Dave_N

Recently there have been a few threads regarding leak down testing. The thing that bothers me is that there are some that claim there is no connection between cranking compression test results and leakdown test results. That just does not make sense to me.

In other words, you can have cylinder(s) that have good cranking compression and still fail a leakdown test.

I recently had a conversation with a local engine builder and he believes that leakdown tests are "a waste of time" because it is a static test and there are ways that a good cylinder can show false readings (compression loss). For example, the ring end gaps happen to line up when the engine was shut down to perform the test. He does think leakdown is a good way to diagnose where compression loss is occuring if a cranking compression test indicates low readings in a certain cylinder.

So my question is do you agree or disagree and why.

Before you answer, consider that nowhere in any service manual or service bulletin does Mercruiser call for or recommend a leakdown test.

Dave

KNOT-RIGHT

A leakdown test is a simple way to find where the compression is being lost.
A crank compression test showing a cylinder variance
of more then 10% would be a good canidate for a leakdown.
EX. Hissing out of the exhaust or intake,carb or the oilpan.
Exhaust valve, Intake valve, Rings-cylinderscoring.

When doing any of the above tests I look for uniform results.

Doing a leakdown and crank compression test on a fresh motor allows to set a baseline for future checks.

A boost vacuum gauge on the dash will also tell
a story of the condition of valves and rings.

Cold big bore motors that are very high HP
can also give erroneous leakdown numbers.

just my02

B BOATER

I personally believe in a leak down test. Mercury warrantied 2 motors after we leak tested them after a season and they had over 70% leak. The early 502 had a ring problem and the motors would lose power even though compression was fine. Only way to prove it was the leak down test. It is also true that large hp motors may have a high leak value because they are blower motors or they were built lose.

articfriends

iTrader: (1)

A leakdown test is a valuable tool,it has been around FOREVER. I recently started freshening my 540 blower motor,the compression was pretty good,155-159,the cyl with 155 had 19-20% leakdown,was leaking past exhaust valve and into crankcase,1 other one was at 14-15%,leaking into crankcase. The good cylinders were still at 4-6% like the day it came off the dyno,After pulling the heads I can see perfect ring finish in the good cylinders and signs of some kind of rusting/roughness on the "bad" cylinder,that cylinder is only going to get worse,I could probably squeese another season out of it BUT I'm looking for top performance from my motor so it gave me the info to justify going all the way thru the motor while its out of the boat. Keep in mind I'm running zero gap rings,a normal motor can be ran and is pretty normal at 15-20%,with zero gap rings its considered pretty high,Smitty

articfriends

iTrader: (1)

[QUOTE=KNOT-RIGHT;2067189]A leakdown test is a simple way to find where the compression is being lost.
A crank compression test showing a cylinder variance
of more then 10% would be a good canidate for a leakdown.
EX. Hissing out of the exhaust or intake,carb or the oilpan.
Exhaust valve, Intake valve, Rings-cylinderscoring.

When doing any of the above tests I look for uniform results.

Doing a leakdown and crank compression test on a fresh motor allows to set a baseline for future checks.


I preach what your saying-A motor should be compression checked and leaskdown tested as soon as your sure the rings are seated,then the info is available for future comparison. I also use j/e ring seating compound,the rings are fully seated after a few minutes on the dyno,Smitty

cooter01

when doing a leak down check do you just spin the motor over until the valves are closed or do you back off the rockers?
thanks

DRIPPINWETII

You can do it either way. Much easier to turn the engine over with a breaker bar. Just hang on to it when the pressure builds.

Dave_N

This is the kind of stuff that puzzles me, a leakdown of 19-20% vs 5% only drops cranking compression by 4 PSI? This engine would have been considered perfect by the OEM recommended testing standard of just a cranking compression test.

Smitty,
Any thoughts on why leakdown testing, although widely accepted as a valid diagnostic procedure, is not recommended by Merc?

Dave

cloudmaster_321

The differences that i know of, is a compressoin test, at least on my tester, it is measuring the amount of compression that a motor can make. The shown value on the guage will not drop unless you push the vent button.

On a leak down test, your measuring the ability to hold that compression for a specified time period. It is only as much of a closed system as there is no leaks. In a compresson test, the guage is a close system, will show a reading wether or not the cylinder leaks........ My opinion. I know up in the maintenance shop where i work, i work at the airport here in madison, they do mostly leak down tests. I believe they put in 80 psi, typically i see 75-76 written on the cylinders when they are done testing. But i'm not the aircraft mechanic, i'm the spark chaser (avionics).

articfriends

iTrader: (1)

Dave,the first thought that comes to mind is they just don't want to know and they don't want to go trying to make production motors perfect,especially under warranty which just cost them money!!! My stock, metric- ringed 502 burned excessive oil by my standards when my2000 272 baja/502 mpi was brand new and stock,I took it back to the dealer who agreed it shouldn't burn 1-2 qts of oil every 10 hours after 40-50 hours. They did a compression test and found 1 cylinder that was 5-10 psi lower than the highest,it had 35 or 45% leakdown. The battle then began with Merc,after my boat sat at the dealer for several months Merc told them to pull the cyl head and examine the cylinder and head (my boat was 2 months old and BRAND NEW!!!). After all was said and done merc agreed to pay to R&I the block and hone 1 cyl/replace the rings. At that point I lost complete faith in having their USELESS warranty and had the motor pulled by a independent shop,rebuilt it correctly myself along with better merlin heads and the procharger etc,the rest is history and I have never looked back. After wards/since then I have heard that Merc does not even want their dealers doing leakdown test and that they claim pretty much ANY amount of oil consumption is "normal" ,As long as their isn't a rod sticking out the side of it they don't want to do anything for customers from what I can tell so it would make sense they don't want leakdown test,Smitty