Yes, it's necessary. After working perfectly fine for years, my '88 Grand Caravan suddenly developed a problem where after driving about five miles on the highway, the temperature gauge would go almost to maximum, sit there a few seconds, then drop down to cold and slowly come back up. It would do that three or four times before it finally smoothed out and stayed in the normal range. It did that for months. It dropped to cold when the thermostat finally opened and cold coolant came rushing in from the radiator. That would cause the thermostat to close again. Finally I had one of my students take the thermostat out and I drilled a 1/16" hole in it. That solved the problem, and today, about five years later, it is still working fine.
The reason for the wide range of fluctuation is the sensor for the gauge on the dash is not real close to the thermostat. It sees the rise in temperature long before that heat gets to the thermostat. With the bleed hole, the coolant can circulate very slowly before the thermostat opens. That allows the hot coolant to migrate over to the thermostat faster.
What you can try first instead of replacing the thermostat is to just remove it, then fill the radiator. When the coolant appears in the thermostat opening you can reinstall it and there will be very little trapped air behind it. The problem is though you will run into the same bleeding problem the next time the system is drained and refilled.
Tuesday, March 1st, 2011 AT 10:36 AM