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Heater problems Toyota Rav4

Heater works when outside temp is 20 and up but is poor when it is colder. Replaced thermostat, flushed everything. Checked all the controls and such, looks good. after 30 min of driving this am still just warm heat, got out and felt upper and lower rad hoses, bottom was cold, top was warm but not hot, somethings not right. Temp guage shows about half within 10 min of driving but does not seem to be telling the truth. It will eventually warm up but takes a long time. Acts like there is no thermostat bu there is, it is new. The new one seems to work no differant than the old one. HELP<

Has the coolant system been checked for a head gasket leak which might cause air pockets in the coolant system?

Um, no. I would think i would have other issues if that were the case though? Water in oil, oil in water, poor running, something? The vehicle runs great, no oil burn, no smoke, gets 29 mpg at our elevation.

You either have a big pocket of air in the heater core or a partially plugged heater core.

Raise the front of the car as much as you can. Use a hill,ramps or jack to get the radiator cap as the highest point. Take off the radiator cap (cold) and squeeze or fles the heater hose and radiator hoses to get the air out, Refill w 50/50 mix.

Thats a can do, a little more info for your pleasure. If i run the car, no rad cap, i can pull the heater core lines off and there is a trickle flow in or out. I flushed core several times on several occasions because everbody says core is the problem. flow through the core is not the issue but air may be. The cores on these vehicles are somewhat higher than the rad

Oh, thanks for your help, i feel i am a pretty decent mechanic but this ones is killing me.

Have you considered blocking part of the radiator with cardboard to see if that helps with the heat?


I did that this morning, the cardboard was about 8x18, i put it in from of the top hose.

Did not seem to make any difference.

Perhaps you need a new water pump? This doesn’t make any sense b/c you should overheat eventually if the pump isn’t working. But if you have the car running and pull the inlet hose to the heater core you should get more than a trickle - unless I read that wrong. You’re lower radiator hose should not stay cold either.

I am with you on this one, if the pump was weak, you would think i would have the opposite issue. That and pump with steel fins dont usually go bad, they start leaking. I think maybe i may have air in the core that will not come out, therefore little flow. but i dont know, i am at a loss. I just parked it on a hill to get the core lower than the rad, maybe i can push the air out.

If you get the radiator neck to be the highest point (hill, ramps, jacks) you should just be able to run it with the cap off to purge the air.

The thing is though, that you still shouldn’t have a cold lower radiator hose - unless when you said you got out and checked the radiator hoses you meant that you checked the heater hoses. A cold heater hose makes sense if your problem is air in the core. A cold lower radiator hose doesn’t.

That is what i am trying to do with the hill, so we will see.

On the lower hose, i do mean rad hose to the thermostat and engine.
It seems like it does not want to come up to temp and open the ther but i have seen it come to temp and open the therm on other occasions. I used one of the laser temp guages

Occasionally a radiator hose can have an internal problem. It may look OK on the outside buy not the inside. The only way I know to check it is start taking things apart.

What would you think the result of that would be?
I just went out and pulled the rad cap off and ran it for 15 min, did not see much air come out, i am parked on a pretty good hill, i live in colorado, not hard to find hills, LOL
Again, thanks for the help.

Have you verified that the new thermostat is working properly?

Easy to check. Boil some water and drop the thermostat in the pot. Does it open? Does it open all the way?

I learned my lesson on that years ago, checked it before i put it in, i does open and close, all the way.

While thinking about this issue another thought came to mind. Maybe the trouble isn’t with the heater system, but with the vent system. How well does the heat work in the defrost mode?

I am torn with all of this, one minute i think it is cooling related and then something else.

It is all mechanical flapper doors, seems to be operating correctly. What does not make since is that when it is very cold outside the heat sucks but when it is above 20 works great just takes a little while to heat up. Above 30 no issues.

Well if the trouble doesn’t seem to be with a certain area you have to think it may be elsewhere. Perhaps a vent door is getting stuck below a certain temperature. I still would like to know how well the defrost mode works.