HA! :biglaugh:
So I picked up a radiator cap at Kragen this afternoon, and tried two things. I pulled the cap off the radiator and wrapped a towel around the opening, and started it up, expecting to have a miniature fountain under the hood. ...nothing sprayed out.
Okay, good sign! :icon_bigg I put the new cap on it and ran it until it got up to running temperature. ...nothing flowed into the overflow tank. :icon_surp
I ran it out around on the highway for a while, got it going, and when I pulled back into the car port, the cooling fan was running... but still, nothing flowed into the overflow tank. :biggrinbo
Problem solved! Culprit: :icon_arro Radiator cap.
Edit: Oh- and I still plan to run up and do that compression test anyway. Can't hurt, right?