91 LX 5.0 Automatic A/C
Stock, no mods.
Highway speeds.

For the first ten minutes, I have no heat and the temperature hits the "L" on "Normal" (about 240 I guess). The heat willkick inand the temperature will drop (maybe 210). I'll lose heat and it will climb again. This happens about 5 times. Eventually the heat stays on and the temperature drops and settles in at the thermostat temperature (180). If I run the car immediately after bleeding the system (cap off, thermostat opens, fill radiator), there is no problem.

I believe I have air in the system, at the thermostat/ heater by pass, that eventually works its way out. The problem is it finds its way back in and this becomes standard procedure. Thermostat is good, radiator is new (2 row G core), pressuretested cap and system (engine off). No dampness on the passenger floor (heater core doesn'tappear to be leaking). No smoke, no noticeable loss of power,no coolant in the oil (head gaskets ok, I hope). Radiator hoses do not have springs- any sources for hoses with springs?

There is coolant in the reservoir (it fills ok when hot)- I top it off after shutting the car off to be sure the system has enough reserve coolant to suck back inafter the pressure drops. It just seems that it is taking in air from someplace else instead of coolant from the reservoir. No visible coolant leaks that I can find so far.

Any ideas would be greatly appreciated.

Thanks
Bryan

 

I had a similar problem and I put a cooler thermostat in it and it fixed it.

 

What makes you think the t-stat is good?

 

take it out put it in boiling water to test it.

 

Thanks for the feedback folks.

I've tried 2, maybe 3 newthermostats, and I did test one (can't say it is the one in there). I have ruled out the thermostat because it does regulate at the set temperature correctly eventually, and correctly from the start if I bleed the system from the start. It is installed in the right direction too (spring in the motor). If the thermostat was bad, I would still have heat.

I think it is air in the system because the heater isn't hot when the engine is.There is a bypass hose, so when the motor starts running hot, I should have heat- not just when the thermostat is open. I used to get heat before the motor reached the thermostat temperature.

I justimagine an air pocket under the thermostat and bypass hose, so no hot coolant circulates through the heater core and no hot coolant reaches the thermostat to trigger it- until it gets very hot and the pressure builds. Some circulates, the pressure drops, coolant level drops, I lose heat, thermostat closes. Heat builds, pressure rises, I get heat, thermostat opens, release pressure,level drops, no heat, thermostat closes. Until eventually the air is worked out and the heat stays and the thermostat is in full contact with the coolant and able to regulate properly.

If something else is happening, I'm open to ideas- I may be way off. I'll switch out the thermostat again. Hoses have at least 100k on them (car has 168k)- does anyone think a small hoseleak (or several)might contribute to air seeping in? Should I look for other problems?

Thanks again.

Bryan

 

Run the car without a tstat and see if the problems keep happening? If they do then that rules out air causing ur tstat not to open.

Just a thought.

 

I have the same problem, I run a 180 stat and i have a new water pump. Mechanic said underdrive pulleys can cause them to run warmer. My next step is to install a hi flow water pump and a 160 stat. And I was thinking of an electic fan, just to free up some extra horsepower.

 

A 160 stat is too cold for your car. Stay w/ the 180

 

The thermostats I've had in there are the 192, 180 and 160. No difference in the problem except for the temperature at which the motor will "settle in" to after the initial peaks.

Without a thermostat themotor stays cool, gets to maybe 150,and the heater works. Motor doesn't get hot at all, which is what I would expect.

The water pump isn't showing any leaks, but that might be my next part to try, along with the hoses.

Thanks for the help, this is my last stop before I just start replacing system parts in the hopes of accidentally nailing it.

 

drill a small pinhole in the tstat. allow it to breathe : )