How to Properly Reverse
Flush a Cooling System
Many people seem confused as to the best way to flush out the cooling system in their classic car or truck. Most people agree that reverse flushing is the best way to go about it but may not know how to achieve a true reverse flush. Reverse flushing will tend to push out most debris that builds up in coolant passages especially those of the radiator and heater core. I will try to explain in the simplest terms how get the desired results. The layout of components that I am using in this procedure will be consistent with what will be found in most vintage V-8 engines and even most 4 & 6 cylinders.
Let’s start with how the antifreeze/coolant mixture goes through your cooling system under normal operating conditions. The easiest place to start and also to remember the flow pattern is that heat rises. Hot fluid will leave the engine at the thermostat housing. On most vintage vehicles this is the highest point on the engine except of course for the carburetor. If you can’t tell the difference between a carburetor and a thermostat housing please proceed to your nearest repair shop to have any and all work done on your car by some one else. From the thermostat housing the fluid goes through the upper radiator hose to the radiator. This may seem rather simple and basic to most of you but please be patient while I go through the motions anyway.
Once in the radiator the fluid gets cooled off as it passes through the core portion of the radiator. This is the part with the tubes and fins. Heat transfers from the fluid, to the tubes, to the fins and the fins get cooled off by the air passing thru the radiator. After being cooled off the fluid is then sucked back into the engine by the water pump. It is at this point that a lot of people are not sure which way the fluid goes.
The water pump actually pushes the fluid through the engine block from front to back. In fact there are many head gaskets that have small or no coolant passages in the front of the gasket just for the purpose of forcing the coolant to the rear of the block. Once at the back the fluid goes up to the cylinder head(s) and starts to go back to the front of the engine through the heads themselves. On a typical V-8 the fluid from both heads will then flow together again in the front of the intake manifold and out the thermostat housing to start the journey all over again.
Since the fluid is at it’s hottest at the thermostat area this is also the best place to tap into to the circuit for some much needed heat in the cold winter months. Most manufacturers have the hose that feeds the heater core start very close to the thermostat but not all do, so you may need to think and search a little if yours is not the norm. And remember you can always go back to the basic scientific law that heat rises. As the fluid goes to the heater core it may or may not go through a heater control valve. This varies from car maker to car maker and even model to model from the same car maker. Once the fluid is in the heater core it actually acts as a miniature radiator and cools off the fluid by transferring the heat from the fluid to the air that passes through it. Only this time we want that heated up air to warm up our fingers & toes. After the fluid has gone through the heater core and lost most of its heat it gets sucked back to the engine through the heater return hose to the water pump. Yes, just like the radiator gives up its coolant to the water pump, so does the heater core. Again, this description applies to most common V-8 engines because I need to stick to something that most of you are familiar with without getting into a description of how newer vehicles and cooling systems are designed.
Okay, now that we know how the normal flow of things goes we can do some preventive maintenance on our pride and joy by throwing it in reverse. It would be best to start this procedure when the engine is not hot so you don’t waste time going to the emergency room with some serious burns. Draining the cooling system of its existing coolant is the best place to start but not really required. Of course you should drain it into a drain pan for proper disposal at the end of your busy day. But do be sure that there is no pressure in the system. Now you should remove the heater return hose from either the water pump or the heater outlet tube. You will want to go into either the hose or the heater core so that you are forcing water to go backwards through the heater core. If you choose to remove the hose from the heater core be sure to use extreme caution so you don’t create the problem of a leaking heater core. Slicing the hose and peeling it off the heater neck will usually assure no heater core problems. In fact now might be a good time to replace the heater hoses anyway.
There are many “Reverse Flush Kits” for sale in just about every auto parts store and most might work just fine. I use a home-made flush gun that combines water and air pressure to force the fresh water into the system and all the old stuff out. If you disconnected the hose from the heater core you will need a scrap piece of hose to connect from the flush gun to the heater core. If you took the heater return hose off at the water pump just put the flush gun into the hose and clamp it with a hose clamp. Now you need to block off the opening just created by your flush gun hook-up. Any simple thing will work since it is only temporary. You can even stick an old spark plug in the hose and clamp it if you have to.
Now that you are ready to push all the old fluid out you need to give it an exit. Let’s go through the system backwards and see where we end up. Through the heater core (backwards), into the engine near the thermostat housing, backwards through the cylinder head(s) and engine block to the water pump, out the lower radiator hose and into the lower neck of the radiator. The fluid will then go backwards through the radiator tubes and come out the top neck of the radiator. This is where all the old stuff will come out. We have now gone backwards through the heater core, engine and radiator. This is a true reverse flush. Remove the upper radiator hose from either the upper neck of the radiator or at the thermostat housing. If you choose to leave the hose connected to the radiator you might want to rotate it so it points down into a drain pan to catch the old stuff. As far as the other opening, you won’t need to do anything as long as there is a thermostat in place and it is closed as it should be with a cooled off engine. The closed thermostat will prevent your flushing procedure from turning into a messy bath for you. If the thermostat is stuck open now would be a good time to change that too. If you don’t use a thermostat for whatever reason you will need to plug up the housing to prevent the aforementioned bath.
The final things to check before starting the flush are that you put the radiator cap back on and the drain plug on the radiator is closed. Another very important thing to check is that if your car has a heater control valve it is now in the hot or open position. Failure to do this can cause excessive pressure build-up in your heater core which may very well cause it to leak. Very slowly open the valve on your source of fresh water and make sure that you get fluid coming out of the top of the radiator. If you don’t you either have a major blockage in the heater core, the heater valve is closed or you have no water pressure in your hose. The fluid will come out whatever color it originally started out at and will gradually change to clear as all the old fluid is replaced with clean fresh water. Assuming you have good flow out of your system you can now add just a little bit of air pressure (if you are using a flush gun like I have shown). I will repeat, just a LITTLE bit of air pressure. You only want to give it enough air pressure to create some bubbling activity in the cooling system to help loosen up any sediment or blockages. Once again I cannot stress the importance of proper disposal of your old dirty coolant. Please check with your local municipality for the acceptable methods in your area.
Ok, now you have a cooling system that is free of all the old antifreeze and hopefully all or most of the other undesirable sediments that have accumulated over the years. Now you can disconnect your flush hose from the engine but do not hook up the heater hose to the engine just yet. You also want to drain out as much of the clean water from the system as possible so open the radiator drain plug to drain the radiator all the way down. After the radiator is empty and you have closed the drainplug (try not to forget this part) you can start putting in fresh antifreeze. But wait, the heater hose is not hooked up yet. No, I didn’t forget. If you leave the heater hose off the engine as you pour in the fresh antifreeze you can eliminate most of the air pocket in the engine to make for an easier and quicker warm-up. When you see liquid just start to come up out of the open heater hose fitting, stop adding antifreeze and hook up the heater hose. Now you can fill up the rest of the way with antifreeze.
A little information here about antifreeze and the proper mix. Some people think that more is better, not so. Try to get as close as possible to a 50/50 mix. If you are in a climate with severe cold temperatures you can go as far as 65% antifreeze and 35% water. Any higher percentage of antifreeze is a waste of money and actually detrimental to the cooling system for at least two reasons. Reason #1; at extreme cold temps a 100% mix of antifreeze will actually start to jell up sooner than a proper mix. Reason #2; pure antifreeze will not dissipate heat in hot weather as well as a proper mix. Think of antifreeze like oil. You have many different types of oil, right? What is the difference between gear oil and motor oil? Would you put motor oil in your differential or gear oil in your engine? Straight antifreeze would be too “thick” kind of like gear oil and will not transfer the heat out of the system the same as a proper mix will. On the sample vehicle we flushed the cooling system capacity as specified in the owners manual is 14 quarts. This comes out to 3 ½ gallons. Putting in two gallons of undiluted antifreeze will provide approximately a 60% antifreeze and 40% water mix.
OK, the radiator is filled almost to the top, the heater hoses are hooked back up and the drainplug is closed, right? Time to start up the engine. Right after start-up the level in the radiator may drop down a bit. If so bring it back up to just below the filler neck and wait for the thermostat to open up. Just prior to the thermostat opening the coolant may rise & fall a bit and maybe even push out. Once the thermostat does open the level will drop and you can top it off. Since the engine will be warm/hot at this point you should fill the radiator right to the bottom edge of the filler neck and put on the radiator cap. If your car has an overflow tank you should fill that at least ½ way up so that as the engine cools it can draw coolant from the tank. Most 1960’s and older cars do not have a separate tank and should not be filled all the way to the top when cold. This lower level allows room for the coolant to expand and pressurize as the engine warms up. If you do fill one of these older cars all the way to the top when cold it will only push out the overflow until it finds its own level.
You have just performed a true reverse flush of your cooling system. Please remember that a cooling system flush is meant to prevent problems, not fix them. If your cooling system has been neglected and the radiator or heater core has become seriously clogged, a reverse flush probably will not solve your problem. A complete system flush every two years should prevent build-up in the system. Due to the many variations of different manufacturers cooling systems I may not have given a precise set of directions for your particular vehicle but these steps should apply to most classic cars & trucks.