Question about Cars & Trucks
Need to know what type and engine you are asking about as not all engines use the same clearances.
Posted on Nov 30, 2014
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Posted on Jan 02, 2017
SOURCE: valve clearances
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Posted on Aug 22, 2008
First of all, you'll want to perform what's called a 'LEAK-DOWN' test. The assumption you make (regarding Intake versus exhaust) is irrevelant. Interferrence is BOTH intake and exhaust valve clearances.
A leak down or "cylinder leakage" test is similar to a compression test in that it tells you how well your engine's cylinders are sealing. But instead of measuring pressure, it measures pressure loss.
A leak down test requires the removal of all the spark plugs. The crankshaft is then turned so that each piston is at top dead center (both valves closed) when each cylinder is tested. Some people start with cylinder number one and follow the engine's firing order.
A threaded coupling attached to a leakage gauge is screwed into a spark plug hole. Compressed air (80 to 90 psi) is then fed into the cylinder. You don't have to use that much pressure... you can use 30 psi... percentages are the same.
An engine in great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble.
The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn.
A leakage test can also be used in conjunction with a compression test to diagnose other kinds of problems.
A cylinder that has poor compression, but minimal leakage, usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc.
If all the cylinders have low compression, but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two.
If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it usually indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire). These are not ABSOLUTE conclusions -- each assumption must be VERIFIED before concluding anything. You mentioned "the lifters seem to be collapsed."... Did you VERIFY this? Since you KNOW the engine is OUT - OF - TIME because you KNOW the timing belt has SLIPPED, then you know you must PUT IT BACK IN TIME... right?...before you can continue diagnosis.... right? Otherwise, what are you MEASURING?
Posted on Jun 25, 2009
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