Leakdown testing when cold

[dantm]

Hello this is not for a Porsche engine but I see a lot of expertise in this forum so I thought I would ask anyway.

I have an engine that was showing poor leakdown numbers when cold. The air is blowing by through the rings as we could hear it through the crankcase/oil tank.

We took everything apart and inspected the pistons/cylinders.

The factory tolerance for piston-to-cylinder gap is between 0.002 - 0.005". However, when measuring the cylinder bores and piston diameters we are finding 0.003 - 0.007". Would this be sufficient to cause poor leakdown in some of the cylinders?

Also for testing this correctly does it make sense to put up to 100 psi when the engine is cold and/or should we consider lower pressure? Would it make sense to create a test plate for the cylinders so that the heads are taken out of the equation?

Thank you all!

[dusyh]

I did a test on all my cylinders after setting the tool to "set" at zero. When hooked up to cylinders I heard the air flowing and the needle moved from zero to about halfway through the "set" area and stopped there with the air flowing. This is a brand new rebuilt 22RE Toyota engine. So does this mean there is "no leakage"? The problem I'm having is it seems the engine is not advancing enough when rev-ed to 3k.






snaptube vidmate

[otto_kretschmer]

What is poor leak down numbers?

For me 20% leak would be poor. I can live with 10%

50% tear the motor down and rebuild

[otto_kretschmer]

this is what you want and this is on a cold engine (my 1970 BMW R50/5)

[dantm]

Thanks guys, the leakdown we were measuring was in excess of 70% on two (out of 12) cylinders.

I'm thinking with the tolerances we just measured a gap of 0.007" in a 3.2" diameter piston is just too large (gap should be 0.001" per inch of diameter so about 0.003" max tolerance between piston and cylinder). I wonder if due to that the rings were just not seated right and we got the numbers we did?

[dantm]

Originally Posted by otto_kretschmer

this is what you want and this is on a cold engine (my 1970 BMW R50/5)

Am I reading this correctly you put in 50 psi and expect 50% leakage when cold?

[winders]

Originally Posted by dantm

Am I reading this correctly you put in 50 psi and expect 50% leakage when cold?

No, you are not reading it correctly. The air input is 85 psi and the air pressure is holding at 81 psi in the cylinder. That is 5% leak down.

[dantm]

Originally Posted by winders

No, you are not reading it correctly. The air input is 85 psi and the air pressure is holding at 81 psi in the cylinder. That is 5% leak down.

Thanks, this is clear now. The tester I have i think calculates the leak down percentage and bins the percentages in a red, yellow, green if I recall, hence the confusion.

For this engine rebuild we're making a cover plate such that we can test the block alone and the heads alone.

[JoeMag]

Regarding leak down after rebuild, you need to run motor and get the rings to seat before you’re going to get what the leak down really is. You’ll have high leakage before rings seat.

[dannobee]

Set the input air pressure to 100psi and read the leakdown in percentage. If you're using 80psi input, the second number is displayed as a fraction of the first, i.e., 78/80. But that format is often used in light aviation, not automotive.

Any leakdown test with lower input pressures aren't accurate and should not be used.

[otto_kretschmer]

Originally Posted by dantm

Thanks guys, the leakdown we were measuring was in excess of 70% on two (out of 12) cylinders.

I'm thinking with the tolerances we just measured a gap of 0.007" in a 3.2" diameter piston is just too large (gap should be 0.001" per inch of diameter so about 0.003" max tolerance between piston and cylinder). I wonder if due to that the rings were just not seated right and we got the numbers we did?

a piston/cylinder gap being over a couple thou will not give a 70% leak down.

a burnt valve or broken ring or huge ring gap will give a 70% leak down

make sure you are doing the test correctly, testing on TDC on the exaust stroke will give bad readings if there is some overlap with the valves (valves will be slightly open)

I test heads by installing the valves and plugs and filling the chamber with water or gasoline and looking for leaks in the ports

[dantm]

Originally Posted by otto_kretschmer

a piston/cylinder gap being over a couple thou will not give a 70% leak down.

a burnt valve or broken ring or huge ring gap will give a 70% leak down

make sure you are doing the test correctly, testing on TDC on the exaust stroke will give bad readings if there is some overlap with the valves (valves will be slightly open)

I test heads by installing the valves and plugs and filling the chamber with water or gasoline and looking for leaks in the ports

Thanks -- we took out the cams so all the valves are fully seated and the TDC question is taken out of the equation.

In the next test we are making a flat plate that will seal on each cylinder and allow the pressure test in the chamber (and separately the plate can be mounted on the heads to test each pair of intake/exhaust valve).

With regards to the 70% leakage the issue was that we were seeing it on 2 out of 12 cylinders, with the others at very low numbers (sub 10%?) with engine cold and a fresh rebuild. But the heads, gasket, were not taken out of the equation. The heads had been tested separately by having them upside down and filling the heads with gas to check for any leaks.

Thoughts?

Thank you!

[otto_kretschmer]

if you have 10 good cylinders and 2 really bad cylinders I would look for broken rings and burnt valves. Another possibility is you have crud on the seats of those two cylinders.

I did a leak down on my 76 911 and cylinders 1-5 were good, around 5% leak down. Cylinder 6 was weak with a 25% leak down. I was expecting to have to pull the motor and do a top end at the least but I've decided to do a Italian Tune up on it when I get it running again. Its been on jackstands for a decade. The last time I had it registered it passed smog in California.

I moved to Arizona two years ago so I have time to work on the car and since its been sitting for so long I'm going to go thru everything and get it back to being a reliable car.

[Walt Fricke]

1) Regulating the air to 100 PSI makes the math easy - the difference between 100 and the reading for the cylinder under testing is the percentage.
2) One of the most important parts of a leakdown test is listening for where the air is leaking out. A rubber tube shoved in an ear and into the exhaust, into the intake, and into the breather will give you a good idea of where the leaks are. If you have a stethoscope, just take the end part off to use it this way. Always a little into the engine past the rings unless you have Total Seal rings. A burnt, bent, or otherwise not sealing valve is pretty obvious, even at fairly low percentages.

[Jeff Alton]

Originally Posted by JoeMag

Regarding leak down after rebuild, you need to run motor and get the rings to seat before you’re going to get what the leak down really is. You’ll have high leakage before rings seat.

We leak down every motor we build prior to it ever being started. Never seen more than 4-5% leakage at 100 PSI. We then check them once they are broken in.


Cheers

[Walt Fricke]

Jeff's approach makes a lot of sense. Simple to leak an engine on the stand (probably before the plugs are inserted, too), just to see if something got overlooked or otherwise isn't right. You will still see how consistent things are, plus where the leaking is coming from. Obviously you'd expect some leakage past the rings at that point. Everything is easier on the stand.

['78 SC]

Originally Posted by Walt Fricke

... Everything is easier on the stand.

This would make a good tattoo. Say, on the back of my hand. Maybe both hands.

[Jeff Alton]

Originally Posted by Walt Fricke

Jeff's approach makes a lot of sense. Simple to leak an engine on the stand (probably before the plugs are inserted, too), just to see if something got overlooked or otherwise isn't right. You will still see how consistent things are, plus where the leaking is coming from. Obviously you'd expect some leakage past the rings at that point. Everything is easier on the stand.

Exactly Walt. Takes about 10 mins to do it on the stand, both without cams installed and with cams installed. Not that we have had an issue, but sure would want to catch it right away before it is in the car. Then we also have a baseline to measure from.

Cheers

[dantm]

Thanks all, so without the total seal rings what would one expect to see for leaking through the rings when freshly built?

We're making a pressure plate to test independently the block through each cylinder and then each set of valves on the heads.

[Walt Fricke]

During the post machining stage of a 911 valve job, the valve closure integrity is typically done with air pressure, either in the port with some kind of plate and fitting for seal, or with a plate over the combustion chamber and the air fitting in it. The seal doesn't have to be fancy - a rubber gasket is fine. Then fill the other side with water, or some soapy water, and watch for bubbles. You can pressurize it by blowing through a hose, and hold things on with your hands. If the valve to seat seal isn't competent, this will show it. It won't show if everything is as you want it - maybe the sealing surface is too narrow or something. But no amount of pressure will show that.

Some just pour water or some equally thin fluid in and see if gravity pushes any past the recently done valve. You can do this stuff without the valve springs, too, just holding the valve firmly in place.

Maybe Jeff remembers what he sees for leakage numbers on the engine stand. I think I did this once long ago, but forget what the numbers were.