Quote:
Originally Posted by Father Of Giants
Uneven sound while cranking is low compression, IMO OP will waste time with a compression test. We already know that cylinder isn't contributing.
Once the leakdown test is done, we'll know immediately just how severe the situation is. As in, whether the valve train failed or the piston failed.
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I like leak down tests however, a compression test is a quick way to determine what cylinder is low and a leak down why it is low. Looking at starter amp draw on a scope is also a quick way to determine relative compression.
We don't know if the cranking goes RRRR , chug , RRRR, chug , RRRR , chug or RRRR , chug , RRRR , RRRR , RRRR , chug
The first could be a battery with a shorted cell ( actual shorted not what some throw at any electrical problem ) causing the cranking to be weak overall. This kind of battery failure is like installing a 10 V battery in a 12 V system , it will crank for long periods of time but cranking will be slower than normal.
The second would be a cylinder with low compression.
A worn cam lobe / rocker / large valve lash can increase compression, this is something that a leak down won't detect. When valves open late, close earlier and intake / exhaust overlap is reduced, compression readings will rise.
While we are on the subject. there are two kinds of compression ratio calculations, static and dynamic. Static is a calculation using swept volume and minimum combustion chamber volume and Dynamic that also takes into account valve events / intake ram effect / forced induction.