Tip & How-To about Heating & Cooling

Why heater dump valve causes car to over-heat when heat is on!

To properly diagnose your overheating problem we need to rule out some things.

First: Is there enough coolant/antifreeze in the radiator? Don't just look inside the plastic overflow bottle, but remove the radiator cap (when the engine is cold) and look inside the radiator. You should be able to physically see the fluid level if it is at its proper level.


Most cars and trucks will hold 1 1/2-2 gallons of coolant and water mixture. If you have to add more than a pint of fluid you should have the cooling system pressure tested for a leak. If you see any obvious fluid loss on the ground or in the engine compartment, you should also have the system tested for leaks.

Second: If no coolant leak or low fluid level is present, then determine when the overheating complaint occurs.
If the engine overheats while at a stop or idle only:
Most front wheel drive cars use an electric cooling fan motor located in front or behind the radiator.


The function of the cooling fan is to improve airflow across the radiator at stops and low speeds. The fan is controlled by sensors that regulate the engine temperature and additional load that might be placed on the engine.

The air conditioning compressor will require the cooling fan to operate at idle as long as the compressor is on. A quick way to check the cooling fan operation is to turn on the air conditioner. The cooling fan should come on with the air conditioner compressor.

Some cars will have two electric fans, one is for the radiator and the other is the air conditioner condenser fan. Usually the radiator fan is closer to the middle of the radiator. The radiator fan is responsible for engine cooling, and the condenser fan is responsible for increasing air conditioning efficiency at idle and low speed.

If your vehicle does not have an electric cooling fan on the radiator it will have a belt driven fan blade and fan clutch. This fan should be pulling a large amount of warm to hot air across the radiator onto the engine.

What you want to determine with either fan situation is that there is ample airflow across the radiator at idle. The radiator is the primary heat exchange for the engine, and airflow is crucial.

What if the engine overheats while at high speeds on the freeway?
Again, airflow and coolant circulation are crucial. At 55 MPH we can assume you have ample airflow across the radiator, so proper antifreeze circulation is the thing to inspect. I compare overheating at 55MPH to jogging with a sock in your mouth.

The faster and longer you jog, the more air you are going to require, and with a sock in your mouth you are going to have to breath extra hard to maintain the proper amount of air to keep you going. At 55MPH the water pump is pumping a large amount of hot antifreeze throughout the cooling system.

If there is a restriction in the system like a kinked radiator hose, a restricted radiator, or a stuck thermostat, it will produce the same affect as the sock in the mouth scenario.


Rust and water calcification can accumulate in the radiator and drastically reduce the flow of coolant at high speeds. Removing the radiator from the vehicle for disassembly and cleaning or radiator replacement are the only two real cures for a clogged radiator.

Using a can of "radiator flush" additive might help as preventive maintenance, but will probably just be a waste of time and money trying to correct a restricted radiator.


Hope this helps, best regards.

Posted by on

Heating & Cooling Logo

Related Topics:

Related Questions:

1 Answer

temp gauge light come on stating hot temp coolant, no steam from radiator, no light runs fine if heat is not on


Your engine is either overheating or your light is malfunctioning. The coolant may not necessarily "steam" from the radiator because the engine is overheating. The purpose of the light is to warn you of an overheating condition before it gets bad enough to cause your coolant to blow out the radiator cap. Severe engine damage can result from overheating. I would recommend having this problem diagnosed by a qualified technician that has the proper equipment required to to test and verify that your cooling system is functioning properly.

Jan 22, 2011 | 2001 Pontiac Aztek

1 Answer

To properly diagnose your overheating problem we need to rule out some things. First: Is there enough coolant/antifreeze in the radiator? Don't just look inside the plastic overflow bottle, but remove the radiator cap (when the engine is cold) and look inside the radiator. You should be able to physically see the fluid level if it is at its proper level. Most cars and trucks will hold 1 1/2-2 gallons of coolant and water mixture. If you have to add more than a pint of fluid you should have the cooling system pressure tested for a leak. If you see any obvious fluid loss on the ground or in the engine compartment, you should also have the system tested for leaks. Second: If no coolant leak or low fluid level is present, then determine when the overheating complaint occurs. If the engine overheats while at a stop or idle only: Most front wheel drive cars use an electric cooling fan motor located in front or behind the radiator. The function of the cooling fan is to improve airflow across the radiator at stops and low speeds. The fan is controlled by sensors that regulate the engine temperature and additional load that might be placed on the engine. The air conditioning compressor will require the cooling fan to operate at idle as long as the compressor is on. A quick way to check the cooling fan operation is to turn on the air conditioner. The cooling fan should come on with the air conditioner compressor. Some cars will have two electric fans, one is for the radiator and the other is the air conditioner condenser fan. Usually the radiator fan is closer to the middle of the radiator. The radiator fan is responsible for engine cooling, and the condenser fan is responsible for increasing air conditioning efficiency at idle and low speed. If your vehicle does not have an electric cooling fan on the radiator it will have a belt driven fan blade and fan clutch. This fan should be pulling a large amount of warm to hot air across the radiator onto the engine. What you want to determine with either fan situation is that there is ample airflow across the radiator at idle. The radiator is the primary heat exchange for the engine, and airflow is crucial. What if the engine overheats while at high speeds on the freeway? Again, airflow and coolant circulation are crucial. At 55 MPH we can assume you have ample airflow across the radiator, so proper antifreeze circulation is the thing to inspect. I compare overheating at 55MPH to jogging with a sock in your mouth. The faster and longer you jog, the more air you are going to require, and with a sock in your mouth you are going to have to breath extra hard to maintain the proper amount of air to keep you going. At 55MPH the water pump is pumping a large amount of hot antifreeze throughout the cooling system. If there is a restriction in the system like a kinked radiator hose, a restricted radiator, or a stuck thermostat, it will produce the same affect as the sock in the mouth scenario. Rust and water calcification can accumulate in the radiator and drastically reduce the flow of coolant at high speeds. Removing the radiator from the vehicle for disassembly and cleaning or radiator replacement are the only two real cures for a clogged radiator. Using a can of "radiator flush" additive might help as preventive maintenance, but will probably just be a waste of time and money trying to correct a restricted radiator. Hope this helps, best regards.

Dec 03, 2009 | Vehicle Parts & Accessories

1 Answer

heater dump valve, causing car to over-heat when heat is on


To properly diagnose your overheating problem we need to rule out some things.
First: Is there enough coolant/antifreeze in the radiator? Don't just look inside the plastic overflow bottle, but remove the radiator cap (when the engine is cold) and look inside the radiator. You should be able to physically see the fluid level if it is at its proper level. Most cars and trucks will hold 1 1/2-2 gallons of coolant and water mixture. If you have to add more than a pint of fluid you should have the cooling system pressure tested for a leak. If you see any obvious fluid loss on the ground or in the engine compartment, you should also have the system tested for leaks.
Second: If no coolant leak or low fluid level is present, then determine when the overheating complaint occurs.
If the engine overheats while at a stop or idle only:
Most front wheel drive cars use an electric cooling fan motor located in front or behind the radiator. The function of the cooling fan is to improve airflow across the radiator at stops and low speeds. The fan is controlled by sensors that regulate the engine temperature and additional load that might be placed on the engine.
The air conditioning compressor will require the cooling fan to operate at idle as long as the compressor is on. A quick way to check the cooling fan operation is to turn on the air conditioner. The cooling fan should come on with the air conditioner compressor. Some cars will have two electric fans, one is for the radiator and the other is the air conditioner condenser fan. Usually the radiator fan is closer to the middle of the radiator. The radiator fan is responsible for engine cooling, and the condenser fan is responsible for increasing air conditioning efficiency at idle and low speed.
If your vehicle does not have an electric cooling fan on the radiator it will have a belt driven fan blade and fan clutch. This fan should be pulling a large amount of warm to hot air across the radiator onto the engine. What you want to determine with either fan situation is that there is ample airflow across the radiator at idle. The radiator is the primary heat exchange for the engine, and airflow is crucial.
What if the engine overheats while at high speeds on the freeway?
Again, airflow and coolant circulation are crucial. At 55 MPH we can assume you have ample airflow across the radiator, so proper antifreeze circulation is the thing to inspect. I compare overheating at 55MPH to jogging with a sock in your mouth. The faster and longer you jog, the more air you are going to require, and with a sock in your mouth you are going to have to breath extra hard to maintain the proper amount of air to keep you going. At 55MPH the water pump is pumping a large amount of hot antifreeze throughout the cooling system.
If there is a restriction in the system like a kinked radiator hose, a restricted radiator, or a stuck thermostat, it will produce the same affect as the sock in the mouth scenario. Rust and water calcification can accumulate in the radiator and drastically reduce the flow of coolant at high speeds. Removing the radiator from the vehicle for disassembly and cleaning or radiator replacement are the only two real cures for a clogged radiator.
Using a can of "radiator flush" additive might help as preventive maintenance, but will probably just be a waste of time and money trying to correct a restricted radiator.

Hope this helps, best regards.

Nov 21, 2009 | 2001 Lincoln LS

2 Answers

my honda is over heatin why


CHECK COOLANT.IF COOLANT TOO LOW IT WILL CAUSE OVERHEATING.IF COOLANT LEVEL IS GOOD.YOUR THERMOSTAT IS STICKING CLOSED.IF ALL IS WELL.YOUR RADIATOR IS CLOGGED UP NEED FLUSHING.ENGINE BLOCK WATER PASSAGES IS STOPPED UP.LEAKING HEAD GASKET WILL CAUSE OVERHEATING. USE A SCANNER TO CHECK TO MAKE SURE COOLANT FAN IS WORKING.AND COOLANT TEMPERATURE SENSOR IS WORKING.COOLANT TEMPERATURE SENSOR IS WHAT CAUSES THE COOLANT FAN TO TURN ON AT 190 TO 200 DEGREES.IF COOLANT FAN NOT WORKING.HOT WIRE IT USING JUMPER WIRE TO BATTERY. IF FAN RUNS YOU HAVE A BAD FUSE OR RELAY.IF NOT FAN MOTOR IS FAULTY.REPLACE IT.ALSO CHECK WATER PUMP IF YOU SEE SIGNS OF ANTIFREEZE LEAKING AROUND WATER PUMP.REPLACE IT.ALSO MAKE SURE YOU HAVE 50 % WATER AND 50 % ANTIFREEZE. IN COOLANT SYSTEM TOO MUCH WATER WILL CAUSE ENGINE TO OVER HEAT.ALL OF THOSE THINGS IS POSSIBLE CAUSES THAT WILL CAUSE YOUR CAR TO OVER HEAT.

Nov 08, 2009 | 1992 Honda Accord

1 Answer

how do I add antifreeze to a Mazda 2000 MPV 2.5L radiator? The only thing I could find that resembles a radiator cap is on the reservoir.


That's it!
You need to drain the radiator enough to add antifreeze/coolant. Thre is a drain plug/screw at the bottom corner of the radiator on the driver side.
You should drain it completely and close the valve then add coolant at the proper mixture.

Sep 09, 2009 | 2000 Mazda MPV

Not finding what you are looking for?

110 people viewed this tip

Ask a Question

Usually answered in minutes!

Top Heating & Cooling Experts

paulcarew

Level 3 Expert

2458 Answers

Dan Webster
Dan Webster

Level 3 Expert

8220 Answers

Donni Steen

Level 3 Expert

659 Answers

Are you a Heating and Cooling Expert? Answer questions, earn points and help others

Answer questions

Loading...