I'm so happy I had to share this with you guys. I've got a 92 with a 4.9 litre six. It has had a very slow mysterious coolant leak for years and the system never really pressurized. It also had problems with never reaching proper engine operating temperature. I tried water pumps, heater cores, thermostats,hoses, flushing-EVERYTHING!!.

A few days ago I went at it again and tried to put a 195 degree thermostat in, but the thermostat housing snapped as I tightened up the bolts. I didn't use much pressure so I thought it was odd I snapped it. Anyway, I installed a new thermostat housing and now I reached proper operating temperature and the system seems to be pressurized, which it never did before. I'll drive for a few days and see if there is any coolant leak, but so far so good.

I can only guess that the old thermostat housing must have had a slow leak and would not keep pressure(maybe a pin hole in the casting?). Anyway, problem solved. I just wanted to share it with everyone in case you ran into the same problem. It's strange that it never would overheat, it just would NOT reach operating temperature.

 

Strange. Who would have guessed? Could you tell if the thermostat housing was in bad enough shape to let coolant bypass the thermostat?

I wouldn't expect that lack of cooling system pressure would be a cause for not coming up to temp. Was the coolant level high enough? If there was not enough fluid, then maybe there was no fluid in contact with the sender.

Thanks for posting so we can all learn.

 

This cooling system is very confusing. All I know is once replaced it worked better. Last week I couldn't get the a/c working. The selector for heat/AC/vent/floor thing did nothing. I found out the little plastic vacuum hose had a leak and the vacuum operated swtiches didn't work. Always something new!