I have changed the Heater Core because it was leaking. After completion, started vehicle and waited for the heat to blow out of the vent. 7 minutes later -- good hot air pouring out of vents. Get in car and start to drive, Water temp begins to fall and goes all the way down to 100 degrees on the gauge. No heat at all coming out of vents. Decide that thermostat has stuck open for some reason. Go home and change thermostat. Start vehicle up and let it idle in driveway -- 7 minutes later you got it hot air pouring out of vents. Take car out for test drive and guess what, Water temp again falls to 100 degrees on the gauge and no hot air coming our of vents. So I check water level and it is all full. Try to purge all air out of system again and decide that maybe Radiator cap is not allowing system to pressurize correctly. Go by new CAP. Idle in drive way and wala-- 7 minutes later hot air pouring out of vents and temp rock steady at 190 degrees. Take car out and drive it and you guessed it , temp fall back to 100 degrees on the water temp gauge and no hot air at all. In the FSM it says that a Overcooling problem is cause by a faulty thermostat, since I have replaced it with the best possible thermostat that can be bought I don't think that is the problem. Has anyone ever seen or heard of this type of problem before?

Any help with this would be greatly appreciated.

Its a 1990 and I have had no problem with the vehicle except that I changed all the fuel injector out about 4 months ago. I hope its not going to start falling apart piece by piece now.

 

You may have the thermostat installed upside down.
Are the spring coils facing up or down when the thermostat is installed?

 

I don't think you can do anything to get a warmed up engine to run at 100 degrees. Temp sensor issue?

 

the thermostat is definitely installed correctly with the springs facing down just like the FSM illustrates. This is the wierdest thing I have ever seen. As the water temp goes down the heat disappears from the vents inside the vehicle. The other thing I forgot to mention is that the Temperatures for both the Water temp and the Oil temp both come up to proper operating temperature while the car is parked and idling. But when you start to drive both temperatures on both gauge decrease to the 100 degree indications.

 

This is what I was thinking also. The oil temperature usually lags or changes slower than the water temperature (up or down). I was wondering how fast the oil temp drops in temp because it should not track the water temperature. Both fluids have different specific gravities.

I would almost say it was a weird electrical problem if it wasn't for the drop in heat out of the vents in the car. What I would do is get an inferred thermometer measurement unit and take some reading that way to get an independent source of information. Take reading when hot and then when you think it is cold. If you have that kind of temperature difference, you should be able to grab a heater hose and even feel that kind of difference. You almost can't hold a 200* hose, but you can hold a 100* hose easily.

Where do you live and what are the outside temperatures?

What temperature stat are you using?

If this is really happening, send some of the coolness to Florida.

 

Just a guess, I'm thinking the heater stops blowing because the temp indicates it to do so. Something is obviously sending false readings to the gauges as there's no way your dropping real oil/water temps while underway.

 

Easy to find out, just feel the heater or radiator hose at the right times.

 

I'm going to cast my vote that your new thermostat is bad. I've had brand new defective ones before. A faulty CTS for the ECM should not cause cold air to blow out of the vents, nor will a faulty sending unit for the gauge. Your L98 will have an oil cooler on it, which flows coolant down to the filter area, and if your coolant is at too low of a temperature then that's going to affect the oil temp.

Do as was mentioned, and when the gauges show they are cooling off, touch the upper radiator hose to see how hot it is. If you've had the car running and driven it, and it's mildly warm to the touch, then your thermostat is not functioning correctly. If it's hot and you cannot touch them for more than a second or two, then you have another issue at hand.

You can also have the car idling and feel the upper hose as well, if it warms up as the gauge warms up, that's another indicator of a faulty thermostat.

Just my thoughts, hope you get it figured out.

Oh, is the heater itself shutting off, or is it blowing cold air with heat selected?

 

Ok

I am in Virginia beach and it is cold here. Average daily temp is around 32 to 40 degrees. The top heater hose and the bottom radiator hose are definitely hot as are the hoses in to and out of the heater core when the car is idling. The oil temp goes down much slower than the water temp but both eventually end up just barely off of the 100 degree mark on the gauge. I drove from Naval Air Station Oceana to Home this afternoon with a stop at the WAWA gas station. When I left the gas station hot air coming out of the vents. Its a 30 Mile drive home. Get on the interstate and with in 5 miles of travel at 65 mph, temp back down to about 110 degrees on the gauges and no more heat out of the vents. Get home let car idle in driveway and water temp increase back up to 180 degrees and the oil temp recovers to normal operating range. The Fan unit continues to blow air out of the vents at all time good air when it is idling and then it gets colder as you drive the vehicle. It is almost as if the coolant is somehow not moving around the system as it was designed to do. But the car still runs great

 

Really sounds like a defective thermostat to me.

 

Has anyone looked at the vent controls and the heater control valve

 

Since it worked before you changed the heater core. This is just a thought, could the hoses be installed backwards on the core?

 

The hoses should be a different diameter at the heater core, just to prevent that kind of thing from happening.

 

No. The heater hoses are definitely installed correctly. There is only 1 way to install them since they are different diameter. Does anyone know if there is a purge screw on this system that is used to bleed trapped air in the system out after you drain and refill the system. I have heard that maybe air is trapped in the system under pressure but I can't find anything in the FSM to indicate how to get it out. Should I get the system flushed at a garage that has one of those machines? I know the 1992 model has this feature on top of the thermostat housing but the 1990 I cant find one anywhere.

 

You clearly have coolant somehow bypassing the thermostat, even if the thermostat is proven working properly. There is supposed to be an O ring that seals the flange of the thermostat. Your engine heats up under idle rpm because the water pump cannot circulate coolant fast enough to attain 100 degree coolant, but it will when you drive. Your coolant will drop to 100 degrees if you can force your electric fan on and increase the idle speed! You need to look carefully if the thermostat is truely blocking the coolant flow. L98 thermostats should not be installed in LT1 engines, make double sure you have the correct thermostat.
Proof that your gauges are reading correct, force the radiator electric fan on after idling up to 190. During idle the coolant temp should remain close to the opening temp of the thermostat with the fan on. ln fact, I would just let the car idle (no forced fan on)and see what happens to the temp. What should normally happen is that the coolant temp will continue to rise until it gets to 228F, the rad fan will come on and the coolant temp will drop back to about 210 F and the rad fan will go off and this will repeat.

 

Before you changed the heater core, the temperatures were normal, Correct? The new core could be more efficient but to lower the coolant temperature to 100 degrees? The low ambient temperatures could possibly if enough air flows through the radiator. Are your cooling fans running? They shouldn't be. If the thermostat is stuck open the coolant temperature will be around 150/160 degrees. Maybe a wire is off a sensor, the system is failed safe to provide extra cooling.

 

Thanks for all the advice. I will do all of these recommendations and see what happens. I have even considered taking everything in the engine compartment( hoses and radiator out and checking everything). I agree it seems to be in some type if fail safe mode. I just don't know where to begin.


Freeze

 

Bad circulation---> water pump
Air in the system---->heater core

 

If you have a 1990 model it's the L98 which has a conventional coolant flow syatem; the LTx engines have reverse flow. You mention the 92 bleeder valves and the FSM. Are you sure you're following the illustration from a 1990 FSM? The FSMs are model year specific.

 

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