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During a leak down test never remove a spark plug until the engine is on TDC of the cylinder to be tested. Then remove only that plug. The reason is the way the 911 2-valve head is constructed, and the exhaust valve is directly below the plug. If the plug is removed, when the exhaust valve is open, a good chance exists that a piece of carbon can fall from the tip of the plug and land on the exhaust valve seat. Then, during testing, when the engine is put on TDC for that cylinder, the exhaust valve will close against that very hard chunk of carbon, and not seal against the valve seat. Result? Faulty number.
What is the chance of this happening? Probably pretty slim, but my shop had cars come in "with 40% leakage in cylinder #x" asking for a second opinion. Of course, the car had been driven since the "bad" test at the other shop, and the carbon that was there was gone, so our test came up clean. The benefit? We gained a new customer - every time.
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Keep the Shiny Side UP!
Pete Z.
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