Hi,

My wife and I inherited a '98 Grand Marquis from her parents. We've had it nearly two years. Just this past weekend I noticed that the temp gauge wasn't moving from below the 'C' and that the heater just blew cold air. The temp gauge suddenly jumped to the normal operating temp and the heater began blowing hot air. The gauge proceeded to fluctuate quite a bit. I figured it was the thermostat and I replaced it. The problem still exists. The only thing I can figure at this point is that the heater controls on the dash aren't working correctly and allowing water to flow through the heater. It seems that this would cause the engine to overheat but that hasn't happened. The gauge has never registered above the normal operating temperature.

Any ideas?

Thanks!

 

air in the coolant system (add some more coolant), some blockages somewhere along the lines in the system (flush the system). That's usually what I've seen to cause these issues. It's probably time to have the coolant system flushed and filled to clean it out.

 

Thanks Slymer! I had trouble with air in the cooling system on an '89 Jeep I had. I was told that it was a pressurized system and that any air in it would cause problems. Is the cooling system on this Mercury a pressurized system? I hope I'm getting that terminology correct, it has been a few years.

 

check the coolant recovery tank. if it has a pressurized cap, then yes. I think as of 1992 they all had that type of system. you might try purging some air from the system with it warm (like first thing in the morning on a hot day or with the temp gauge reading just on the high side of the cold mark on the dash). this will keep it from being fully pressurized and prevent getting sprayed when releasing the pressure from the system. then top off the system with coolant. replace the cap. next time it does a full temp cycle, it will help purge any excess air from the system. if the coolant level in the tank is not low, then there's probably goop in the system keeping the coolant from flowing properly and needs to be flushed. You may also be having issues with the heater flow control valve, but I'm thinking there's just not enough coolant in the system.

 

Slymer- You were correct, the coolant was low. I feel like a knucklehead for not checking the level first and then not even checking it after I replaced the thermostat! Thanks again.

 

Happen to me once. It ended up being the thermostat was stuck open.