Strange cooling system issue.
 

Any ideas?

1989 22r - Bizarre cooling issue.

Bad head gasket, so exhaust is leaking over and pressurizing the coolant passages? Happened to me on a subaru I had once.

This is actually the direction I'm leaning. If it's getting worse it would explain the poor performance this morning, too. Nothing else I've done to the truck recently should cause it to stutter the way it did today.

Whenever I screw up something, I always go back to the last thing I did.

But here's a story for you. Years ago, I was pulling a 25-foot trailer through Jerome, AZ with an early-80's Suburban. The fuel pump started cutting out in narrow, twisty streets. What a fiasco. I brought it to a garage in Wickenburg and watched the mechanic fix it. He said there was a pushrod that pushed the lever on the fuel pump. The pushrod could get cocked so he had to be careful.

I'm pretty sure that Toyota copied the Chevy engine. The pushrod runs off the cam, and something like the water pump could also run off the same pushrod or something.

Not saying they couldn't be related, but the 22r's water pump is belt-driven from the crank. It is a strange coincidence that it started overflowing coolant right after replacing the fuel pump, however.

I would think of trapped air before a gasket burn-through, but the gasket would be #2 on my list.