pinging e36

["david macdonald" <freightdawgusa@xxxxxxxxxxx>]

Interesting theory Aaron, at least it shows your thinking.  However, for 
enough oil to seep past the plugs to cause what your theorizing the plug 
would have to be very loose. It would probably blow gasses out on 
compression. In 15 years of working on horizontally opposed aircraft 
engines, we were often told that theoretically enough crankcase blowby could 
sufficiently dilute the fuel air mixture to reduce the octane rating of the 
mixture to the point of allowing detonation which is what pinging is.  I 
never saw it. Usually the spark plug will oil foul first and show a very low 
compression test.  It can show a high compression test if bad rings are 
pumping enough oil to seal the rings for the test.  A low compression test 
can be isolated to the rings or valves by squirting a few shots of motor oil 
into the sparkplug hole and rechecking the compression. An increased reading 
signifies bad rings and no change points to the valves.  if the reading 
increases, your rings are leaking. If no change its the valves.   You can 
isolate which valve by using a differential compression tester where you put 
in 80 psi of air and see how much it leaks down. You can tell if it is rings 
if you hear air blowing out the oil filler hole or crankcase vent.  If it is 
an intake valve, you can here the air rushing out the intake when you pull 
the air filter and open the throttle valve. You may be able to hear it out 
hte exhaust pipe if it is an exhaust valve.  On an aircraft engine its easy 
to hear since they have open exhast.  Put a timing light on the ignition and 
see what your timing is, even if you can't adjust it.  You may have a sensor 
feeding a bad signal to the computer calling for advance.  You could also 
have an lean condition from an induction leak leaning your mixture causing 
the detonation.  Give your engine compartment a thorough visual exam.  Also 
make sure you are not running too hot a plug or too low an octane fuel.
Dave MacDonald
Nebraska Autobahn Society

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