Preventative Maintenance Tech Note Cooling System Performance v3.1

Information provided is given free of charge in good faith without prejudice Sonic Tech Note: 


Nov 2 1998


   The purpose of this tech note is to discuss some tips for diagnosing the cooling system and also some ways to monitor the cooling system performance.  This text will also help to explain how to see the signs of problems with the cooling system on any car before they cause major problems.  This tech note is applicable to any water-cooled automobile with an overflow bottle connected to the rad neck using a two-way pressure cap.

   There have been far too many club members worrying for no reason about their engine's health and a pending head gasket failure.  My intent with this tech note is to show the reader how to diagnose his own system and monitor its condition alleviating the worry and confusion about the status of the engine and cooling system. 

   Please read the Caution note at the top of this tech note, and remember it for the rest of your life.  Over the years I have seen many people badly burnt by cooling systems at the side of the road and racetracks.  People just don't realize the danger of a hot pressurized cooling system, and furthermore never think that their friendly little car could put them in the hospital burn unit for an extended period of time.  It can, and has.

   In the PMTNMA70.v35 file I have explained other ways to test for cooling system problems.  IE: the Snap On test is the sure-fire way to find out whether you have a problem with the head gasket.  This tech note will show you some other common denominators with correctly working systems and warning signs of a cooling system with problems.

   IS WHAT MY CAR DOES NORMAL?    When a club member explains that his engine makes steam and smoke when it is started this is not conclusive or useful information.  This is because the steam he sees could be the same steam (or what looks like steam) all cars make when the engine is running in cold climates.  However if the car is pumping out steam into the summer heat this is an obvious problem!

   The key to getting a correct diagnosis of any problem is to fully understand the owner's complaints and explanation of the condition he is experiencing.  For example, when it is warm outside and he is seeing this steam there is a problem with the engine without question.  Because all cars make what looks like steam when they are run in cold weather, we need to ask the car owner for more information to draw a conclusion from his first explanation.  Also some oil smoke on startup is normal if the car has been sitting for a few days or overnight.  But if the car in question is billowing out oil smoke all the time and never stops, this is also not normal for the engine.

   Some club members have asked questions regarding the level in the coolant bottle and the necessity to top up the overflow bottle on a regular basis.  This is another example where we need to know more information from the car owner.  It is only normal for the coolant level in the radiator overflow bottle to go down if the coolant system was recently serviced or the car had any other type of service work requiring some or all of the engine coolant to be drained out of the system.

   The coolant level in the overflow bottle will always go down after the system has been drained down and refilled.  This is because as the engine cools down from normal operating temperature the coolant liquid will contract in its physical volume. Because the radiator cap is two way this contraction or sucking will cause some coolant to be pulled back into the radiator from the overflow bottle.     A cooling system that is functioning correctly will have this sucking action happen every time the engine is shut down and allowed to cool off to ambient temperature.

   After the engine is restarted the heat from the engine will cause the coolant to heat and expand to a larger physical volume; this excess volume is pushed out into the overflow bottle.  This volume of liquid that is expelled due to the expansion of the liquid will always remain the same as long as the radiator is completely full when it is cold.

   As this small amount of expanded coolant is pushed out of the radiator through the two way pressure radiator cap into the coolant overflow bottle, any air that may have been trapped in the cooling system when the rad was drained for service is also expelled into the coolant bottle.  Later, after the engine is shut off and allowed to cool again this expansion volume of coolant will be drawn back into the radiator keeping the radiator full of liquid.  This overflow bottle system keeps the radiator completely full of coolant when the radiator/engine is hot or cold.

   Without this overflow bottle setup a completely full radiator would just push out the small amount of expanded coolant onto the ground causing the radiator to be slightly low on coolant after the engine cooled down.  This is how the old cars in the 60 and early 70's did it.     They just left the level down in the rad by this amount so that when the car was running the liquid would have just enough space inside the radiator to expand into.

   Note:     This expansion happens very slowly as the engine heats up.  You should never see any bubbles in the coolant overflow bottle, unless you are a real propeller head type that would stand there to watch it like a hawk during the 15-20 minute warm up time.  P.S.  (If you do this do not let your friends catch you!  They will ask you what you are doing and if you have family members in institutions or loony bins. Hee hee).  (As you may guessed-I have been caught doing this).

   When the cooling system and engine is working correctly together,    after about 10 or more complete hot to cold temperature change    cycles the cooling system will be bled of any trapped air from    servicing and will now function as a totally closed system.  This    will be the end of the topping up of the overflow bottle for many    months.  After many months it is possible to add a small amount of    coolant (one or two ounces) to the overflow bottle because some of    the water in the coolant will evaporate over such long periods.

   Remember that this is only the case with a cooling system that is    working correctly and that has not been serviced or had the rad cap    removed for many hot cold temperature cycles.  This healthy system    must have good rad cap, hoses, heater core, head gasket, water pump,    etc to achieve this stability in the level of the overflow bottle.

   Remember that the level in the overflow bottle will only change    when the engine temperature changes from cold to hot and back to    cold.

   COLD:    When the engine has been cold for many hours, the coolant level in    the overflow bottle will be at the lowest point. The best time to    check this level is before you start the engine in the morning.

   HOT:    When the engine has been running for more than 20 minutes and is at    normal operating temperature, the overflow bottle will be at the    highest coolant level point.  The change between the cold and hot    level is the amount the coolant in the system expands due to the    heat.  This amount should always be the same. 

   NOTE:    Keep in mind that if you check the cold level in the afternoon and    your car has been sunning all afternoon in the 90F degree heat the    level will be somewhat higher than the cold morning level, but not    as high as the hot engine running level.  This is because the heat    from the sun has expanded the coolant to some degree.  The engine    has a thermostat to control the coolant temperature to 88 degree C/    190 degree F.  When the engine is running checking the hot level is    the most accurate level to look at,  the hot level will consistently    be at the same level in the overflow bottle from day to day on a    healthy system.

   When the cooling system has stabilized like this, the change    between the two level points will remain consistent from day after    day to week after week and year after year.  The low and high points    will stay the same.  On my Supra GTE, I have the hot and cold levels    marked on the overflow bottle with a  permanent fine point black    felt pen.  Some cars have bottles that have the hot and cold levels    marked in the plastic, I would recommend that you make your own    marks. 

   As I have mentioned above, the cold-level and hot-level in the bottle    will always stay the same when the system is not tampered with.    Following the sun-tanning example above, the cold level should always be checked at about the same ambient temperature.  This consistent up/hot and down/cold coolant level movement is the best example of a correctly working cooling system and engine you will ever see watch for it.

   An engine with a bad head gasket, or any other type of coolant leaks in the system, will never achieve this consistent long-term overflow bottle level movement.  Such an engine will always show that the system needs more coolant to maintain the hot and cold levels in the coolant overflow bottle.  It is not a closed system because coolant is being forced out onto the ground, or coolant is being sucked into the combustion process.

   The following three case examples are conditions experienced with an engine with a bad head gasket condition.  The engines in these cases have no other cooling system problems.  The thermostat, clutch fan, water pump and radiator are all in correct working order.

   #1 (severe symptoms) The engine has run a very short time and is still bone cold, but the rad is pushing so much coolant out into the overflow bottle it is overfilling!  If your car does this you know you have a very bad head gasket without question.  This is happening because the compression gases from your engine pistons are pushing the coolant out of the rad and into the overflow bottle leaving you with an engine full of air rather than full of coolant.  Without coolant your engine will overheat very badly in a few minutes.

   #2 (less severe symptoms) The engine has been running for more than 20 minutes during which time you have noticed some abnormal temperature swings, nearly hitting the red mark on the temperature gauge without overheating.    Then you notice later when you are checking the hot level as we have discussed in this tech note that there are some bubbles coming up from the coolant overflow bottle or you can see bubbles in the neck of the radiator when the car is running.  You have a head gasket that has started to fail.

   #3 (hard to tell what is wrong) You are cruising down the highway on a long trip and two hours have passed with no problems whatsoever.  Then you look down and notice that your temperature gauge has started to climb and there are signs of coolant coming out of the overflow bottle.  There has been no change in the speed you were travelling and the ambient temperature is the same.  In this example it is safe to assume that your engine has a very small leak in the head gasket.  When there is a slow leak in the head gasket allowing compression gases into the coolant system it takes time for the compression gases to force enough coolant out of the radiator into the overflow bottle and cause this condition.  Driving this car in the city for a short 30 to 40 minute trip, where you stop the car and let it cool down, completely when you get to your destination will mask this condition. When the car cools down the engine will suck back in most of the coolant that it started to push out.

   All the cars in the above example would fail the Snap On test.     Example #3 would take a longer time to show that it had failed the test.

   I hope this helps you understand more about the cooling system.     Please forward any questions or typo reports to regr@me.com

    Written by Reg Riemer Copyright Sonic 1997 (free to distribute and copy in this original format)


Reg Riemer


Supra Owners Network In Canada FROM THE HOME OF THE SUPRA GTE 
Information provided is given free of charge in good faith without prejudice
Visit my Supra Page at http://suprasonic.org
Calgary Alberta Canada. 


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