How to Flush...

This has nothing to do with pulling a handle!





Don't Rush the Flush!



Over the years I have had a chance to learn a great deal about cooling systems and in particular, contaminated cooling systems. I have seen some of the worse cooling system cases imaginable and have been able to work out some flushing ideas and techniques. It is my hope that I can share some of these ideas with you and that you can also experience the same measure of success that I have when dealing with contaminated cooling systems. Whether it be electrolysis, oil, rust, dex-cool mud, silicate drop out, aluminum oxide or iron oxide, these problems can be cleaned up, treated, and restored in most cases.


The following is a list of some of the more popular flushing ideas. Some of them have been around for awhile, some are fast, some are quite messy, and some are targeted for very specific problems, using common sense and basic physics to reverse problems that have developed over a long period of time. Each flushing technique has advantages and disadvantages over others.


For the purposes of this website, the terms Coolant and Anti-freeze are used interchangeably, and have the same meaning.



The Water Flush

Good only for routine maintenance, the water flush is really only a rinse, and might be better called a Water Rinse. It is simple basic step that is good when changing spent coolant. After the old Antifreeze is drained out, plain water is used to fill the cooling system, and the engine is operated until it reaches operating temperature. After draining the water, new anti-freeze is added and the engine is warmed up. This does not fix anything, and is not a cure or treatment for any cooling system problem. It is only an extra step in the maintenance process, but it is an important one: a clean cooling system is a healthy cooling system.

  • Advantages: Quick, easy, and the perfect first step for a new anti-freeze change, especially if you are planning on using concentrated anti-freeze. Eco friendly, uses only a couple gallons of water.
  • Disadvantages: Not effective in electrolysis, aluminum oxide, or oil contamination removal. Does not repair anything.
  • Conclusion: While this flush does not fix anything, it could be the most important step in preventing future problems. A water flush is best used in a maintenance situation, as it takes an extra 20 minutes, uses a couple gallons of water, and yet has the potential to save you thousands of dollars...



The Tee Flush

We have all seen this one, it is the plastic tee fitting that is installed into the heater inlet hose. Fresh water from a garden hose is attached to the plastic tee allowing the pressurized water to push out the old coolant, rust, dirt or other contaminates. This needs to be done with the engine cold to avoid engine damage from thermal shock. By "tapping" into a heater hose, household water pressure can be introduced into the heater and top of the engine without the thermostat removed. Better results can be achieved by removing the lower hose at one end.

  • Advantages: Bypasses thermostat. Provides a reverse engine flush. Once Tee fitting is installed, follow up flushing is easy.
  • Disadvantages: Heater inlet hose is not always easily accessible. Flush results may be less effective on vehicles with a fully plugged heater. Plastic Tee fitting is cheaply made and will leak, sometimes in just weeks or months after installation. Not particularly effective in flushing the radiator. Not particularly eco friendly unless a capture & contain plan is in place.
  • Conclusion: The results of this type of flush will be better than The Water Flush above, however it does not require the engine to be running. Newer designs of the Tee limit the amount of flushing pressure available, which makes it slightly safer for old rusted systems, but also less effective.


The Fast Flush

If your idea of a quality cooling system flush includes the words " Five Minute Flush" or if you have the idea that you'll complete this flush during half-time, then your flushing results will not be very good. The usual active ingredient in these flushes is Sodium Citrate, which may act as a neutralizer in acidic conditions. Typically, the fast flush chemical is added to a cool radiator, the engine is brought up to operating temperature, the radiator is drained, and a new coolant mix is added. A fast flush is not much better than a coolant drain and refill.

  • Advantages: Quick, easy and slightly better than nothing. Eco friendly, only uses a couple gallons of water.
  • Disadvantages: Doesn't really do much good, does not fix anything.
  • Conclusion: While Good for regular anti-freeze maintenance, but don't expect a fast flush to reverse any cooling system problems.


The Internet Flush

Made popular on certain internet car forums, this type of flush involves a garden hose stuffed in the lower hose feeding into the water pump. With the thermostat removed and with the engine running, water enters the engine block, up the heads and out the intake, enters the radiator, then shoots out of the lower radiator connection. This type of flush utilizes the combination of the water pump agitation and pressure from the garden hose.

  • Advantages: Great for removal of particles of rust, sludge, mud and spent antii-freeze.
  • Disadvantages: It makes a huge mess and is not eco friendly unless a containment area is established. All the junk is flushed out of the engine and into the radiator. Uses a lot of water if the cooling system is dirty. Engine damage may occur from thermal shock if the engine is hot when the flush is performed. The thermostat must be removed for this flush to be effective.
  • Conclusion: Although messy, this is a great way to clean up a dirty cooling system. Particularly effective for rust, dirt, silicate drop-out and great for removing all traces of old anti-freeze, however most of the bigger particulates will end up in the radiator.


The Oil Flush

Oil contamination can be one of the toughest clean up situations in a cooling system. The primary concern involves finding the source of the oil. Is it from the engine block? Could it be a cracked head? The engine oil cooler? The Trans oil cooler? Once the source is found and repaired, the clean up can begin. Start by replacing as many rubber cooling system hoses as you can. Clean out the overflow reservoir or degas bottle with soap and water. While replacing the radiator is not always required, it will greatly reduce your clean up effort. Remove the thermostat and re install the outlet with a new gasket. Flushing with an oil emulsifying chemical like L-11 and water, allow engine to warm up (as best you can) and run for at least 30 minutes to an hour. The oil should be emulsified (dissolved) and drained out with the flush water. In extreme cases, repeat with an additional L-11 flush. If the oil mess does not disappear, the oil leak may still be active and need further repair or replacement. Once the cooling system is found to remain clean, another water flush should be considered (like a rinse) before the thermostat, and new coolant mix is installed. Don't be overly alarmed or disappointed if after a few days a small amount of "milkshake" is found at the cap or in the reservoir. It is inevitable that there will be a bit leftover that just needs to work itself out.

  • Advantages: A petroleum based oil emulsifier like L-11 works better than detergent based soaps.
  • Disadvantages: Any type of oil clean up is a lot of work.
  • Recommended Chemicals: Use L-11 for Oil Contamination.
  • Conclusion: By replacing all the rubber hoses, the thermostat, and possibly the radiator, the amount of oil to be removed is reduced to that which is in the engine and heater. Replace as much as you can, and flush the rest. A shortened version of the 3 day power flush (below) will yield better results since a bucket is involved.



The Live Flush

Also know as the Freeway Flush, this entails letting the engine do all the work. Start by draining the old coolant, adding the desired flush compound with water, and then driving the vehicle for 15 - 20 miles or more. This type of flushing uses the engine to supply agitation, and temperature. By choosing the correct chemical, most cooling system contaminates can be dissolved and then promptly drained out.

  • Advantages: Uses the engine to create the agitation and heat. This will let the engine "flush itself" if given enough time. For the best results on the really tough problems, usually I recommend driving at least 100 miles and letting the flush stay in the cooling system for up to 3 days. This will give the chemistry of the flush a realistic chance of working. This flush can be done over several days, and the car can be driven by the customer during this time. The thermostat does not need to be removed.
  • Disadvantages: You have to drive the vehicle in order to flush it. Is the vehicle drivable? Is the dash removed? Who is paying for the gas? The radiator may be subject to blockage if the contaminates are not fully dissolved in the flushing solution.
  • Recommended Chemicals: Use 420CF for Electrolysis, Rust, Dexcool muck, or Silicate Drop-out. Use L-11 for Oil Contamination.
  • Conclusion: I like this flush because vehicle down time is minimal. Particularly useful in electrolysis cases, this flush makes sense for the DIY folks while still yielding professional type results.


Hose Drop Flush

General Motors has a very interesting flush technique for removing certain unwanted components from an engine block. It seems that the abrasiveness of aluminum oxide is blamed by GM for repeated failures of heater cores and radiators. GM claims that the aluminum oxide is resistant to normal flushing procedures, as it tends to settle down in the engine block. GM asks that the lower hose be quickly removed on a warm engine to suddenly empty the engine block.

  • Advantages: Uses temperature and agitation to remove aluminum oxide sediment.
  • Disadvantages: Messy and a potential of be burned by hot coolant.
  • Conclusion: I have never done this type of flush, and have no plans to try. It carries to much risk of being burned, and is not for me. Why not use a coolant filter?




3 Day Power Flush

Also known as the Bucket Flush, the 36 Hour Flush or the 3 Day Flush, (yes I know that there are more than 36 hours in 3 days...) this type of flush is very useful for electrolysis removal, oil emulsification, aluminum oxide removal, rust removal, and will clean up any contaminated cooling system . It takes maximum advantage of 5 essential elements: Time, Temperature, Chemistry, Agitation and Settling. If the Power Flush is done correctly, the results have proven to be very satisfactory for even the most difficult electrolysis contamination problems. This flush needs to be done with the thermostat removed. The upper hose needs to be removed at the radiator and directed (extended) into a 5 gallon bucket. Inside the bucket resides a return pump (sump pump) that directs the flush water back into the top of the radiator. The return pump is situated about 4" from the bottom to accommodate particulate settling. With the cooling system filled with water, turn on the pump. If possible, operate engine 5 - 10 minutes every hour, allowing engine to "rev up" to 2500 rpm in short bursts will enhance agitation. Monitor fluid level in bucket at all times, especially when engine is operating. If it is not possible to run engine during this flush, results will still be satisfactory.

  • Advantages: Uses chemistry, agitation, temperature, time, and primary settling. This is the best type of remediation for a cooling system contaminated with electrolysis. Eco friendly, very little mess, uses only a couple of gallons of water. Particles removed do not get stuck in the radiator, they move to the bucket and stay there until cleaned out.
  • Disadvantages: Engine must be capable of running, thermostat must be removed, 110 volt pump (sump pump) must be purchased, must be flushed several hours to really make a significant difference in electrolysis contaminated cooling systems. Pump needs to be capable of running continuously for several hours.
  • Recommended chemicals: Use 420CF for Electrolysis, Rust, Dexcool muck, or Silicate Drop-out. Use L-11 for Oil Contamination.
  • Conclusion: By far, this is the slowest and most costly flush you can do. It is also the best. Having performed this type of flushing for my customers more than a hundred times, I can tell you without a doubt that this power flush is more than up to the task of reversing any contamination problems you will face. In my experience, pumps with an inch (1") discharge thread or larger are ideal suited for this process. Larger cooling systems, or severe contamination problems may require doubling the flush chemical concentration.


Help Me Choose

Below is a chart that suggests the best matches for a flush technique and a given situation.


  Water Flush Tee Flush Fast Flush Internet  Flush Oil Flush Live Flush Hose Drop Flush 3 Day Power Flush Recommended Chemicals
Maintenance X X X X X RMI-25
Rust   X   X X X   X 420CF
Oil X X X L-11
Dexcool   X   X X X   X 420CF
Silicate Drop Out   X   X X X   X 420CF
Electrolysis           X   X 420CF
Aluminum Oxide             X X Use Filter
Stop Leak X X 420CF




In my experience, being very deliberate about properly identifying a cooling system problem, and then flushing will bring about the best outcome. Here are some highlights:

  • Flush with the right chemicals. Electrolysis problems need completely different chemistry than oil contamination.
  • Flush the right way. Determine what your flushing goals are (yeah, I know that sounds funny...), then make a plan.
  • Be deliberate! In the majority of cases I see, the vehicle is 10-15 years old, which means the problem has had quite a head start on you. Choose a flushing procedure and then follow it wholeheartedly.
  • Replace all the rubber hoses! This will greatly increase your flushing results. 99% of the vehicles with contamination problems have unperformed maintenance. The texture of the rubber traps small particles which works against the cleaning process. Replace as much rubber as you can, and flush the rest.
  • Clean the plastic! If your vehicle has a overflow reservoir or a plastic coolant reservoir (degas bottle), paying special attention to removing trapped sediment is crucial. If it can not be thoroughly cleaned, it should be replaced. 
  • Flush for the right amount of time! We need to make enough of a difference inside the engine and cooling system to significantly reduce or eliminate the amount of harmful contamination, and remember,


whatever you do, please...


Don't Rush the Flush!

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