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Home > Video > Video Display > Engine coolant system and compression testing

Engine coolant system and compression testing

There are lots of ways to diagnose a head gasket or to check the general health of an engine, but this video is focusing on 3 major systems.1) The coolant system
2) The combustion chamber
3) The valvetrainCOOLANT SYSTEM PRESSURE TEST:The boiling point of water (coolant) RISES under pressure.  It’s imperative that the coolant system has NO AIR in it, no leaks, a known-good thermostat, and a good radiator cap in order to function properly.  If the pressure stamped on your radiator cap is not being achieved, Boyle’s law is not taking affect, and your coolant will boil when the car is being driven at it’s normal operating temperature, and may exhibit signs of a blown head gasket.  Many have cursed the process of burping all the air out, sometimes even taking several days and heat cycles to complete the process.COMPRESSION TEST:The point of a compression test is to diagnose the health of each combustion chamber.  A compression test is a crude test to determine if a ring, valve seal, valve, or a head gasket problem exists.  It will show immediately in this test because the gauge will not rise to the pressure specified for your engine.  There are 3 numbers you need regarding your car’s setup.High limit, Low Limit and Range.  The high limit is specified by your car’s manufacturer and reflects the equipment they used to build the engine.  If you EXCEED the high limit, chances are it’s caused by excessive carbon build-up, and a Seafoam or MCCC treatment will usually remove the carbon and return it to an acceptable value.  If that build-up isn’t caused by cheap gas, then it’s likely the car has an EGR problem or excessive oil blow-by leading to this situation.The Low Limit represents the point in which the combustion chamber is not able to compress the engine’s specified fuel to the point where it can ignite it.  Gasoline and air need to be compressed together in order for the energy stored in that fuel to be released.  If it isn’t achieved, it will only burn, and not explode.  Lower-than-Low Limit pressure means it’s time to rebuild the engine because one of the sealing components in the combustion chamber has failed or is in need of servicing.  Either way, it’s expensive because working on the combustion chamber requires extensive disassembly or unusual tools that the average mechanic doesn’t have.Range specifies how many PSI of variance there can be between ALL of your compression numbers.  If there’s a large variance on one cylinder, then the engine will not run smoothly.  If a cylinder is below spec, it may feel like a misfire even though the plug is getting spark.  If all the values are close, the engine idles and revs more smoothly.Aftermarket cams, pistons, head gaskets or machining will affect the results of your tests.  A higher compression piston or thinner head gasket can raise the compression numbers slightly.  A thicker head gasket or longer-duration cams will lower compression slightly.  I don’t know how to determine exact numbers because I’m not an engineer, but you should remember that during testing if your engine is modified.So in a nutshell, you want your numbers to be close and between the high and low limits.  If a test fails, the only way to determine WHAT failed is to perform a leakdown test.  Modifications to the factory internals or machining WILL change the OEM compression limit values, but NOT the range value. LEAKDOWN TEST:I’m not using a real leakdown tester in this video.  A real leakdown tester has a regulator, a pressure gauge, a restrictor, and another pressure gauge.  You measure the value of the post-restrictor gauge vs. the regulated supply and mark the percentage of pressure-drop.  As a general rule, most cars are healthy around a 20% value.  Race engines should be between 1-10% for highest performance.I’m using the ghetto method for this test.  Get the piston roughly at Top Dead Center, apply air pressure, fine tune TDC for that piston by turning the crank with a wrench to see if you can stop all of the airflow.  Listen to the intake for leaking air to determine if there’s a bad intake valve.  Listen to the tailpipe for leaking exhaust valves.  If neither are leaking, but air is still flowing, remove the tool and pour a cap of oil into the spark plug hole so that it coats the rings to make a better seal.  After oiling, re-install the tool and repeat the test to see if you get better results.  If you do, you have bad rings.  If you don’t, you have bad valve stem seals, a blown head gasket, or a cracked head.  If you have a cracked head or bad head gasket, then there will likely be pressure venting through your coolant system.  Taking off the radiator cap and watching for air rising to the filler neck will point you in the right direction.
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