My 2 month old Stant thermostat went out on me 2 weeks ago. I babied it home from work, watching the temp gauge go up and down, and I could feel the water in the system flowing and stopping because the heater was on and it would get hot and then cold. It seemed to always flow making right turns and get hot making left turns (pushing water to/away from the pump?) The temp gauge would swing back and forth like it would when there's an air bubble, and it did this about 20 times. The temp got about 1/8 inch from the red once before I pulled over and stopped, but the rapid temp cycling was probably not too good either.

I changed it with a stock item and changed the upper and lower radiator hoses at the same time and it ran fine for a week, until it began to miss this past Thursday from a cold start.

Here are some symptoms and observations:

Cold starting produces a single cylinder misfiring for about 20 seconds, then it fires intermittently before coming fully on line.

Loss of coolant, but not getting blown out of reservoir so it's going into the engine. No mixing of oil and coolant (thankfully...). No hydrolock symptom when starting the engine cold.

Number 4 spark plug was wet from both coolant and fuel.

Warm start after about 10 minutes off produces white smoke from tailpipe for only a short burst, but a significant amount.

Missing at full throttle, but no overheating.

Pulled the exhaust header and found number 4 port to be somewhat "steam cleaned" with a little residue of coolant. Other ports were darker with carbon from running slightly rich.

Pulled the blower and and found number 4 intake port to be the dirtiest and oiliest. My guess is that since it's closest to the bypass valve, it gets the straight shot of the oil mist from PCV when cruising.

However, here's the weird part -- there were distinct coolant traces on both sides of the septum where the port splits in two, but it began about an inch into the port. The backs of the intake valves show coolant and oily residue. The backs of the intake valves of cylinders 1-3 are mostly clean, maybe very slight oil or fuel stains.

Am I seeing reversion in the intake port, and this is what is causing the port to get wet, or would this only happen if the head is cracked above the port and it's dripping into there? Since I have no pressurization of the coolant and the oil hasn't mixed, this seems likely. The temp sensor is also away from cylinder #4, so it was probably hotter than what I was seeing on my dash gauge.

Has anyone experienced this before? I'll have the head off this Saturday and can inspect the head gasket then, but I'm hoping it's only the gasket and that the head isn't cracked. I haven't done a compression check yet because my friend has my gauge. I guess I now have a good excuse to have Portflow work some magic if it's still in shape, but that's a good $1000+ that I didn't need to spend right now...

BTW, I have a 97 GSR running 10psi from a JRSC and Hondata managed with 172,000 miles. It was only consuming about half quart of oil every 5000 miles, which is my change interval with Mobil 1 10W-30.