How To Compression Test?
01-24-2009, 11:27 AM
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How To Compression Test?
Hey guys I tried searching but no luck. I need to compression test the truck since it misses at idle and makes some kind of knock under throttle while torqued. I don't wanna use internet guidlines since any retard could have typed them and I need this to be LS engine specific. I will prolly rent one from auto zone. Any help appretiated
01-24-2009, 12:51 PM
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Remove all spark plugs, remove fuel pump relay, unplug both coil pack connectors. Screw hose with correct end(it will match spark plug threads) into spark plug hole and connect the gauge. Read the gauge while somebody cranks the engine for you. While they are cranking, they should have the throttle slightly cracked open. Let the engine spin three times(the gauge will jump 3 increments) and that is your reading.
01-24-2009, 01:14 PM
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I do all of that listed above, but throttle is at 100% open and I crank until the gauge stops moving.. max peak compression... once you find the low, faulty cylinder, I would do a cylinder leak down to verify the actual cause of the low compression...
01-24-2009, 03:48 PM
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A leak down test is not too hard to do but you need the gauge set and a compressor. Put the cylinder you want to check on TDC on the compression stroke and connect the gauge. You screw in an adapter similar to a compression test and connect with a quick disconnect. Plug in the air to the gauge and set the input pressure around 100 psi. Then you read the gauge on the output side and it should be within 10% of the input pressure. You can tell were the leak is by:
1.) Listen to the exhaust pipe and if you hear air leaking out then you have a poor sealing (or burnt) exhaust valve.
2.) Listen to the intake with the throttle cracked open and if you hear air leaking out you have a poor sealing (or burnt) intake valve.
3.) Remove the PCV valve from the valve cover and if you hear air leaking out you have a crankcase leak and poor sealing rings or a burnt piston. If your PCV does not have a check ball in the line then you have to unplug it before you do test 2.) or it will pressurize the intake manifold.
If you do not have the piston on TDC for the cylinder you are going to check the air pressure will likely turn the engine over when you connect it to the leakdown tester. If it is not on the compression stroke a valve will be open and the test will fail.
1.) Listen to the exhaust pipe and if you hear air leaking out then you have a poor sealing (or burnt) exhaust valve.
2.) Listen to the intake with the throttle cracked open and if you hear air leaking out you have a poor sealing (or burnt) intake valve.
3.) Remove the PCV valve from the valve cover and if you hear air leaking out you have a crankcase leak and poor sealing rings or a burnt piston. If your PCV does not have a check ball in the line then you have to unplug it before you do test 2.) or it will pressurize the intake manifold.
If you do not have the piston on TDC for the cylinder you are going to check the air pressure will likely turn the engine over when you connect it to the leakdown tester. If it is not on the compression stroke a valve will be open and the test will fail.
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01-25-2009, 09:32 PM
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Yup passed smog. Fixed the tick under throttle. Think it was a lifter floating. I have a hunch that the missing during idle is a vac leak or bad coil. Plugs are clean, new plug wires. More problem solving to come tomorrow