We have some heating problems with our Toyota Previa. The heat only works sometimes - only when we are driving and not when we are stopped. And only sometimes when we are driving. Even if we are idling there is no heat. Also Our air conditioning has not worked for the last year so I don't know if that can effect it as well. Any help on what to look into would be greatly appreciated. I already have a Previa repair manual so if someone could point me in the right direction that would be great Cheers Blair Mac
What does the coolant temperature gauge on the instrument cluster show? If it shows low, then the thermostat may be stuck open. Check the engine cooling section of the repair manual.
If the coolant temperature gauge shows normal temperatures, then the heater control valve may not be opening because it is blocked or the control wire is disconnected or broken, or the heater core may be clogged, or the damper control wire may be disconnected or broken. Check the HVAC section in the repair manual.
Most likely cause is the thermostat is stuck open. Bet the engine temp fluctuates quite a bit too.
MIGHT be the water pump impeller has corroded away as well.
May just be low coolant level. Could be the main rad hoses or heater hoses are soft and restricting flow.
Separate problem, commonly caused by the O-rings in the a/c hose fittings leaking. May be the hoses themsleves. Get a quote on refurbing your system, converting to r143a if required. Sit down befoe you get the numbers...
Your starting point is a new set of hoses and clamps, a thermostat, and a flush / refill of the whole cooling system.
I'm writing in generalizations because I'm not terribly familiar with the Previa van but here are some generalizations which may help you diagnose the problem.
Your heater core is a mini-radiator behind the dash. Hot coolant passes from the engine through the heater core; the heater fan blows air through the fins which warms it. There's a valve, activated by the dash "cold-hot" lever through a cable, which opens to let coolant flow through the core.
There are three things that can go wrong with this system: The coolant flowing through the core isn't itself hot, the coolant is not flowing through the core, or the air to be warmed is not passing through the core.
The most potentially damaging is the first, as it likely means that your engine is not running at the correct temperature. If the temperature gauge is reading low you may have a thermostat stuck in the open position or perhaps completely removed by a previous owner / inept mechanic.
The coolant may not be flowing through the core properly. Find the valve - look at the firewall and there will be two hoses (about the size of a garden hosepipe) coming from the dash. One will go to somewhere on the engine; the other will go to a device on the firewall - that's your valve. See if you can manipulate it to the open position manually and check whether that solves your problem. If so it's just a matter of adjusting or replacing the cable control. If not, you can bypass the valve altogether. If you get heat the problem is with the valve itself.
Still no heat? The core itself may be clogged. You can try removing the hoses from the dash and running water through the heater core through a hosepipe (this all from the firewall side of the engine compartment). Don't use a lot of pressure; just see if a stream of water flows through the core. If so, your problem likely lies elsewhere. If not, you'll have to replace the heater core.
Finally, it's possible that the hoses going to the core itself are clogged. I have not personally seen this happen but it is a possibility.
If it appears that hot coolant is flowing through the core, your problem may be with airflow. The core's fins are fine and can become clogged with hair, dust bunnies etc., particularly if you have pets. I replaced an evaporator (the air conditioning equivalent of the heater core) in my 95 Tacoma last year and was dismayed at how much cat hair had accumulated, simply from being carried into the truck from my clothes.
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