re: leak down test/compression readings 2L

["Tony Sims" <SimsTony@domain.elided>]

Giuseppe wrote:

>my 73spider is burning too much oil.

How much is too much?  And how many miles on your engine?  Anything up to a
quart every 1,000 miles could be considered pretty normal unless you've seen a
significant increase in consumption over a short period, say a month or two,
or your engine was recently rebuilt.

>i did a compression test,first readings
>were done dry,then the 2nd with a little a bit of oil in the cylinder.across
>all four cyl. i'm getting 145-150 dry,175psi wet.How does sound???

Sounds like worn piston rings.  Usually, seeing a significantly higher reading
on a wet vs. dry compression test indicates worn rings.  The other thing it
would seem to indicate is that your valves are sealing pretty well and your
head gasket is sound.  Generally you look for the lowest cylinder reading to
be no less than 75% of the highest reading.  Any difference beyond that would
suggest a cylinder with leaky valves or a leak in the head gasket.

As for the actual readings, you don't say if you tested the engine cold or
warm (warm is best), but I just tested my 2 liter cold and I got
140/145/140/160 dry, the same wet.  Yours are similar and more consistent,
which sounds pretty good except for the dry/wet differential.  I'm installing
a new head gasket and will lap the valves and clean out the carbon, so
hopefully I'll see a slight increase on 1/2/3 and more consistency when the
engine is reassembled.

>secondly,how do i perform a leak down test(suspecting valve guides) and what
>kind of readings to expect.

Not sure if a leakdown test would help identify worn valve guides, since you
perform a leakdown on each cylinder at TDC with the valves closed.  The intake
guides (the only ones that really contribute to oil consumption) aren't ever
exposed to compression.  And, as mentioned above, your compression test
results don't really point to leaky valves (or at least, your valves are all
leaking in a remarkably consistent manner).  Leakdown testing requires some
special equipment and an air compressor, unless you plan to do these tests
frequently I'd just take it to a shop if you really want it tested.

If your car smokes a lot during hard acceleration, that can be a sign of worn
piston rings.  You can judge the condition of your valve guides and seals by
doing the following.  Drive to a place where you can accelerate and coast
without interfering with traffic  (i.e. in the boonies or on the freeway when
it's not busy) and make sure your engine is fully warmed up.  Accelerate at
full throttle to 55-60 mph, then let off the gas suddenly and coast for 8-10
seconds.  Do this 2 or 3 times in a row, and then when you step on the gas at
the end of your last coast, look in the rearview mirror to see if your car is
belching blue smoke.  If it is, you likely have worn valve guides and seals.
When you back off the gas suddenly, it creates a strong vaccuum in the intake
runners, which will draw oil around the guides if they are worn.  Then when
you hit the gas again, it draws the extra oil into the cylinder and burns it,
giving you a nice cloud of blue smoke to confirm your suspiscions.  My old BMW
1600 would blow so much smoke after a long decelleration it looked like it had
caught fire...

Another indicator of worn valve guides and seals is excessive smoking when you
first start the car after it sits overnight.  Again, if the guides are leaking
then oil will seep past the guides into the intake runner, and when you start
the car it sucks in all this extra oil and burns it.

Cheers,
Tony
74 GTV

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