RE: Piston - valve kissing

["Rich Wagner" <drsavage@domain.elided>]

Phil wrote:

>After these two experiences I always make sure and have the spark plugs
out
>and am VERY careful to not turn the engine backwards. Funny thing is when
we
>were changing the driveshaft donut we thought we never turned the engine
>backwards. We could only assume that we must have when we found the timing
>had jumped.

I'm beginning to see a pattern with a number of posts wherein V6 owners are
seeing their timing jump.  In pretty much every case so far, the owner has
said that they turned the engine backward, and then looked to see that the
timing had skipped a tooth or two.  In Phil's post, he's seen that his
timing has jumped, and is convinced that, even though he never turned the
engine backwards, he must have anyway.  The problem I'm having, is that
none of these folks can guarantee that the timing hadn't skipped to begin
with.

Let me relate a little story.  Recently, my GTV6 started exhibiting some
problems.  I won't go into what the symptoms were.  I spent the better part
of a month chasing my tail, trying to figure out what was wrong.  After
conducting test after test, I finally decided that the problem was inside
the engine itself.  I checked the valve timing and, voila, it was retarded
by one notch.  Both cams were still timed with respect to one another.  The
timing had slipped one notch at the crankshaft sprocket.  After thinking
about it, I could tell when the slip happened, too.  It happened while I
was driving, on a trip when I took the engine up near redline.  It was at
the end of that trip that I first noticed that the car was running poorly. 
After stripping the front of the engine, I carefully checked the
tensioner's adjustment before changing anything.  Sure enough, the tension
was low.  The pointer was just barely reaching the top of the index mark. 
I reset the timing, and retensioned by the book:

- -  set the tensioner to MAXIMUM tension (pointer should be PAST the index
mark)
- -  turn the engine over AT LEAST four revolutions to seat the belt
- -  reset tension by REDUCING tension from maximum--NOT by increasing from
low tension

Using my fingers on the belt, I could easily feel a difference in tension,
compared to the way it was before.

It appears to me that the pointer on the tensioner isn't the most sensitive
instrument in the world, since a difference of half an index mark is
tangible.  And as such, I'm wondering very seriously whether many of these
belt slips are actually happening while the car is being driven, rather
than when the engines are turned backwards.

An important thing to note:  if a slip occurs when the engine is being
turned backward, it is VERY unlikely that it will slip over the passenger
side cam.  The most likely point of slippage is the driver's side cam, the
crankshaft being slightly less likely.  Now here's the most important part:
 a slip at the crankshaft when turning the engine backwards will result in
both cams being ADVANCED.  A slip at the driver's-side cam when turning the
engine backwards will result in that cam--and very likely ONLY that
cam--being retarded.  If this is what people are seeing, I'll be convinced
that their slip happened when the engine was being turned backwards. 
Otherwise, I'm skeptical.

If, as in my case, they found both cams to be retarded by the same amount,
the slip almost certainly happened at the crankshaft while the engine was
running.

Anybody care to share what you've seen?


Rich Wagner
Montrose, CO
'82 GTV6 Balocco

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End of alfa-digest V7 #733
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