Join Date: Feb 2009
Location: Northeastern BC
I suppose it's always possible there's some air trapped. Take off the radiator cap and squeeze the hoses with your hand to "burp" it. Eventurally, you'd think the air would make it's way out to the overflow bottle, and be replenished, assuming you keep the bottle topped-off.
But if it were air, I'd think this would cause over-heating, not over-cooling.
It definitely sounds like the coolant temperature isn't being properly regulated. I don't think it has anything specifically to do with your heater, etc. -- that's just the symptom.
If you think about it, there are only 2 things that regulate coolant temperature: the thermostat -- which you've already replaced, and the thermostatic fan clutch.
The fact that the fan is constantly spinning is deceptive. On the viscous coupled fan clutches, they'll always freewheel when the engine is running. It doesn't necessarily mean the clutch is engaged or not.
The way I used to check this on my old Nissan, was to carefully grab the fan blade with a wadded up rag, wearing heavy work gloves, while the motor was running, but still cold. Then I'd observe if the clutch started to grab, as the engine warmed up.
***DO NOT DO THIS!!! IT IS VERY DANGEROUS -- LIKE STICKING YOUR HAND INTO A BLENDER!***
I only mention it to demonstrate how it works, and I won't do it anymore, now that I've got the drinking back under control.
The manual gives a method of using a laser tachometer to check the fan rotation speed, which is far safer, and will actually tell you if it's within spec.
I'm assuming you don't happen to have a laser tachometer rattling around in your kitchen drawer, so for the money, I'd just swap out the fan clutch. The good news is a fan clutch can be had on Ebay for somewhere less than $75 w/shipping. Canadian Tire wants $124, if you need it pronto.
Perhaps a scrapyard one would be fine, too? It's probably pretty easy to pull, and I suspect the same clutch is used on a lot of different Ford vehicles.
If Princess Auto were a real girl, I'd ask her to marry me.