My parent’s 1998 Town Car seems to be overheating and venting coolant.
The general behavior is this: While driving from a cold start, the temperature gauge gradually rises to the full H mark. A couple of minutes after reaching H, the temperature gauge drops back down, and holds slightly below midrange for the rest of the drive. There is significant coolant loss, all of it appearing to vent from the coolant cap on the degas bottle. A recent drive of approximately 6.5 miles resulted in about a quart and a half of coolant venting from the pressure cap. The thermostat and water pump were replaced, but this had no effect. This behavior occurs when driving. Tests where the engine was left to idle for an extended period did not result in venting.
I performed the pinpoint test in the shop manual for engine overheating. After 4 minutes of idling, I felt the inlet and outlet heater core hoses, near the firewall, and one was noticeably warmer than the other. This results indicated a failed heater core. As I understand it, the heater core is responsible for cabin heat. Why would this result in overheating, and what else should I check?
One possibility is a large air pocket in the cooling system. I would say this is the most likely explanation for all the symptoms mentioned. With the engine cold, make sure the coolant level is at least up to the mark on the reservoir. Check again after a cycle of fully warming up and fully cooling down, and top up as needed. Repeat until the level stays high enough.
A clogged heater core will not affect engine cooling. An air pocket will affect flow and often the heater core is the first component to lose coolant flow.
Usually I don’t get heat until after the temperature spike. When I drove it last night, I had heat soon after starting, then lost heat as the temperature gauge rose toward H, then it came back after it dropped back down.
I have done some more checking. About an hour after I posted, I ran the engine again for 10 minutes. The heater hoses were close in temperature, so either I misinterpreted the temperature difference or the engine did not run long enough. I also had cabin heat. The upper radiator hose was cold, however.
Several hours later, I drove the .5 miles to the drugstore up the street and went in to buy more antifreeze. I then drove back to my house, total distance 0.9 miles. I felt the upper radiator hose, which was cold. No coolant had leaked from the cap. I then drove around the neighborhood, for a distance of 2.4 miles, when I lost heat. When I pulled over a few moments later, at 2.6 miles, I checked the engine again. There was still no coolant leakage from the cap and the upper radiator hose was still cool. While idling for about 2 minutes, the temperature gauge continued to rise and eventually reached H. I observed coolant begin to flow from the cap and felt the upper radiator hose become warm, as the temperature gauge returned to its normal range, which occurred 8-10 minutes after I pulled over. I drove back to my house, for a total distance of 3 miles. I was unable to tell if more coolant had flowed from the cap. I had driven this 3 mile circuit about a week ago, before my dad replaced the water pump, with the same result. The car also overheated at about the same location. I didn’t have the heat on at that time.
Sounds like an air pocket or possibly a lower rad hose that is sucking flat, and not allowing coolant to enter the engine. Feel the lower rad hose, if you can squish it flat with your hand with out too much resistance, replace it.
I felt the lower radiator hose, as Rick suggested, and it was easy to squeeze, at least easier than the upper hose. I will replace it tomorrow and report back on the results.
When I found out that a replacement hose is $50, probably because the hose has a permanent T fitting and can’t be cut from a bulk length, I decided to work on the air pocket assumption again, before buying a hose. I had already drained the system and removed the lower hose, so I removed the upper hose and degas bottle too, and rinsed them. I flushed some water through the radiator, since it was open anyway, and I checked the thermostat with boiling coolant, which opened properly. I put everything back together and refilled, running for 5-10 minutes at a time and checking the level as air bled out, and everything seems fine now. Thank you for all of your help.
If the engine is being starved for coolant, eventually bad and expensive things will happen. While not familiar with your specific vehicle, a $50 hose replaced now can be money saved when measured against engine damage.
A closer visual inspection of the hose after I removed it showed no cracking or other wear, and observing the hose in the daylight while the engine was running did not show any pinch points or other signs of hose collapse. I drove it tonight for a total of 13 miles with no problems, which would have been impossible earlier.
I, and the other people who drive it, will of course continue to watch for any more issues. We’re very observant of engine temp problems in general, after my mom destroyed the engine in my dad’s Altima because she didn’t notice anything after the upper radiator hose broke off the radiator while she was driving.