1998 Subaru Forester-Thermostat

I have come here as my last resort. Recently, my car had started having cooling issues. It was running hot, into the danger zone, and it would not blow hot air. I took action and replaced the thermostat myself, drained the coolant, and bled out any air bubbles, filled it up with coolant and the problems still persisted.

Please help!!!


Head gasket.

With it being Subaru +1 on the head gasket.

Or water pump.

New development. When the car is in park and heats to normal temperture the heat does blow hot. I rev the engine and the temperture needle does not move. Once I start driving the car that is when it heats into the danger zone and no heat coming through.

How would a faulty head gasket explain the no passenger compartment heat? It would explain the overheating possibly, but I don’t see the relationship to the inside heater. It seems like the inside heater air would be extra hot, if anything, b/c the coolant is extra hot.

If the engine is overheating, there’s several possibilities for that. Head gasket, engine cooling fans malfunctioning, water pump, collapsed or plugged hoses, faulty thermostat, radiator cap, air bubbles, temp is actually ok but the gauge is wrong, etc.

If the heater isn’t working, there’s several possibilities for that, heater controls faulty and not allowing coolant to flow through the heat exchanger, the blower fan not blowing air across the heat exchanger, vent flaps not working, heat exchanger is clogged, etc.

So given what you’ve already done, you know the heater works fine at idle, so what’s in common? Somehow while there’s plenty of hot coolant available, at highway speed it isn’t going to the heater … hmmm … that’s a tough one. I’m thinking it has something to do with an air bubble still remaining in the coolant , or some kind of weird pressure distribution within the cooling system, which might be due to a faulty water pump or a failed radiator cap.

I might just replace the radiator cap, try that as a flyer. Since it is inexpensive to do.

Still a no go? There should be a large volume flow of coolant into the top of the radiator when the engine is warmed up, and it should flow even faster at higher rpms. Does it?

Still nothing to go on? Test the amount of flow through the heater core by opening the two hoses. Plenty going in? Plenty coming out?

If all that experimenting doesn’t work, I’d ask a shop to do a pressure test of the cooling system, and check the head gasket.

When I had head gasket problems, I would get no heat at low RPM but once driving, the water pump would over-come the air bubble and fill the heater core again. Of course then it would go through the temp gauge spiking and then returning to normal. On the last trip, I finally had to loosen the radiator cap to drive the final 30 miles home. Any time you get no heat and spiking temps, you gotta suspect either low coolant or head gasket. Plus correct me if I’m wrong but in the old days thermostats used to stick closed as they aged, but now have the tendency to stay open. So bad thermostats are more likely to result in a cooler temp than over-heating.

Ok I think I see what you mean Bing. The bad head gasket may be introducing an air bubble into an otherwise air-bubble free cooling system. Maybe in the OP’s case the air bubble doesn’t appear until the high speed driving starts, and somehow it disappears or goes somewhere innocuous when returning to idle.

Edit: OP could look at the coolant in the radiator maybe, and see if there are any signs of air bubbles popping up to the surface. That could indicate a bad head gasket.

A bad head gasket will blow gasses into the cooling system from the combustion chamber during the combustion/power stroke and will draw coolant from the cooling system on the intake stroke. That creates gas pockets in he system as well as heating the coolant with the combustion gasses. The effect is highly magnified at speed over idle because the explosions are a lot hotter and more intense.

This description has all the aspects of a blown head gasket.

My recommendation is to

  1. check the coolant visually. If it’s cloudy and murky, you probably have a blown head gasket.
  2. if when cool bubbles come up out the radiator fill hole when the engine is running, you probably have a blown head gasket. The bubbles will be the combustion gasses migrating to the system’s highest point… the fill hole.
  3. get a test kit to check the coolant for the presence of hydrocarbons. It’s a cheap lab test kit, available at parts stores.
  4. if you’re still unsure, do a pressure leakdown test on the engine. Again, a kit with instructions is available at eth parts store.

If all of these tests are negative, congratulate yourself on having dodged a bullet and begin looking at
a) radiator flow
b) even heat dissipation by the radiator (easily checked with an infrared thermometer)
c) water pump flow
d) replace hoses (inner linings can collapse and bloc flow)

My guess is that once you solve the engine cooling problems your heating system problems will disappear too.

Oh, and when you purged the air out of the system, did you do so with the heater turned ON? If not, you won’t purge the air from the heater core.

A few questions.
While driving does it overheat during in town driving, highway driving, or both?

Have you checked radiator cooling fan operation; both as to engine temperature operation and during A/C operation?

Subaru has had their head gasket issues but I’m not quite ready to condemn them just yet.

Ok4450…driving in town the car is fine heat blowing and everything. Once I get into the 45 50 mph range is when it begins heating up and the air turns cold. The real odd think is that I can sit in park and rev the engine high for as long as I would like and there is no increase in temperture into the danger zone.

I am going to run through the tests suggested above and see what the answers are.

Offhand, sounds like the radiator is clogged up or possibly a cooling fan issue.

At idle and with the A/C off one of the fans should cycle on and off when the temp climbs up to about 200 degrees.
With the A/C compressor engaged both fans should run all the time.

If fan operation is ok then a new radiator might be in order.

The latest description sounds like a radiator cap to me. Its cheap so its worth a try.

A chemical test kit can detect unburned hydrocarbons in the coolant, if present. So can an emissions sniffer. It would be a quick and inexpensive way to determine if head gasket failure is a possibility.

If you buy a radiator cap, ask about the test kit while you’re at the store. You could save yourself a trip to the store, and a lot of messing around with coolant, hoses, spark plugs, etc., if you find out the easy way if you do or do not have unburned hydrocarbons in the coolant.

Yup good luck with only a cap or radiator. I believe a head gasket will give a spike in temp not a more gradual increase in temp. So into the hot zone very quickly, then back down to normal again. At least in my experience. I suppose once you are driving, putting the extra load on the engine might cause the over-heating where it wouldn’t at idle. Best just have someone look at it after trying the new cap. They can use a temperature reader to check the temp at various places on the radiator to see if there are hot or cold spots.