"Wet" Compression Testing...

[IO2...@maine.maine.edu]

As suggested by others, I performed a dry and "wet" (1/2 oz
of oil in cylinders) compression test to help diagnose a slight
oil burning problem on start-up. Directions with the gauge
stated:
"Dry Test: Max pressures should be within manufacturers
specifications. If they are considerably under, proceed to
"wet test"."
...then...
"Wet Test: -If compression increases to nearly normal, the
rings, piston and cylinder are bad."
-If pressure does not increase, the valves need
servicing."

SEEMS straight forward, except: On the dry test I got readings
175-190 psi vs. and spec. of 178 psi. This looked great, but
I did the Wet test for the hell of it, and got 200 to 210 psi.
Does this 10 to 30 psi increase demonstrate bad rings? Or should
I accept the dry readings as in spec and consider the rings good?
(Basically, will a healthy engine with good dry compression
always read higher on a wet test? Even 15% above spec.?)

Also, with all other variables constant, does engine timing
have a significant effect on compression readings?

Any comments/advice/experiences greatly appreciated!

-Mike, '86 MR2

[Andy Dingley]

The moving finger of <IO2...@MAINE.MAINE.EDU> having written:

>Does this 10 to 30 psi increase demonstrate bad rings?

Well it sounds like you have an engine with 90K on it. What do you
expect ? 8-)

How much money do you have ? If you want to stop the oil burning, and
return performance to "as new" condition, then pull the head and maybe
do the rings & bearings too. If you pay a garage to do this, you're
talking about major money. If you want to do it yourself, then it's a
fair bit of work, an awkward lift to get the engine out of an MR2, and
you might even enjoy it.

If you don't want to do any of this, then just leave it. It's not
broken, so why fix it ? I'd top the oil up with something
non-synthetic though.

--
Andy Dingley din...@codesmth.demon.co.uk

"Cut the second act and the child's throat"
- Noel Coward, on seeing the young Bonnie Langford on stage

[C & K Claxton]

Picture 2 'C's one on top of the other, and the open ends of each facing in opposite
directions. This is what your rings are like. There is always going to be some leaking
past the rings. If you fell into the pressure range for a "dry test" that is
acceptable, then you shouldn't have to worry about your rings. The "wet test" seals the
gaps in those 'C's, and so would give you a higher pressure. You should be OK.
Happy Motoring!

Chris Claxton
('67 Camaro SS)
('95 Firebird Formula)

[Mark Mathys]

In article <317E88...@sound.net>, C & K Claxton <brn...@sound.net> wrote:

> Picture 2 'C's one on top of the other, and the open ends of each facing
in opposite
> directions. This is what your rings are like. There is always going to
be some leaking
> past the rings. If you fell into the pressure range for a "dry test" that is
> acceptable, then you shouldn't have to worry about your rings. The "wet
test" seals the
> gaps in those 'C's, and so would give you a higher pressure. You should
be OK.

The oil does help seal the ring gaps but more significantly it creates a
better seal between the rings and the clyinder wall. When compression is
initialy low and then increases after adding oil to the cylinder it
usually indicates that there is ring/clyinder wear. This is one way to
determine if rings or valves are the cause of low compression. If the
valves are bad there will be little or no increase in compression after
adding oil.

If the compression is good when dry then there is no need to worry.

Mark

[C & K Claxton]

Sorry - You're right on. It was a bit too late for me to be thinking when I wrote
that.....

[Earl Shimaoka]

In article <mmathys-2404...@sl15.tic.bismarck.nd.us>,
mma...@tic.bisman.com (Mark Mathys) wrote:

> The oil does help seal the ring gaps but more significantly it creates a
> better seal between the rings and the clyinder wall. When compression is
> initialy low and then increases after adding oil to the cylinder it
> usually indicates that there is ring/clyinder wear. This is one way to
> determine if rings or valves are the cause of low compression. If the
> valves are bad there will be little or no increase in compression after
> adding oil.
>
> If the compression is good when dry then there is no need to worry.

The last statement isn't always true. The dry test can be affected by
the number of compression cycles it took to get the maximum cylinder
pressure reading. If it took only 2 or 3 cycles, things are fine; if
it took 5 or 6 cycles, you need to investigate further. It stands to
reason that, even if the dry readings are at spec, if the wet readings
are higher there is a leak. This very scenario happened to an engine
I had several years ago where it produced dry readings at spec but a
leak-down test showed that most cylinders were losing about 15% of its
pressure. The rings were bad.

Earl

[JBlessing]

In article <shimaoka_earl-2...@earls.patch.tandem.com>,
shimao...@tandem.com says...

--
When I do a compression test I always let the engine get up to full
cranking speed and release the pressure and see what it is on the next
pump. Repeat several times to make sure. All that really matters is the
first pump. If you get 75 PSI on the first pump that is way too low, it
doesn't matter if it gets up to 130 after several strokes.
Jordan Blessing L1 Master Tech