Compression. How low can you go?

[AlfaNeely@domain.elided]

In a message dated Thu, 10 Oct 2002 15:47:51 +0000, owner-alfa-digest@domain.elided writes:

> 
> Date: Thu, 10 Oct 2002 03:46:12 EDT
> From: ALfactor@domain.elided
> Subject: Compression.  How low can you go?
> 
> Accepting the fact that a quick compression check is not the "gold standard" 
> engine diagnosis. If all cyliners were at about the same 
> level, at what point 
> would you start to be concerned?  150?, 125?, or ???
> 
> Mitch Fisher
> Alfactor@domain.elided
> 74 Spider


     I recently posted that I consider any cylinder with less than 100 pounds to need work.  An engine with 125 in all cylinders is starting to be marginal.  I have never seen even cylinders with that low a reading as normally one or two cylinders will drop off faster than the rest.  Anything 140 or above is in reasonable shape, if they are even.
     Again, this assumes normal camshafts, an open throttle during the test and an engine that has been run recently.  An engine that has set for an extended period could have all sorts of things happen to give one cylinder a low reading.  The rings could be rusty or stuck.  The valves could have been open and become rusted, etc.  I agree with those who wrote that an engine that had set for a while should be driven to see if the condition improves.  A cylinder with less than 50 pounds is probably not going to get better.
     Another thing that happens and can give a false reading is carbon on a valve seat.  When you crack a spark plug that has been in an engine for a long time, carbon can drop loose.  That carbon could fall onto an open valve and cause it to leak during the test.  It is best to crack each spark plug loose a couple of turns, tighten them back and start the engine to blow the carbon bits out the exhaust.  Then do the compression test with all four spark plugs removed.

Ciao,
Russ Neely
Oklahoma City
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