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A leakdown test is...(much too long) (archive)

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Posted by michael on July 18, 2001 at 07:48:59:

In Reply to: Need info. regarding leak down test for '95 M3 ... posted by SilverM3 on July 17, 2001 at 21:03:32:

a differential pressure test. It mesures the difference between the pressure going into your cylinder vs. the pressure that your cylinder can hold. The reading is mesured as a percent. 10% leak means that at 100 psi going into your cylinder, that cylinder can hold 90 psi. The loss will occure around your rings, through your head gasket, around your spark plug seats and between your valves and their seats. This loss represents combustion loss. A new engine with Total Seal piston rings that have been seated (run for 50 or 100 miles to mate the rings to the cylinder walls) and a fresh valve job will leak between 2% and 4%. This is a very tight engine. A new engine with "normal" seated rings and a fresh valve job will leak in the 4% to 7% range. For a used but solid engine, you can expect up to 10% leak down. The leak down test should be done on a warm engine, at the top, middle and bottom of the piston sweep. All cylinders should be within 10% of each other. Also this test needs the piston to be moving up to seat the rings before the test is done.
If one cylinder is notably low, it can indicate several things. On the high leak cylinder, spray some oil in and see if that improves things (bad rings or ring gaps in your pistons). A broken ring will leaak like mad. If your two compression rings have "walked" to be inline with each other, they will leak. This indicates a possable out of round bore. Retest a "bad" cylinder, making sure the piston was not stoped going down in its bore. this sets the rings at the top of the piston's ring slot. The ring will not seal if it is not seated at the bottom of its slot.

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