🚀 go-pugleaf

>BTW, the compression was between 170 and 180 psi on all cylinders; does
>anyone know what the spec is?  The shop manual doesn't seem to have it!!

I feel factory spec compression figures are not very useful as an engine
indicator, and I suspect that's why they're often not featured prominently
in service manuals.

Consider that these measurements are taken over a compression stroke
that is of vastly longer duration than an operating stroke.  Ring sealing
depends on combustion chamber pressure, which simply doesn't build very 
quickly at cranking rpms.  At the same time leakage past the rings takes 
place for a longer time.  Therefore the pressures observed are not the 
same as those you would see during operation.

Since the components involved are assembled to specified tolerances,
manufacturers are often able to give ballpark compression figures for
a new engine.  As the engine wears the compression figures drop but
are not a reliable indicator of power production ability.  When the 
compression figures fall below the "specs" they invite the conclusion
that the top end is "worn out", which is not necessarily true.  
 
Of course a serious problem like a stuck valve or a warped head can be
diagnosed with a compression test, but in that case the magnitude of the
figures involved is not important, just their difference.  And these
sorts of problems are pretty obvious to begin with.

I once saw a Toyota 18RC engine that wouldn't start.  The compression had
been measured between 30 and 50 psi on all cylinders.  The spec was,
as above, 170 psi.  Engine junk?  Well, not entirely.  The starting
procedure had to be adapted to give maximum compression - i.e. throttles
wide open.  Once started in this fashion the engine made normal power
and burned no oil.  Since then that car has piled up another 20,000 miles.

So it seems to me that a compression test only validates a claim that
an engine top end is relatively new.  I think their use in estimating 
the "life" of an engine is overrated.

   Jan Prins   UUCP:   {vax135, ihnp4, decvax}!cornell!prins
               Csnet:  prins@cornell
               ARPA:   prins@gvax.cs.cornell.edu
               BITNET: PRINS@CRNLCS

	Which two plugs were they ?  Cyls. 7&8 often do this on small blocks
when the valve guide oil seals go out . Due to the tilt of the engine in the 
chassis (about 7 degrees in most vehicles ) , the oil in the valve covers
has to run past the valve guides to get to the drain hole to the crankcase .
If the valve guide seals are gone (they usually crack and break off ),then
oil can be drawn in on the intake stroke and burned .
	Due to the path it takes in the combustion chamber (ie in with the
fuel , across the plug gap due to swirl ) it tends to build up on the plug.
Why is it white instead of black ? Because it is a very small amount of oil ,
and it is being concentrated on the hottest part of the combustion chamber .
The result is not oil residue per se , but the ash left from burning the oil
 ADDITIVES at high temperatures .
	Another unpleasant result is pinging when the engine gets hot , as the 
deposits retain enough heat to pre-ignite the mixture before the spark is 
applied . The deposits also form on the faces of the valves , and are most 
resistant to any combustion chamber cleaner . Head removal is usually the
only method of cleaning at the advanced stage .Prior to that , try changing the
guide seals (can be done without removing the heads) and a fresh set of plugs.

					tmorris@convex ,disclaim,disclaim!

In article <1083@cornell.UUCP> prins@cornell.UUCP (Jan Prins) writes:
>I once saw a Toyota 18RC engine that wouldn't start.  The compression had
>been measured between 30 and 50 psi on all cylinders.  The spec was,
>as above, 170 psi.  Engine junk?  Well, not entirely.  The starting
>procedure had to be adapted to give maximum compression - i.e. throttles
>wide open.  Once started in this fashion the engine made normal power
>and burned no oil.  Since then that car has piled up another 20,000 miles.
>
One friend sold another friend his "junk" Toyota 18RC for 5 dollars. The thing
ran terribly. Besides tuning it up, we checked the compression, which was 140
on all the cylinders. We adjusted the valves, and checked it again. 180 on all
cylinders. I find it hard to believe your 18rc is getting normal power. Try
adjusting the valves. You might get a nice surprise!

gary cottrell				cottrell@nprdc (ARPA)
Institute for Cognitive Science		sdcsvax!sdcsla!cottrell (USENET)
UCSD

*** reface this slime with your message ***

It sounds like ash deposits formed by burning oil additives.  If the car
has been burning oil recently, you may want to do one or more of the
following:

- Stop using an oil additive, if you're currently using one.

- Try a different brand of oil; one with less additives in it.

If the car's burning oil it's probably getting to the combustion
chamber via worn valve guides (this is a chronic problem with small block
Chevys).  A valve guide job is in order if you're real fanatical, but
since small blocks don't die (they just fade away) you may want to
wait the 50k miles or so until the engine really needs it...

Probably the best thing to do is just put in new plugs and see if it
happens again.

About the compression check- don't worry too much about that either.
Compression readings can fluctuate wildly with changes in ambient
temperature and humidity.  As long as they're all within 10% of one
another things should be O.K.  If you're still concerned you might
want to take the car to a good tuneup shop for a leakdown test.

R.M. Mottola
Cyborg Corp.
Newton, MA.

(Reply posted for me by a friend, but not for long)