MAZDA MPV 2001 no heat from rear heater?

Answer 1

Found solution on this site-> http://forums.mazdaworld.org/index.php?showtopic=18013

Have you changed your coolant by draining the system with the radiator drain plug and the drain plug for the rear heater (a fitting under the van), then refilled with new coolant only to find you can't get any heat out of the rear heater? If you get the engine up to temperature and go around to the right rear wheel and feel the two hoses (actually insulated metal tubes) above the wheel, you will probably find they are cold because you now have an airlock in the rear heater coolant path.

The workshop manual suggests you can cure this by getting the engine up to temperature and then running the engine at 4000rpm for 1 minute, idle for 1 minute, then keep repeating this cycle until happy. THIS DOES NOT WORK unless you are patient enough to go through many cycles.

First some basics. When the engine is up to temperature and the thermostat is open, the output of the water pump sees 4 parallel circuits; the radiator, the engine circulation, the front heater core and the rear heater core. There are no valves closing the two heater core paths, so coolant is flowing through them all the time regardless of the cabin heat setting. What you have to do is increase the pressure and flow to the rear heater to get rid of the air lock, and you can do this by closing off some of the other parallel loops. Start with the engine cold, the closed thermostat will then block circulation to the radiator. Now get a pair of needle-nose vise grips and reach over the right side of the engine (passenger side) to where the two rubber hoses go through the firewall to the front heater core and pinch one of them off. They are about 1" outside diameter and are rubber, DO NOT pinch any metal refrigerant lines. If you now run the engine at moderate speed, say 2000rpm for a few minutes, it should get rid of the air lock. Recheck the hoses above the right rear wheel, if they are reasonably hot (remember they are insulated), you should now be getting heat out of the rear heater and you should have to top up the coolant recovery tank when the system is cold.

The rear heater and tubing takes almost 2 liters of coolant so you could end up with a lot of air trapped in the rear circuit. If the above method still doesn't give success, you might have to remove the plastic trim panel from the rear of the inside of the van to gain access to the heater unit and then burp air out of the coolant hoses where they attach to the heater core. Do this by sliding the hose clamp back then CAREFULLY and slowly easing the hose off the heater core fitting to let air out. As soon as coolant appears, push the hose back on and reclamp it. If you are careless, you could end up with a lot of coolant in the back of your van.

there is an easier way... you clamp one of the heater hoses going to the back, under the hood and run the van. It doesn't take long and you'll get heat. I know since I've done tons of those rear pipe