Donb,
Well, ya got me there; I HAVE been running Pennzoil 10/30 (a quality Pennsylvania grade crude, unlike the garbage that comes from the West coast). Unlike quite a few other members, I use oil rather than synthetics, but religiously change oil every 6 mos (and rarely ever go over 3000 mi in that period).

I have to agree that a lighter oil certainly could lead to greater chance of heat build up, but my overheating issue was far beyond anything minor; it screamed "something is drastically wrong!" Lighter weight oil could lead to minor increases in temp, but not what I saw.

The thing is, I don't have a mechanic I trust to accurately diagnose my problem. There are some gifted mechs out there; and then there are a lot of guys who earn money working on cars. Too many mechanics do the same thing that I would do; try one thing, and if that doesn't work, try something else. It's possible that engine build-up got into the new radiator core; but I could see this problem being the thermostat, the water pump, or something (worse) else.

The heater doesn't seem to lower the overheating temp as it should, so that might point to a bad pump; but then I felt the hoses and didn't feel pressure after the car got to operating temp, and that sounds like a thermostat. But when I took the thermostat out, it opened at 167F (but maybe not all the way? I don't know...). I do know that I hate randomly replacing parts; but at least if I'm the idiot doing it, and I don't have to pay myself labor, and can thereby afford to buy more parts, and increase the chance that I'll replace the right thing. It's such times that one realizes that skilled mechanics are truly God's gift to car owners.

I've heard that a blown head gasket can cause overheating (and vice versa); that a slipping trans can create overheating; retarded timing; air pockets in the coolant from improper burping; and several others. I had no idea that so many things could cause overheating! I've been hoping to come across something that said, "when you experience x set of symptoms, y is the cause." But so far, no avail.

I'm out of town, and therefore away from my tools. I will replace the thermostat, and hope that takes care of it. If not, I will limp it 100 mi home (pulling over twice for an hour break) and start eliminating candidates.

I still hope that someone has had a similar problem, and has "the fix."

__________________
Love driving my '77 450 SEL!
124,000 miles