Compression test & cylinder leakdown test
12/29/12, 10:19 AM
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Compression test & cylinder leakdown test
Compression test and Cylinder leakdown test:
Compression test.
A compression test measures cylinder pressure made when the piston travels from bottom dead center ( BTC ) to top dead cemter ( TDC ) with the valves closed. The reading is taken at the spark plug with a compression tester.
Before starting the test make sure all the plugs are out and the ignition and /or fuel system is disabled for saftey.
Instert the comp tester into the spark plug hole and crank the engine 5-6 times. Note the 1st and final reading and also note the amout of times you cranked the engine. if you test cyl. #1 and cranked 6 times then do all the same crank each 6 times, also make sure you have good battery health you want the motor to turn over at the same speed ( all of these could effect readings).
Repet above on each cylinder.
The most import thing in a compression test is consistency between cylinders. The rule of thumb is to have less than 10% difference between cylinders. If a motor has an average reading of 150psi there should be less than 15psi between the highest and lowest cylinders.
Cylinder leakdown test.
The compression test is a dynamic test ( engine moving ) a cylinder leakdown test is a static test ( engine at rest ) The compression test measure how much pressure the motor can produce while cranking. the cylinder leakdown tests how much pressure is lost in the motor.
Bring the motor up to TDC on the cylinder in question. Hook up the leakdown tester and pressurize the cylinder. The tester then measures the volume of air needed to maintain a predetermined pressure in the cylinder. The reading is expressed in a percentage, a good reading should be below 5-8%.
The great thing about the leakdown test is that it deals with how well the cylinder is sealing and nothing else.The readings are not effect by carbon deposits, timing or egnine cranking speed.
The other thing about this test is that you can hear the air leaking out of the cylinder, making it more of a pinpoint test. When there is a high percentage of leaking 1st open the oil cap, if you hear hissing air may be leaking by the rings. Air coming out of the exhaust or intake, then it may be burnt valves. 2 ajoining cylinders have a low reading or air leaking into the next cylinder , could be a failed head gasket.
Being able to pinpoint the exact source of compression loss will tell where the problem is not just that you have low compression.
Compression test.
A compression test measures cylinder pressure made when the piston travels from bottom dead center ( BTC ) to top dead cemter ( TDC ) with the valves closed. The reading is taken at the spark plug with a compression tester.
Before starting the test make sure all the plugs are out and the ignition and /or fuel system is disabled for saftey.
Instert the comp tester into the spark plug hole and crank the engine 5-6 times. Note the 1st and final reading and also note the amout of times you cranked the engine. if you test cyl. #1 and cranked 6 times then do all the same crank each 6 times, also make sure you have good battery health you want the motor to turn over at the same speed ( all of these could effect readings).
Repet above on each cylinder.
The most import thing in a compression test is consistency between cylinders. The rule of thumb is to have less than 10% difference between cylinders. If a motor has an average reading of 150psi there should be less than 15psi between the highest and lowest cylinders.
Cylinder leakdown test.
The compression test is a dynamic test ( engine moving ) a cylinder leakdown test is a static test ( engine at rest ) The compression test measure how much pressure the motor can produce while cranking. the cylinder leakdown tests how much pressure is lost in the motor.
Bring the motor up to TDC on the cylinder in question. Hook up the leakdown tester and pressurize the cylinder. The tester then measures the volume of air needed to maintain a predetermined pressure in the cylinder. The reading is expressed in a percentage, a good reading should be below 5-8%.
The great thing about the leakdown test is that it deals with how well the cylinder is sealing and nothing else.The readings are not effect by carbon deposits, timing or egnine cranking speed.
The other thing about this test is that you can hear the air leaking out of the cylinder, making it more of a pinpoint test. When there is a high percentage of leaking 1st open the oil cap, if you hear hissing air may be leaking by the rings. Air coming out of the exhaust or intake, then it may be burnt valves. 2 ajoining cylinders have a low reading or air leaking into the next cylinder , could be a failed head gasket.
Being able to pinpoint the exact source of compression loss will tell where the problem is not just that you have low compression.
Last edited by skunk21; 12/29/12 at 11:16 AM.
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