Hello, I have a problem with a heat, mechanic replaced thermostate, water pump, radiator, also blowed air out heater core. But problem still there. It gives me cold air, water gauge moves a little from cold toward hot, then drops down ( several times) then it goes to middle and I get hot air and again can drop to cold. Seems mechanic changed everything. Does anyone knows what might be causing this... Thank you

Is your air locked slacken off top hose and make sure your getting a good flow of water no air

The lack of heat in the car has little to do with the car's cooling system, other than the hot coolant is the heat source. 95% of all "no-heat" issues are related to heater controls and/or the heater core itself. Since you get some heat, and then it goes away, this indicates a probable vacuum leak in the heater controls. When you select heat, vacuum is directed to a vacuum motor. This motor operates a vacuum actuator that moves a linkage that moves a door inside the heater box to direct the interior air through the heater core. If this actuator has a leak, it will activate, then as the vacuum leaks off, deactivate. You probably need to have a part replaced in the heater actuator mechanism.

When the van is warmed completely check the top radiator hose by squeezing it together. If you can squeeze it without any resistance replace the radiator cap. The coolant is not circulating properly and can keep the heater from working. If you cannot squeeze the hose together than you may have a bad water pump. The water pump has fins inside of its' case. Those fins may be rusted completely off to where the water cannot circulate. You can check the circulation by feeling the top and bottom hose coming off of the water pump to the radiator. Both those hoses should be the same temp. If not I would consider changing the water pump.

I would try flushing heater core to see if it's plugged. I don't think it's waterpump, your rig would be overheating if it was. Leaking is most common waterpump failure.

try flushing your coolant system. if you have air compressor.remove hoses to heater core and try ro blow it out. also with the car running see if the hoses are the same temp. if not try blowing out the heater core. also start the car and see if the water pump is pushing water out of the hose goimg to the heater core. also a bad radiator cap could keep it from heating up.let the car run and see if the top hose gets hard to where you can not squeeze it. if you can than replace the radiator cap. hope this helps leave comment.

is water flou ower the heater you must let the air out from the radiator

Put your gloved hand on the radiator in the front of your car - if it is hot - then the water in your heater core will be hot. If it is hot in the radiator - and not in the heater core - make sure your water level is up to the top in the radiator itself. 2 - make sure the 'heater' switch inside the car is working. 3 - look under the hood for a shut off valve - after market as some people put these on to make the ac colder on cars that begin to go bad.

Chances are you're low on water in the front radiator -

Also a simple check which is cheap should be done by your regular mechanic. If you don't have a professional Mechanic - get one.

Be careful you dont over heat soon. I posted this in a different web sight I hope it helps you.

Re: Overheating and blowing cold air
I have been a mechanic for 20 years. Your problem is the heater core is plugged. The cold water from the radiator blows directly at the thermostat keeping it closed even when you are overheating! BAD DESIGN!! If you look at the water flow charts in AllData you will understand. If you rev the motor to 3,000 rpm's the turbulent HOT water at the water pump impeller (near the thermostat) will reach the thermostat to open it and the car's temp will fall very fast to the correct temperature. If you remove the thermostat it will not overheat but will run way to cold. If you bypass (loop) the heater core it will not over heat.

The hot water that leaves the heater core returns to the engine at the thermostat. It is the only hot water introduced to the thermostat to keep it open and allow the water to flow from and to the radiator. I normally just back wash the heater core and get large chunks of rust flake out then its fixed. Back wash then froward wash repeatedly many times. Also flush out the motor so you don't re plug the heater core. I hope you have now blown the head gaskets or cracked a head yet. Make sure to run the proper coolant in the car so you don't promote more rust flakes. And bleed the air out as mentioned in detail by other posters.

Nobody knows this little secret. Not even Subaru mechanics at the dealer, no TSB (Technical Service Bulletins a special note after the fact to mechanics). After days and days of trouble shooting a 1993 Subaru Impreza (2.0?) I looked at the water flow chart and then it was clear what was happening. This phantom overheating problem stumps the best mechanics and sends many Subarus to the crusher.

It sounds like your water pump may have gone bad. The water pump pushes the antifreeze through the engine, heater core and Radiator which is regulated by the thermostate. Sorry for the bad news but i hope this helps, and Good luck

I had problems with a similar truck,a 1998 GMC diesel after checking to make sure the control cables were operating the blend door and blowing the mess of the heater core,I replace the thermostats.Still no heat turns out there was a small amount of debris collecting in the fitting for the inlet hose at the thermostat housing after removing it and blowing out the lines the heat was fine.