Leak down test question
 

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

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

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.

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

[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

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

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. :D

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

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).

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":rolleyes: ,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

If you want to know the truth about the condition of your engine do a leakdown test.
If your not ready to spend money on engine work do a compression test.
Just remember that many engine failures begin with valve problems, if the valve is not fully seated it will not tranfer heat properly. The excessive heat will cause the valve head to break off and rock the piston and crack your cylinder.
It is much cheaper to freshen a set of heads than to build a new engine.
Jeff

I heard that!

what is the # to look for for compression on a 500 hp carb motor mine has 370 hrs now and i did valve springs at 260hrs. and when doing a leak down does the motor need to be warmed up or not.

thanks

one other thing I thought of is you should do a leak down test when the motor is cold and hot there is a huge difference if the motor is tired.

Funny. My Merc 575SCi service manual even has a spec
for cold engine leak down test (30% by the way).

At 430 hours and 5 years one of my two 575SCi's was
at 33% so I'm re-doing the heads on that one. The
highest cylinder on the other one was 28%, it can wait
until winter.

Pesky Varmint

For a regular comp test, you would want to see 140 or so with less than 10% variance between the highest and lowest. It also should be done on a warm engine to get accurate results.

Your right, there is a spec listed for leakdown in the 575Sci service manual, I had not seen that before. They list 30% as the max cold leakdown, but then say it should be checked warm, and don't list a warm engine spec.

Dave

Anyone have the answer to this?

140 to 150 but you are more interested in the varience between cylinders. 10% max.

I've done leakdown warm and cold, with numbers pretty close. If you do it warm, by the time you get to the last cylinder, its not going to be very warm anymore.

Plus, the most accurate leakdown numbers that I've gotten have been with the rocker arms backed off. Then I KNOW that the valves are shut. you can rotate the engine all you want, but if its not something you do every day, it may take a while to get the valves closed 100%.

We did a leak down test and checked the valve spring pressure after each race. These two test combined will give you a good indication of the condition of your engine. Both these procedures are worth doing at least once per season.

Leak down is a standard test on an airplane engine because the cylinder is tapered (tightest @ the top). When you perform this test on any engine you need to bring the piston up to but not past TDC as the ring seat changes from bottom land seal to top lan @ TDC. You can get very different test results if you go past TDC. As stated above leak down is an excellent diagnostic tool. One other technique used on aircraft is to stake the valve, meaning a small particle on the face of either valve can show excessive leak. A socket placed on the valve spring and hit with a hammer can clear the partical and improve the test results.

Look at this link. Good poop and a real world example of why leakdown is a necessary part of engine troubleshooting.

http://vmaxoutlaw.com/tech/leakdown_tester.htm

Happened to stumble on this thread and find some of it interesting.

I have comp tested and leakdown testing so many marine engines in my day but sometimes found neither one to be completely 100 % solid at pin pointing internal engine problems that you would think it could or should have.

Anyways thought it was a good thread if anyone wants to add experiences or anything to it.

In the last 6 years many outboard techs leakdown 2 STROKE outboard engines and claim to others, throw out the comp gauge as leakdown testing by far is the way to go. I am not a believer in leakdown 2 stroke outboard engines. Just adding this info on top of.

Not one comment at all ? Speechless up here huh ? Anyways thought someone would have something to say about experiences good, bad or indifferent.

I'll add to it and say again what Ive said in past, when you build a NEW motor after rings are seated after dynoing it a leakdown test at that point is VERY good for future reference!! If you come off dyno and ypu have a cylinder with high numbers would be a REAL good time to tear in and find out WHY! My recent 540 I freshened had 3-4% leakdown after thrashing on dyno both NA and with blower, if it was going back in a boat every season when I was winterizing it I would again do a leakdown, gives you all winter to address issues vs hoping for the best and wondering, fwiw, Smitty

I like that.