How to perform a compression test

baktasht

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Compression test time! For all you guys who have never done one before. Here is the proper method. Cold engine, disable fuel injection, pull ignition lead, and then go at it. I like to do 2 rounds. 1 round with shorter crank interval and a second round with slightly longer crank interval.

EDIT: Some folks mentioned to run another test per the FSM procedure with engine at operating temp. On a factory "street" engine I always run compression tests cold so that the piston rings are not expanded. It gives me the best reading of what the compression is at worst case scenario. When the engine is warmed up the pistons/rings expand and you have better seal and better compression. For a commercial engine or race engine, always have to run the test warm since they are built "loose" by design. BUT, I am not very curious to see the difference. So I'll be doing a follow-up test and video with engine at operating temp and throttle at WOT. I'll post that once I get it done this week.



Step by step video for ya'll here on the first round of testing with engine at cold temp and throttle plate closed:

 
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baktasht

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A lot of good back and forth discussion on this on another platform where I posted. I thought it relevant to post a brief follow up here as well on the reason for a the dry compression test.

"Dry compression test is exactly as it sounds. Piston rings are dry against the cylinder walls, and valves are dry against the cylinder head, and headgasket sealing surface is dry between the head and block. It gives you the overall "real" compression results based on all those factors. If a dry compression test result is good then all systems are good. If a dry compression test is not good then you move on to a wet compression test. The reason for a wet compression test is to isolate the piston rings as a root cause of low compression. You apply one cap full of oil into each cylinder before testing for compression in each, the cap full of oil helps the piston rings create a better seal against the cylinder walls. What does this do for you? It allows the piston rings to seal up, and if at that point you have a good wet compression test it means that your piston rings are failing. If at that point you still have a bad compression test, then it means that either your head gasket is blown or that your valves are not seating properly in the head. "
 

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