As the irish rover says they will very likely all read the same when oily but
why is one plug oily at the getgo?
that aint right so - possible reasons:
oil scraper ring on that cylinder is broken but the compression rings are still ok.
one or both valve guides on that cylinder are worn or valve stem oil seals mislocated or missing.
mayo indicates water in the oil is evaporating off and condensing somewher cooler like the oil filler cap, so why is there water in the oil?
head gasket leak
cracked block.
Try a leak down test, similar to a compression test but measures pressure loss (over a fixed period) instead of pressure.
Requires removal of all spark plugs, crankshaft turned so each piston is at TDC on compression stroke for each cylinder test.
Leakage gauge screwed into a spark plug hole in turn, 80 to 90 psi compressed air pumped into the cylinder.
A very good result is 5 to 10% leakage. Fair is up to 20% leakage. But more than 30% indicates toast.
Air coming out of the tailpipe means leaky exhaust valve.
Air coming out of the throttle body means leaky intake valve.
Air coming out of the breather means rings cylinder worn.
Air bubbling from the cooling system means a gasket/block problem.
Compression and leak test results compared together, can be used to diagnose valvetrain issues such as a worn cam, broken valve spring, collapsed lifter, bent push rod, etc.
Low compression, but minimal leakage, may indicate incorrect valve timing, timing chain slipped.
Good compression and minimal leakage together with misfiring will indicate bad injector or a missing spark.
Ramon
Vintage Model Airplane and Rover SD1 3500cc Twin Plenum Vitesse
