To properly diagnose your overheating complaint we need to rule out a few basic 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.

Does the engine overheat 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.

Of course there are more technical issues that could produce an overheating complaint, but the check list described above will identify the most common. Anything you can do to help the mechanic diagnose the problem will probably result in less diagnostic charges to you, and might help insure a proper diagnosis of the problem.

Flushing your Radiator and Cooling System...

The first question that comes to mind is WHY do you want to do this? Red flags go up in my head when customers ask me a "how do I..." question. Some people not only like to diagnose their car problems themselves, but they want to perform the operation as well. This can be a great thing, but only if the diagnosis was correct. If it was not you just wasted your time and money. So my question back to the customer would be "what is the problem you are experiencing, and why do you think this operation will solve the problem?" Boy do I get a blank stare then!

"Flushing" a radiator sounds like a wonderful thing to have done periodically to your vehicle, but what does it actually do? You probably have a mental picture of this high powered jet blast of water mixed with some kind of detergent that removes all the gunk that has accumulated in the radiator, and after doing this procedure your car will not only run "cooler" but... "better", right? I mean this gunk has probably been the source of your "engine robbing performance" in your mini van for months, right? I hate to put a damper on your parade, but we need to talk.

Most radiators today are small, made of light weight aluminum, and crammed so tightly in the front of the car you can barely see it let alone "flush" it. The neck of the radiator (where you pour in the antifreeze) is usually angled in such a way that it is impossible to pour in the antifreeze, or even SEE the antifreeze for that matter. The inside of the radiator is made up of a honey comb maze of rows, or "tubes" that sends the hot antifreeze on a long meandering journey from left to right of the radiator. Air is being forced through fins on the outside of the radiator to cool down the antifreeze inside the radiator.

Ok, I hope you are still with me because here is the answer to the question. Where does dirt and sediment accumulate in the radiator, at the top or the bottom? The bottom of the radiator will trap the majority of the rust, dirt and sediment. You can try as hard as you want to, but you will not be able to remove enough of this compacted material to do any real significance in engine performance. The way the radiator is designed internally prevents the access of any high pressure action that you might be able to insert into the small opening of the radiator neck located at the top of the radiator.

At my shop the term "flushing" the cooling system has been replaced with "draining and refilling" the cooling system. Removing the lower radiator hose, or if equipped use the radiator drain cock to drain out the old antifreeze and replace with the new fluid is essentially "draining and refilling the cooling system." This of course will only remove any minor surface debris along with the old contaminated fluid, and will probably NOT cure any overheating complaints you might have been experiencing. Calcium and rust build up within the tubes are the main causes of radiator stoppages, and will cause a over-heating complaint. If this is the case, removal of the radiator from the car for disassembling and rebuilding, or replacing the radiator are really the only two viable options.

Yes, there are many "radiator flush" additives on the market, but most are not to be used in aluminum radiators (which all newer vehicles are equipped with), or just flat out don't work. There are very few (ok, probably only one or two) problems with a motor vehicle that can be solved by the contents of a can.

So, in a nut shell...draining and refilling your radiator with new antifreeze every two to three years WILL help maintain and extend the life of your vehicle, but will probably NOT have an impact on the way it drives, overall fuel economy, cure a major over-heating problem, or improve handling in wet weather. Ask your mechanic to inspect the radiator and heater hoses, and test or replace the radiator cap when replacing the antifreeze.