Just pulled the thermostat out of my wife's 1995 3/4 ton suburban 7.4 liter and it was stuck open. It wasn't overheating just the opposite, it wasn't heating up completely. There was a couple of tabs on either side holding it in place and returned to the closed position when I applied pressure with a screwdriver. So I was just wondering why it's like that. Thanks in advance for replies.
Brian

Its just a sign that the thermostat is getting old. The metal will wear and causes the thermostat to have spots in it that will stick it open and cause the vehicle to never get up to temp. In some cases debris get into the system and stick the thermostat also.

They over heat when thermostat sticks closed.

Fuel mileage should be a little better, since it will go into closed loop mode now.

Here's a pic, see the little tabs on the side, they were actually holding it open. Tested it on the stove and it seemed to work fine.

Hopefully the wife will be happy to have a little heat in there now. I thought maybe it was a high pressure situation and maybe the block might have some kind of blockage in it.

That's a "failsafe" thermostat. They are suppose to lock open in the event that the engine overheats.

This is exactly what I would think.

Also I hope you put a good thermostat in it. I had to replace mine in my old '94 chevy when it was about 10 years old. I put a cheap one from auto zone in it and had to replace that thermostat every fall when it started getting cold, whent to NAPA and got a good one and it worked for another 6 years till I sold the truck.

matt

Flushing your cooling system and putting in fresh anti-freeze will help prevent this problem.

Might do that sunday if I get time. Now I remember why it did that, the wife was driving it home and the bottom radiator hose popped off while she was driving.