I-6 really weird "overheating" story

[66Submarine]

Just got done chasing this over the last couple days. I swapped the 283 in my truck out for a 250 I-6 I had. Used the same temp gauge; worked fine when I drove the truck into place to pull the engine, and didn't do anything to damage it.

Got the 250 in, filled it up with coolant and drove it maybe 1/2 mile down the street and back; it wanted to peg the 250º temp gauge. Winding it up to 3,000+RPM would cool it down a bit, and it seemed like it would hold something like 210-220º IIRC; idling or slow driving and it would shoot right up. Damn it all! So much for a victory lap.

I'd never run this engine in a truck with the cooling system hooked up, only with an old spare radiator loosely attached leaking water everywhere (adding as needed), and only for a minute or two. So, a few thoughts came to mind. The first was that thermostat was stuck; the second that the head was cracked by being run without water too long before I bought it; the third that it was full of some crap rats packed in there, and not flowing water.

Changed thermostats; same result. Head didn't seem to be cracked, as there was no bubbling visible running it with the waterneck off and waterpump unhooked. For kicks, I left it out after that; seemed to act somewhat normally, and would idle at 172º. So, must be a problem with the flow, right? Something that restricted flow enough that it wouldn't work with the thermostat in, or something. I could see the flow through the radiator with no thermostat.

So, I took everything apart and flushed it all out with a hose. Some rust came out, but not much, and no evidence of rats. (The heater core was more plugged than anything else.) Flow seemed just fine. WTF? Put it back together, same deal.

So, maybe a bum waterpump or something? Nope. Wiped some aviation sealer and the old waterpump and put it right back where it was. I also repeated the "flush", and probed around with a hooked wire; no evidence of plugged cooling passages.

So, maybe somehow the exhaust is doing something? Grasping at straws here, but the muffler was somewhat plugged by rats when I put everything together. So, being confused and frustrated, I pulled the exhaust off and ran around the block without nothing off the manifold. Louder, but otherwise unchanged. Really getting frustrated at this point.

It doesn't seem to be overwhelming the radiator, so I decide to plug the exact thermostat I ran for over 5K miles in the 283 in there. Same. Damn. Thing.

So, everything is OK, but it can't get out of the driveway without pegging the temp gauge!?!

So I just let it sit there to see how high it would go idling. Only got up to 210º or so; the other stuff I had done showed slight improvements for whatever reason. But I had a revelation when feeling around; I could actually hold my hand on the head! No damn way that thing is 210º and I can touch it! I also then noticed that the gauge needle would move a little if I moved the truck (getting in it, for instance).

So... you can guess where this led. I pulled the gauge for another one that that seemed to work OK when it was pulled; the truck was "fixed" at that point! The old gauge seemed to work/act normally, but it obviously didn't.

Just thought some would enjoy a long, rambling story of stupidity, and maybe I could save someone else some trouble. Moral of the story; if the gauge says it's overheating and it shouldn't be, make sure it actually is!