Over heating heater blows cold air

Asked by Mar 20, 2018 at 08:55 PM about the 2000 Subaru Forester

Question type: Maintenance & Repair

I have a 2000 Subaru Forester brand new radiator
and thermostat car keeps overheating and the
heater blows cold air. Sometimes when the RPMs
are around 3500 the temp will drop to normal
instantly and heater will be hot. Any thought please

15 Answers


heater core


Air in the cooling system.

1 people found this helpful.

I have gotten some air out and I was thinking water pump next then heater core this has been the ONLY mechanical issue I have had with my car


First purge the system by parking on a slight upward incline, or jack up the front of the car. This will allow the rear heater core hoses to purge air as you refill the radiator while idling (keep the AC on to maintain iso-thermality). Squeeze the top rad hose to purge air as you add the last few ounces of coolant mix. Fix the rad cap and fill the (outer) expansion tank 2/3 full. Drive around a while and allow to cool. Add coolant after squeezing air from top rad hose if necessary. Repeat if necessary. If unsuccessful, in that you overheat repeatedly first look for blockage of either rear heater hose by reverse-flushing each hose (don't drain the whole system), as you may have a blocked heater hose (although fairly rare). Are you SURE that the replacement t-stat has the same LARGE orifice size of the old one? This is critical, as cheap aftermarket t-stats are often smaller, and will NOT allow full flow. Most are now Subaru-spec. Compare as necessary. (Note that the culprit here is almost NEVER the OE t-stat, as it's very robust. Why did you replace the radiator? Leakage or overheating? Regardless, if the system cannot be purged, AND the rear heater hoses are clear, AND the t-stat is OE spec, AND your "new" radiator doesn't have a manufacturing fault (blockage...also rare), then have a "percolation" test to check for blown head gaskets. I mention this last, because at this age the $1500 repair cost kills your chariot unless you've deep pockets for this beast. But before getting alarmed check the aforementioned. To perform the percolation test easily WHILE you're purging the system simply add a close-fitting kitchen funnel to the radiator top hole and add coolant while at hot idle (and maybe tilted up on an incline a bit) until the funnel is filled, allowing you a visual "lake" to observe. (Keep the ac fans on to stabilize temp, and hence coolant expansion/contraction volume range.) Since you have a 2000 you can reach back and blip the throttle a bit and watch air bubble up through the cone. If eventually the purging seems complete (may take several minutes), then rev the motor to maybe 2-3k briefly and watch for more bubbling. (BE CAREFUL TO PROTECT YOUR EYES FROM A SUDDEN ERUPTION OF HOT COOLANT!) . If you never seem to be able to finish the bubbling activity...and the motor overheats, vent goes cold, or temp gauge spikes, then, as a last resort, repeat this test with an exhaust gas scanner for hydrocarbons. If positive, that means that the continuous bubbling is hot EXHAUST gases from a blown head gasket rather than simply residual air trapped in the system from the rad/t-stat service. A positive test for HC is definitive for writing a $1500 check or tossing the car. I've seen HUNDREDS of clients describe this scenario where an overheating 2.5i errantly leads to replacing a good t-stat...and then a radiator...before a proper diagnosis of blown head gaskets. But you may get lucky and discover just a blocked heater hose (just ream it out of crud), bad or small t-stat, radiator (unlikely), or a stubborn slow-purger (be patient). Good luck. Oh...to check the new t-stat size take the old one over to the store and compare to a second new one for hole size. The OE (old one) is big and with a chunky spring. Further, you can buy a second and compare their opening temp in a pot of hot water coming to boil on your stove. They should open at the same time. If so then you're old one was ok!

2 people found this helpful.

Note that I posted a version of the above a couple of times over the years. How about a FAQ section, CarGurus?

1 people found this helpful.

Old radiator cracked hats why the replacement I did compare the new Tstat to the old one and they looked the same in size I have been going around in circles so I took it to a mechanic today (normally do my own work) and they asked about the passenger floor being wet and it's not so they were going to pressure test the system. As I said before this is the only actual mechanic issue I have had other than regular maintenance car is very well taken car


Ok...good. You weren't overheating before. So just proceed diligently with the purging sequence. Note that the wet pass floor is probably WATER from a blocked AC drain hose, as the coolant system is almost NEVER leaking from the heater core. Nevertheless, check the carpet to confirm that it's water and not coolant mix just to be sure. But since you didn't have an overheating problem BEFORE the rad relplacement, you're probably dodging a HG bullet and simply need to purge air completely. Be patient and thorough.

Another symptom I forgot to mention after I replaced the Tstat it started over heating faster and more frequently


Yes, but is that because your new t-stat is defective or you weren't purged? Maybe put the old t-stat back in if you cannot purge the system as proscribed....

Got it back last night at 9 30 from the mechanic he got all the air out and car has been running great blowing hot air all the time without over heating however the check engine light popped on for an o2 sensor slow response is that at all related

Also the heater stayed hot but the temp gauge started to read hot for a minute but not all the way on hot then dropped down is this okay


Air pocket still in system. If you're lucky it percolated to the top radiator hose and can be squeezed out as you top off the radiator (again). Do this when COLD. You may have to repeat up to five times with highway runs in between, as some of these can be stubborn.

Can this just work it's self out I have done this a few times it will not over heat anymore but the temp will raise then drop back down on occasion


Maybe it will work itself out and maybe it won't. Purging the air is really not hard work so keep trying. Maybe try a steeper slope and do what the Guru above says.

I don't have a jack at the moment but I have a steep drive way and I've been doing with the front at the highest point while purging

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