Could somebody tell me what is involved in this test, and what does it actually test?

besides leak down test and the the cylinder compression t est, are there anyother usefuld tests on a engine before purchasing?

 

Leak down measures minute airflow past piston rings and valves. It's good for checking the condition the function of the above mentioned parts.

Make sure you check that the sensors are intact. If you can visit the motor a couple times. You'll be amazed at how much you can catch on the second look.

 

hey, thanx for the info rocket, so basically the leak down tests the same thing as the compression test? why do engine vendors do both?


[Modified by SilverCIVIC96, 7:27 AM 6/6/2002]

 

That's a good question, I just talked with a guy at nipponmotors - he explained to me how the leakdown test works. He basically told me that the leakdown is more reliabe versus a compression test. Because compression numbers can be altered by oil thickness. As for why places do both tests.. I dunno, just gives a better overall look at the condition? I'd rather see similar numbers on both tests, other than just good numbers on one test, they could have rigged it somehow maybe?

 

Compression tests can be changed to look good by pouring oil in to the cylinder so that it seals the rings. You are supposed to do a dry compression test first and if the numbers are low you can do a wet test (by pouring oil in the cylinder) and if the compression comes up you know their is a ring problem and if compression is still bad its a valve problem. Compression tests are done by cranking the engine.

A leakdown test is just another way of checking the same thing. A leakdown test is done with the cylinder being tested at TDC. Then the cylinder is pressurized with compressed air and you get your reading. If their is a leak you can tell it is valves if air is coming out of the oil fill cap and if its rings air will come out of the exhaust. If air start coming out of the coolant system youve got a big problem.

Both tests done together can tell you a lot. On my car it was burning oil but dry compression was 200. A leakdown test showed 20% and air was coming out my exhaust. The compression rings are ok but the oil control rings are not.

 

A compression test tells you if you have compression when you crank the motor. This test involves your cam timing, battery strength, etc.

A leak down lets you know where your leaks are.