408 getting hot

[fsstnotch]

Yes, drilling the 1/8" hole allows any air trapped in your engine to escape

Quick explaination i found on the net

The Thermostat Bubble Connection
By Steve Litt
A.S.E. certified master truck and auto mechanic Dennis Buler emailed me, mentioning he's solved many overheats by drilling a 1/8" hole in the thermostat. Dennis explained that if any gas, be it air, combustion gas, or steam, gathers around the thermostat, that often the thermostat can't detect the coolant heat, and therefore doesn't open. If only the thermostat would open, the gas would be "blown" into the radiator. And of only the gas were "blown" into the radiator, the thermostat would open. Catch 22.
Dennis explains that his 1/8" hole provides an escape route for the gas without passing so much coolant that the engine runs too cool. In other words, the thermostat will simply open less in order to maintain temperature.

Reading Dennis's email, my mind drifted back to all the emailed symptoms where the car overheated in the first few minutes, then magically dropped back to normal temperature throughout the rest of the trip. Could it be that when cold, air surrounded the thermostat. The thermostat therefore didn't open at the proper temperature. However, when the engine temperature reached overheat levels, maybe the thermostat opened, blowing out the air. The opened thermostat would allow the cooling system to do its job, returning to normal temperature. The partially open thermostat would prevent a gas buildup. Until the car was shut off long enough that the thermostat would completely close :-)

Dennis's findings might suggest that running for an hour with no thermostat might be an excellent test. If overheating occurs with a known good thermostat but doesn't occur without a thermostat, it's a strong suggestion for Dennis's air bubble scenario.

I don't know enough yet to make drilling the hole a suggestion. I don't know its side effects and risks. But it sure sounds reasonable. Add that to the fact that Dennis says he's seen several foreign thermostats come from the factory with a 1/8" hole, and the fact that another Troubleshooters.Com visitor mentions in an email that VW Rabbit owners often drill such a hole to eliminate engine airlock.

If you want to try it (and once again, I don't know enough to recommend it), you can see the location of Dennis's proposed hole in the diagram below. Here's how Dennis describes the location verbally:

"i drill the hole in the flat disk part like half way between where the gasket would seal and the center opening mechanism" Pink dot represents the 1/8" hole