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Engine Coolant

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Information on Coolant Test Strips 
     Engine coolant is a mixture of water and antifreeze.  As a technician you will be testing and mixing antifreeze with water to protect the engine and it's cooling system.  Antifreeze protects the coolant from freezing and has additives to protect the cooling system from corrosion.  Any coolant with a freeze point warmer than -10°F should be replaced as it will not be strong enough to provide corrosion resistance!
    Mixing 40% antifreeze with 60% water provides a freeze point of -10°F. 
    50% antifreeze with 50% water lowers the freeze point to -34°F. 
    60% antifreeze with 40% water lowers the freeze point to -62°F.
  Up to a 70% mixture of antifreeze can be used in extremely cold climates but the trade-off is reduced cooling efficiency.  Ethylene glycol carries heat less efficiently than water.   If the concentration of antifreeze to water is too high, your engine may overheat! For most locations a 50/50 mix provides the best combination of boiling and freezing protection.  For testing the freeze point of coolant the most accurate tool is a Refractometer.   If the freeze point is not cold enough, the coolant is not clean and clear, or it has been longer than the recommended service interval, you should recommend a cooling system service.  Servicing the cooling system includes removing as much of the old coolant as possible and replacing it with new antifreeze of the proper type at the proper concentration.
     Antifreeze recommended by manufacturers is 96-percent ethylene glycol and four-percent additives.  Ethylene glycol never wears out and this is what lowers the freezing point of coolant.  The chemical additives in antifreeze does wear out and additives are what protect the cooling system from corrosion.   If the freeze point of your coolant is good it does not mean the additives are not worn out!  ALL coolants must be replaced, or cleaned and reconditioned with fresh additives to protect the cooling system from corrosion.  
    There are 3 classifications of additive packages used by manufacturers 
          IAT = Inorganic Acid Technology 
          OAT = Organic Acid Technology 
          HOAT = Hybrid Organic Acid Technology 
IAT = Inorganic Acid Technology
    Traditional green coolant uses inorganic acid technology (IAT) for the additive package.  IAT coolant was used by Ford, GM and Chrysler vehicles until the mid-1990s.  It works well to protect all cast-iron engines and bimetal (cast-iron/aluminum) engines.  It protects both copper/brass radiators and aluminum radiators.  IAT uses silicates that provide metal parts (especially aluminum ) a high level of fast acting protection against corrosion and pitting.  
     Silicate protection is good for about 2 years and then silicates start to drop out of the coolant.  If IAT coolant is not changed every 2 or 3 years deposit will build up in radiators causing engine overheating.  Heater cores can get restricted and lined with deposits that keep the heater form getting as hot as is should.  After 2 or 3 years corrosion increases and all metal components begin to deteriorate.  
IAT coolant should be serviced every two years
OAT = Organic Acid Technology
     Organic Acid Technology (OAT) does not use silicates allowing a longer service life.  OAT-based coolants are usually (but not always) dyed a different color to distinguish them from green IAT coolant.  General Motors, Japanese and European manufacturers have all developed their own OAT formulas, with the Japanese adding phosphates while the Europeans took out phosphates.  
     GM’s OAT-based Dex-Cool is orange and uses 2-EHA in the additive package.   
     Volkswagen/Audi use similar additives and is dyed pink. 
     Honda uses dark green coolant with OAT additives that do not contain 2-EHA.  Many manufacturers have found that 2-EHA  attacks some of the materials they use in their cooling system.  They have decided to use HOAT coolants explained below.
OAT coolant should be serviced every five years

HOAT = Hybrid Organic Acid Technology 
     Hybrid Organic Acid Technology (HOAT) is also known as G-05.  HOAT coolants use organic acids, but do not contain 2-EHA.  HOAT coolants do use some silicate to increase protection for aluminum surfaces and helps repair surface erosion caused by cavitation in the water pump.  Hybrid Organic Acid Technology coolants are used by many European vehicle manufacturers as well newer Chrysler and Ford vehicles.  1985 and newer Mercedes, BMW, Volvo and Mini Cooper use HOAT coolant.
HOAT coolant should be serviced every five years
Do Not Mix Coolants
     It is important that you use the correct type of antifreeze when servicing cooling systems.   Mixing various types of additive packages together can cause the additives to quickly deteriorate allowing corrosion and damage to the cooling/heating system.  If they are mixed, the antifreeze can become cloudy.   If you mix IAT with OAT  the silicates will drop out and begin to plug radiators and heater cores.  Mixing different types of coolants will cause the coolant to lose its extended life properties. 
Choose the Proper Coolant
          Manufacturers carefully research what type of additives to add for the best corrosion resistance and chemical compatibility with materials used in their cooling systems.  The cost of coolant is insignificant compared to the damage that the wrong type of coolant can cause.  Generic coolants may not provide protection that comes from a unique additive the manufacturer has developed for their specific cooling system.  It is best practice to use the manufacturer specific coolant and it will be available from the dealership.
     Many shops use aftermarket coolant.  It is important to know if you need IAT - OAT - or HOAT coolant to get the unique protection that each type provides.     Older vehicles need the silicate concentrations used in IAT coolants to protect their water pumps from cavitation and OAT coolants can damage old style radiators using lead solder. Do not use OAT coolants like Dex-Cool on older vehicles.  
     If you are not purchasing coolant directly from the Dealership you should figure out which type of antifreeze to purchase.   Zerex has published a chart to help you decide if you should use IAT, OAT or HOAT coolant.  The chart is at 
     The aftermarket also sells a “Universal” coolant that claims to be approved for all types of cooling systems.  Manufacturers carefully research how coolants react to the metals used in their engines and would not use a special coolant if it was not needed to make their vehicles last longer.  Take the time to make sure the coolant that the parts store sends you is the correct type.   Do not use universal – one type fits all – antifreeze.  
50/50 vs Pre-mix coolant.
     Many technicians use tap water to mix with antifreeze to obtain the 50/50 mix. Mixing antifreeze with tap water can be hard on the vehicle cooling system and shortens how long the additives will last.   If your tap water is softened it will contain corrosive salts.  Non-softened tap water often has minerals such as calcium, magnesium and iron.  All these minerals and salts will wear out the additives at a faster rate than distilled water.  Purchasing pre-mixed antifreeze will provide a 50/50 mix without additive destroying minerals found in tap water.  Distilled water is CHEAP.  Always use distilled water or “Premixed” coolant.  Premixed coolant contains a 50/50 mix of distilled water and antifreeze.  You should not add water to premixed coolant.   Premix works well with coolant exchange machines.  If you are flushing the cooling system with water as part of your service, the 50/50 premix will get diluted with the water left in the engine block and heater core.
Changing the Coolant
     Draining the coolant from the radiator and then adding fresh antifreeze is NOT a good idea.   A simple drain and fill will leave about half of the old coolant trapped inside the engine block and heater core.  To protect the cooling system you must get rid of ALL the old coolant.
    The best method to get rid of all old coolant (recommended for neglected cooling systems) is to remove the thermostat and remove a heater hose.   Removing the thermostat allows water to circulate through a cold engine.  Removing the heater hose lets you flush out the heater core.  Using a garden hose, flush clean water through every opening in both directions.   When you are done with this process there will still be water left in the system.   IF the engine block has coolant drain plugs removing them will let several quarts of water drain out.  If the engine block does not use drain plugs there will be a significant amount of water trapped in the engine block.  Consider removing a freeze plug to get water out of engine blocks that do not use freeze plugs.  If you flush a cooling system with water, be sure to add pure antifreeze first until you have reached 50% of the entire cooling system capacity.  Then top it off with distilled water.  Be sure to re-check your freeze level after running the engine through a complete warm-up and cool down cycle with the heater on HOT.  Upon occasion the capacity published is incorrect, or an average of various capacities used in that engine.  A true 50/50 mix will show -34°F on your coolant spectrometer.  
     If the old coolant has not been neglected use a coolant flush/exchange machine to replace the coolant. The machine connects to the radiator hose or heater hoses with adapter fittings.  A coolant exchange machine will replace almost all of the old coolant (90-95%) with fresh coolant.  Because the machine is replacing liquid with liquid, air is kept out of the system preventing air pockets that can cause overheating and engine damage. 
     Any time you drain coolant or remove hoses, thermostats, water pumps etc. air will get into the cooling system.  Some engines are very sensitive to this trapped air and will quickly overheat if air is not removed.  Removing air from coolant is called “Bleeding” the system.  Some engines have bleeder plugs that you loosen or remove while adding new coolant.  When the coolant begins to flow out the bleeder hole close it off and then top of the radiator and overflow.  If there is no bleeder you can remove a heater hose and use it just like a bleeder screw.  If the heater hose if hard to get to try jacking up the front of the engine to encourage air to flow up and out the radiator.  Running the engine with the radiator cap OFF and watching to see the coolant circulating from the opening thermostat can help get rid of trapped air. 
     Many engines are very sensitive to trapped air and can be difficult to bleed.  You can never be sure all air is gone until the engine has gone through several warm-up and cool-down cycles with the heater on HOT to help air out of the heater core.  Always run the engine through a warm-up, cool-down cycle and it is best to drive it to make sure it does not overheat on the customer.  
Check for the cause of low coolant
     Modern cooling systems use an expansions tank designed to keep ALL air out of the cooling system.  Air in the coolant will encourage corrosion in the cooling system!  The only place you should find air is at the top of the plastic expansion tank.  Hot coolant expands into this overflow bottle  and as the engine cools it should get sucked back into the engine and radiator.  If there are coolant leaks air can get sucked into the engine and radiator as the they cool down.  It is common to find  the coolant overflow/expansion tank at the proper level with low fluid levels in the radiator.  In systems that have a non-pressurized overflow it is important to check the coolant level in BOTH the overflow and the radiator.
      If the radiator is not completely full you should check for leaks.  When the engine is hot and the coolant is under pressure it is dangerous to remove the radiator cap.  Feel for pressure in the upper radiator hose to help decide if it is safe to open the radiator.  Cooling system pressure testers are easy to use once you find the proper adapters.  Never pump more pressure into the cooling system than what it is designed for.  The pressure cap should have its rated pressure printed right on it.  Small leaks will let in lots of air over time but may be very difficult to see.   Before you decide there are no leaks, leave the pressure tester on for a long time to make sure the pressure does not slowly drop.  Radiator pressure caps also fail.  Always pressure test the radiator cap when looking for leaks.   It is possible for a radiator cap to pass the pressure test but still allow air to get sucked past it instead of the heavier coolant from the expansion tank.  Cheap insurance to just replace it if you have low coolant in the radiator and can't find other leaks.
     If you find a leaking hose you might recommend replacing ALL coolant hoses.  Radiator hoses tend to wear out faster than heater hoses so if one radiator hose leaks it is best practice to replace them both.   If you remove hoses, especially heater hoses, I encourage you to flush the heater core and engine block out with fresh water to remove any old coolant. 
Take the Cooling Systems Lab Quiz

Other pages in this section: ASE Refrigerant Handling Certification | Bleed the Hydraulic Clutch | Classroom Presentations | Course Information and Links | Drive Train Service | DriveTrain Safety Lab | Final Vehicle Inspection | Homework and Lab Grading | Homework Assignments | How to Ruin a Drill Bit | Login To CDX | MOPAR - CAP Certification | MVAC_Lab | NATEF Lab Activities | Shop_vehicles_for_lab | to-do | Unit 1 | Unit 2 HVAC | Unit 3 | Unit 4 Auto Tranny | Unit 5
Other sections in this website: Syllabi | Electrical Systems & Engine Performance | Applied Electrical | Mechanical Processes | HV3.297 Electrical & Electronic Systems | AU3.296 Steering Suspension & Brakes | Introduction to Engine Performance | Introduction to Braking Systems

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