I just installed a milodon hi-flow aluminum water pump, and the hi-flow mr. gasket 195 degree thermostat, along with all new hoses. I've refilled the radiator, now suddenly the engine is fluctuating temperature. I haven't flushed the system yet, but I can feel the water flowing in the hoses. I left the cap off, and filled it as it went down. I ran it about 30 minutes with the cap off. Some times it would spit some back out while bubbling, but I doubt a head gasket - no problem before pump and t-stat replacement. The original problem was a leaking pump - problem solved. Do I have an air-lock or something? The temp will run fine for a couple of minutes, then quickly spike to near H, hold for a bit, then return to normal. Almost seems like a sticking t-stat (new). I have a 2nd stat, but damn, I spent 8 hours pulling the pump, fixing the broken pump bolt, changing timing chain and gears, cleaning off rock-solid gaskets, etc. Hate to mis-diagnose this one, and do more work unnecessarily. It doesn't hold hot for long, but it is abnormally spiking from the 0 in normal to l in normal. It jumps in about ten seconds.
I've only had it running less than 45 minutes so far. Runs like a champ except for the troublesome temp spikes. Sure looks like the t-stat.