🚀 go-pugleaf

Many, many people have posted articles to the effect that blue smoke
on closing the throttle is a symptom of valve-guide wear, not ring
wear.  I think these statements are misleading.

In my experience, the symptoms are exactly consistent with ring wear.
There are a number of reasons why the smoke appears on closing the
throttle.  First, there is less fuel being burned, so that for a
constant amount of oil getting into the cylinder, the oil-to-gas
ratio is much higher.  Second, the combustion is not nearly as
complete, so the oil that gets in is not burned well.
Third, as has been pointed out, the vacuum tends to draw more oil
into the cylinder.  This happens whether the leakage is at the rings
or at the valve guides.

I have not conducted a statistically valid survey (I am sure the other
posters have not, either), but in my experience high oil consumption
is almost ALWAYS caused by ring wear and almost NEVER caused by valve
wear (5-0 on cars I have actually repaired).

A couple of tests have been suggested.  These tests can positively
indicate ring wear, but if they are negative they indicate nothing.
One test is the wet/dry compression.  If your compression improves
by adding a teaspoon of oil to the cylinder, the rings are certainly
shot.  A simpler test is to remove the oil filler while the engine
is running.  If puffs of partially burnt gas and exhaust come
out, the rings are shot (there should be a slight vacuum, assuming
your PCV system is working).

I won't argue too strongly against replacing both the rings and the
valve guides, but I think it is holding out false hope to suggest
that fixing the valves alone is likely to cure heavy oil consumption.

ASIDE:  It has also been suggested that it is impossible to remove
the oil pan on FWD cars.  In the vast majority of cases, it is indeed
possible, and is often easier than with RWD cars. 

> *Sorry...I lost the original posting from the person who thinks he needs
> to replace the rings in his engine, so I'll post it and hope he sees it.
> Apologees to those who are bored with this drivel.
> 
> Are you sure it is the rings that are failing, and not the valve guides?
> 
> If your exhaust puffs mostly (or only) when shifting, I assume you mean
> when the rpms are falling. If this is, in fact, the case, I would suspect
> the valve guides. The reason is that, when the rpms are falling (going down),
> the vacuum generated in the cylinders will have a tendency to pull oil down
> from the head (through the valve guides).
> 
> (OK, I know it's a low pressure zone, and not a vacuum...no flames please).
> 
> Before purchasing tools and/or parts, I would suggest you perform a 
> compression check. You'll need a compression tester (a hose that generally
> screws into a sparkplug hole with a dial to register the pounds per square
> inch generated by the rising piston within the cylinder.
> 
> Follow the instructions for using your tester (keep in mind that the closer
> to normal operating temperature the engine is, the closer to reality your
> readings will be). 
> 
> First, check the reading for all the cylinders (write them down, note any
> large changes between cylinders). Then, pour a teaspoon or so of engine oil
> through the sparkplug hole, and re-test the cylinder. If the reading goes up,
> your rings are shot (the oil helps seal the piston ring-cylinder wall 
> interface).
> 
> If the reading stays about the same, it the valve guides, and/or the associated
> oil seals. Replacing guides is generally something best left to machine shops
> and the like...it is very easy to turn a head into an expensive door-stop.
> 
> Good luck.
> 
> John Rosberg
> !ihuxi!rosberg

An added comment:  To confirm a valve guide problem, go to some
reputable repair place and ask for a cylinder leakage, or leak-down test.
This involves blowing compressed air into each cylinder in turn, through
the spark-plug hole, and checking for air escaping, either past rings, or
past valve guides.  With a stethoscope, the mechanic can usually pin-point
the trouble area.  This test, especially when done with a traditional
compression check, should tell you where to concentrate.

(I had very bad smoking during de-celeration with a Fiat Spider, and it was
bad valve guides, not rings.  Valve guides are especially troublesome
with some kinds of overhead cam engines, depending on the way the
cam "boxes" are designed.  It is also fairly common for valve stem
seals to be worn, even if the valve guides themselves are OK.  Some
designs rely totally on these rubber seals to keep oil burning low.

Bob Schleicher
ihuxk!rs55611

If you decide to replace valve guides yourself, get a good shop manual, as
there are often lots of "tricks" that make this easy or impossible
(things like heating the heads in an oil bath to remove press-fit
guides; putting the new guides in the freezer to let them slide in easier, etc.)
:wq

> Many, many people have posted articles to the effect that blue smoke
> on closing the throttle is a symptom of valve-guide wear, not ring
> wear.  I think these statements are misleading.
> 
> In my experience, the symptoms are exactly consistent with ring wear.
.........
> I have not conducted a statistically valid survey (I am sure the other
> posters have not, either), but in my experience high oil consumption
> is almost ALWAYS caused by ring wear and almost NEVER caused by valve
> wear (5-0 on cars I have actually repaired).
................
> I won't argue too strongly against replacing both the rings and the
> valve guides, but I think it is holding out false hope to suggest
> that fixing the valves alone is likely to cure heavy oil consumption.
............

One confusing part of the above article is whether valve guide wear
or valve seal wear is being discussed.  My own experiences, working
with cars that are reasonably maintained, (oil changed regularly) is
that rings are almost never the problem but valve SEALS very often
are.  Whenever my 1971 GTO started going through more than 1 quart of
oil in 1000 miles, I knew it was time to replace the valve seals.  One
time I took the heads off thinking the valve guides were bad causing
the seals to self destruct.  The shop asked me why I bothered to bring
the heads in as the valves were fine, just new seals needed.  They
recommended teflon seals for replacements.

My 1980 Turbo Mustang, used to go through valve seals constantly due
to the higher engine temperatures.  The seals got brittle and
cracked.  The second set went bad so quickly that I let a mechanic
convince me I had a cracked ring and I didn't recheck to valve seals
until I had the engine disassembled.  No cracked piston rings, but
lots of cracked valve seals, and lots of cussing at the mechanic.
The engine didn't even burn the oil, it got right past the seals
directly into the exhaust manifold.  3 out of 4 exhaust port were wet
with oil.

My advice, try the valve seals first!!!!!

					Rick Schieve
					...ihnp4!ihu1g!rls

In article <728@ihu1g.UUCP> rls@ihu1g.UUCP (r.l. schieve) writes:
>My own experiences, working
>with cars that are reasonably maintained, (oil changed regularly) is
>that rings are almost never the problem but valve SEALS very often
>are.  Whenever my 1971 GTO started going through more than 1 quart of
>oil in 1000 miles, I knew it was time to replace the valve seals.

I agree absolutely.  I was in a situation that required the
head to be removed.  Just for kicks I lapped the valves and
replaced the valve stem seals.  Up to then I had been losing
about 3/4 qt per 1000 miles.  After the new seals the oil loss
was unnoticable.

I *had* thought my problem was uneven cylinder bore (the shop that
rebuilt my engine said that this was a problem that would eventually
crop up.  I didn't rebore the cylinders 'cause it was a wee bit
expensive for my student tastes).  But, after 145,000 miles she's still
going strong.  Oh, yes, I forgot to mention the car.  1970 volvo 145-s,
with B20 engine.

			Kevin

-- 
Kevin Szabo' watmath!watvlsi!ksbszabo (U of W VLSI Group, Waterloo, Ont, Canada)