| A compression test is supposed to be done with the engine warm. For a compression test you are using the starter to turn over the engine at a certain RPM and I wouldn't suggest trying to replicate that with air tools, not going to be accurate. I'm guessing your using an impact wrench to turn over the engine? With a leak down test you are compressing air into the cylinder with the valves closed (TDC) and finding the % leaking. It can be done on a cold engine and will provide you with an accurate assessment of your engines overall health. Here is a good article on how leak down tests work: http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/viewall.html Here is another good article on variances between different leak down testers and how to interpret the numbers you are seeing during a leak down test: http://www.hotrod.com/techarticles/engine/hrdp_1203_budget_leakdown_testers_tested/viewall.html
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