Andy and listers....and Bob,
High RPM miss.... "eh"....HMMM? (Thats a Canadian Technical Term) Andy there
is a whole series of tests to do to determine the wear of an engine and
especially the rings. The oil squirt and then redo the compression test is a
good oil to narrow it down and based on that result I woul have stopped and
started removing the engine (assuming the test was done correctly, all plugs
out, Wide open Throttle, once with all cylinders cold, once with all
cylinders hot, then try the oil to narrow done the diagnosis) A leakdown
test will tell you the severity of the presumed ring break (If I recall
correctly 5% is acceptable warm) although I had a cracked piston which acted
the same way as your condition by increasing cylinder pressure when oil was
squirted in and causing a high RPM miss under load. I did not recall if you
indicated how low the weak cylinder was (45lbs with the others at 110lbs)
and whether you tested it hot or cold. If the cylinder does have broken
parts, like a ring or piston, it may be that some piece of foriegn matter
has caused this damage or departed from the ring/piston and may have exited
through a valve creating a marginal seating condition where at high engine
speed the seat just does not perform a good seal and this would be more
noticeable under load again creating the higher RPM miss. The compression
test will indicate ring condition but the leakdown test will tell you if a
valve seat is also weak. With respect to the ring seal it would be have a
more pronounced leak or Blow By when the engine is hot as the clearances
increase further reducing the ability of the ring to seal. This increased
clearance and poor ring seal should also allow oil into the cylinder and if
so there would be an obvious trail of "Blue" smoke as the car starts
missing. You should also notice some oil moisture on the plug as it is
extracted from the weak cylinder. Bob alluded that it appears things
indicate exploratory surgery and you should probably tear down the engine to
investigate or run the risk of further damage or something more serious and
embarrasing like breaking down at a club event! ... and then the resulting
tow home! I have to agree with Bob as I will drive something ugly before I
drive something that does not run perfectly.
Based on what you said about the low compression improving with oil I would
tear it down and investigate. Too bad cars aren't more like people and just
prescribe a little garage rest until it can "heal a little".
Sorry for the low tech solution,
Regards,
Tim Ronak
B382000680
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