I recently changed the antifreeze in Gypsy's 2GM20 and had a bit of trouble with the overheat alarm telling me that something was now not quite right. Following lots of good (and some not-so-good) tips on the internet, I heated some water and confirmed the thermostat and sensor switch were OK but I was at a loss beyond that. I had seen an earlier thread on changing the antifreeze where a couple guys mentioned burping and guy #3 thought they were goofy. Initially I thought burping was a little far-fetched, but it turned out to be exactly what I needed to do. I had been adding the A/F into the heat exchanger. This traps air in the thermostat housing until the thermostat opens, as mentioned by guy #3. The trouble is that the thermostat is surrounded by air and fails to open before the sensor switch trips and the engine cries out for help. I suppose the thermostat might eventually start to open if things got hot enough, but I did not want to take that chance. Topping off the thermostat housing is what I ended up doing, but loosening the hose to the water heater should also do the trick. I am a believer in "burping" and wanted to thank S. Sauer and Gary Y. for mentioning it. Sail on, Ken