vacuum hose, in- stalling a choke cable with it’s push/pull knob on it, making sure it would fit my size heater hose or using hose adaptors of some type. BTW, what was your rate in the Navy? I was HT2 at time of discharge, on the USS Albany CG-10 out of Mayport, Fla.& the USS Fulton AS -11 out of New London, Conn., discharged 1977
Retired AQC in 1990, to many duty stations to list here.