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Search the web and you will find huge amounts of mis-information on engine coolants

This is especially common in the case of the orange GM coolant, Dexcool.  Statistically, there are no less problems with green, yellow, red or any other coolant on the market.  Properly serviced, all coolants work well.  Improperly serviced all coolants have problems.

Small sample of extended life coolants in current use

Most coolant is ethylene glycol based. This is the freeze protection side of coolant and only a small part of the total work it does. Despite a similar chemical makeup, the differences in corrosion protection strategies, make each coolant unique. Many formulations are in use and we should not mix or interchange different coolants. Dexcool is a hybrid-organic acid technology (HOAT) coolant. This concerns the corrosion protection strategy and this article will not go into the specific details. Other coolants use various other methods. What is important is always to use what the manufacturer specifies and to do it in the proper manner.

Dexcool mileage/time label

GM has used Dexcool coolant in many millions of vehicles since 1996. In thousands of cooling systems, I am yet to see a problem caused by Dexcool. Most problems with Dexcool as other coolants, stem from over extending its capability. GM states five-years and a differing amount of mileages on their vehicles. Watching only the mileage is a big mistake. As with most coolant, age is the more important factor. Chemical reactions (loss of corrosion protection) occur over time. This occurs around the clock, whether we drive the vehicle or not. Coolant depletion is largely independent of mileage. Unfortunately, people still think more about mileage rather than the more important factor of time

The coolant in a one year old vehicle, even with
100,000 miles is likely still  usable.


A six-year old vehicle with 20,000 miles
may be seriously depleted and causing damage.

Five years is the maximum life of Dexcool.  It is completely depleted by this age.

Five years begins when the vehicle is manufactured, not delivered to the customer.

Five years applies to the initial factory fill, when everything is new.

Other manufacturers specify three to five years and Dexcool reverts to three years after the first change.  More accurate is to determine service life based on pH and specific gravity measurement.  A pH below 7.0 means the coolant is acidic.  This can rapidly destroy the cooling system.  This sometimes occurs closer to three-years than five.  Several factors can also shorten this considerably:

  • Diluting the coolant by adding water, such as with leaks
  • Air entering the system, such as with leaks or a radiator bad cap
  • Contaminating the coolant with corrosive chemicals and minerals that can be in tap water.  Distilled water or factory premixed coolant should be used
  • Improperly mixing the coolant, at less than 50/50 concentration

  • Failure to pre-mix the distilled water and coolant before pouring it into the system

Never add water to coolant, premix water with coolant before adding.Coolant should always be premixed, 50/50 before pouring into the cooling system.  Since coolant is considerably heavier than water, the two will not mix in the cooling system.  This leaves heavier coolant (which ironically does not cool) in the lower part of the system.  The lighter (and corrosive) water tends to circulate in the top of the system, where it eats away at the radiator, heater core and cylinder heads.

As mentioned earlier, pure coolant does not cool the engine. It provides corrosion resistance, freeze and boil protection, but water removes the heat. Mixing coolant, in a concentration stronger than 50 percent, can reduce cooling. Using less than a 50 percent mixture will not give adequate protection from corrosion.

Draining only the radiator, instead of the entire system, causes a similar problem. This removes about 60 percent of the old mixture. This leaves 40 percent depleted coolant in the system. Refilling with a fresh 50/50 batch can result in almost 20 percent less protection than needed. Not premixing the coolant, before pouring it in, will make this much worse.

Improperly servicing the cooling system is as bad as no service at all

Under ordinary conditions, flushing a cooling system is not necessary and can harm the components. Replacing the coolant before 100% depletion will help a great deal. Being proactive with a cooling system and following simple guidelines will prevent most problems.

Flushing water through a corroded system does little more than further disturb the final mixture. A corroded system requires professional cleaning, by removing the engine block plugs or core plugs, as needed. This also allows removal of more of the contaminated coolant.

A few signs of serious corrosion are stuck thermostats, leaking heater cores, leaking freeze or core plugs. Simply replacing the corroded part will offer very short-term results as we are addressing only the symptom. Such problems require a cooling system specialist.

Thermostat that is corroded and stuck in the open position

Corroded engine thermostat has stuck open due to bad coolant

This copper heater core is leaking due to corrosion from improper coolant

leaking heater core caused by coolant corrosion

Freeze plug or core plug rusted through by corrosion from improper coolant

 Corroded and leaking freeze or core plug

Air entering the cooling system will drastically aggravate corrosion

Typical radiator cap

Cooling systems are sealed to prevent the introduction of air.  The initial seals are on the radiator cap.  Radiator caps are fairly complex, do a lot of work and often fail.  Checking the cap annually and replacing and worn or damaged cap can prevent a great deal of cooling system issues.

Radiator cap parts

The cap is also designed to control pressurization of the system.  Pressurization raises the boiling point of the coolant.  With a precisely calibrated spring and valve, the cap keeps air out of the system as the coolant expands and contracts, with heating and cooling.

Some systems accomplish this by use of a reservoir.  When the coolant expands, it flows to the reservoir.  As the system cools the coolant returns to the radiator.  This system depends on a syphon effect.  Unfortunately if the system gets too low, the syphon can be broken.  Filling the reservoir will not fill a low radiator in this case.

Radiator reservior system
Many vehicles now use a surge-tank system.   With this system the filler cap is normally on the surge tank, rather than the radiator.  In this system, coolant basically flows all of the time, helping to purge air from the system.  Expansion and contraction of coolant is allowed by the pressurized surge tank.

Radiator surge tank

Bleeding air after service

Typical bleeder screw used to remove air after service

Modern styling dictates that the engine is often higher than the radiator on modern vehicles. Such vehicles may trap air and overheat after servicing. After service, these bleeder-screws are opened to allow any trapped air to escape. Simply open the screw and tighten after no more comes out. Allowing the vehicle to reach full operating temperature and then cooling back down, helps remove any remaining air. Once fully cooled, check the coolant level again and fill as needed with premixed coolant. While doing this final bleeding, the heater should be kept on maximum heat with the vehicle doors left open.

Regular, proper service can prevent almost all cooling system problems.  Cooling system inspection should be part of your annual general inspection.  Let AGCO show you other ways to prevent major problems.  AGCO, it’s the way to go!

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