99 300m heater blown cool air - Cars & Trucks

Make sure no fuses are blown. It could be the actuating motor that controls the door the mixes the warm and cool air. This is usually located next to the heater core.

suspect a blown hose, heater hose probably behind the engine. now when you fill it back up you have to bleed the air out of the system and the bleeder valve is usually a brass screw close to the area of the thermostat... find the leak first and foremost or catastrophic engine damage can and will occur

does the engine run smoothly or do you detect a slight miss in it..if air is getting in to the cooling system its normally a blown head gasket..or cracked housing somewhere..

If you have a blown head gasket, surely you will overheat because some hot combustion gases are entering your cooling system. No way you can remedy it by bleeding the system. Bleeding the system is just removing the trapped air in it. Running the engine with out the radiator cap will do the trick.

Hi James,

First, I guess you realize that the heater is part of your engine cooling system. Then it also has its own parts inside the vehicle that have absolutely nothing to do with the engine cooling system.

The trick to getting your heater working is to first determine which part of the heater system has the problem.
First, you need to verify proper engine cooling system operation. There is more to this than just replacing a thermostat. If you have a problem like a blown head gasket, you r heater will not work. What SHOULD be done is to operate the engine while watching the computer data for the engine coolant temperature sensor (ECT). Doing This, you can verify that the engine is warming up to operating temperature. You can also verify that the thermostat is opening at the correct temperature. Then take an infrared thermometer and take a manual reading from the engine near the temperature sensor. The reading you get from the thermometer should be within about 5 degrees of the reading that the computer is getting from the ECT. The engine cooling fans should come on between 220 and 228 Deg. F. When they do come on, the air that is blowing through the radiator should be HOT. If it is not, this is an indication that the radiator is stopped up, the water pump is not pumping correctly, or there is a blown head gasket that is filling your cooling system with combustion gasses. This can be done with just the thermometer, but it is best to also verify the ECT computer data.

If all of this is working properly, then it is time to look INSIDE the car for the problem. The blend-air door in your A/C-heater housing may not be working. When you switch the temperature control inside the car, you should be able to HEAR a change in the air flow. This applies to ALL vehicles, whether they are equipped with cable, vacuum, or electronically controlled blend air doors. If you cannot hear a difference in the sound of the air moving through the A/C-Heater housing, then chances are pretty good that the problem is with your blend-air door. If this seems to be the problem, let me know and I can give you further instructions on how to check this and/or repair the problem.

P.S. Just thought I'd mention that a pretty good infrared thermometer can be purchased at any parts store for around $40.....This is a very good investment because you can use it for many other things and is a lot cheaper than an hour of diagnostic time at most repair shops.

First check and make sure that your coolant(antifreeze) level is full, the hot coolant runs through the heater core and than air is blown over the heater core heating the air for the heater. if the coolant is full you than need to check the thermostat and make sure it is functioning properly. finally if both of those are not the problem it is possible that your heater core is bad, however if the heater core is bad you will notice coolant leaking onto the passenger side floor. so if the floor is dry I would say the heater core is not the problem

Check heater valve selonoid by removing electrical connection and then remove the 2 lower hoses.Then use a garden hose and water in one of the bottom hose connection.If water comes out the other end, it's good. If good flush out heater core.Make sure the engine is cool and not hot before opening up cooling system. Had same problem in 95 xj12 there was air in the heater core.they call it air lock.

the cheapest way is to replace your thermostat and flush your system.it might have a restriction in the cooling system.good luck

thermostat stuck??

sounds like a blown head gasket to me.  Does the coolant smell like exhaust?