Odd leak down numbers... help?

[Ryan1938]

Hey guys, so I am looking at a 2.4L '72 with MFI and got these numbers back from the compression/leak down:



To me, these numbers look unhealthy at first glance. Only savior being the car hasn't been driven much (1,000 miles over the past decade or so) and at least they are consistent relative to each other.

Then, I read that first paragraph... "A cylinder that is not leaking will read app 20%" Wait, what???

Does this make sense to anyone?

[pkabush]

I’m sure others who are more experienced will chime in but, good leak down numbers would be
2-3% across all cylinders. To state that “up to 40% leakage is good to go” is ludicrous.
It’s also ridiculous to say that a cylinder with 20% is not leaking. A cylinder that reads 1% is leaking.
My understanding is, any reading above 8-9% is ready for a rebuild.

[Jonny042]

Those seem like really high numbers. It's possible the leakdown tester is a non-standard type with a very small orifice that creates a much larger delta in pressure due to leakage?

I have a relatively standard unit (but the directions still call for 100psi when most standard units used in the aircraft industry use 80%

[Ryan1938]

Originally Posted by pkabush

I’m sure others who are more experienced will chime in but, good leak down numbers would be
2-3% across all cylinders. To state that “up to 40% leakage is good to go” is ludicrous.
It’s also ridiculous to say that a cylinder with 20% is not leaking. A cylinder that reads 1% is leaking.
My understanding is, any reading above 8-9% is ready for a rebuild.

This was my thinking as well.

[Solamar]

Unlike aircraft leakdown tests, the automotive testers are not particularly standardized.

I used to have an older Snap On LD tester and it was similar to what your report states, up to 40% was good to go and 20% readings were normal. I replaced it with a new Lang LD tester (who I believe now makes the SnapOn LD testers).

It also says 20-40% is LOW, but after a top end job my 3.2 engine barely moved above 0%.

What this all means to me is ideally you want to see trends on a motor, using the same equipment (I now do a LD test at every valve adjustment for documentation).

But, much more important than the LD # is WHERE it is leaking. Heard at the exhaust? Valves. Heard at the intake, Valves. Heard at the breather hose, Rings. A report without the WHERE isn't worth the paper it's printed on IMO.

[Solamar]

Found these pics -

This was me screwing around trying to determine leak on just the rings. The old SnapOn was reading ~ 15 and the new Lang ~ 2

[Alan L]

The only actual standard is the aircraft one - which is for big cylinders. The orifice from memory is 40thou. If you want more 'accurate' numbers on smaller bores (<4") then you would make a smaller orifice (I have done this). But then you are comparing apples with oranges.
But even at the standard 40 thou, those leakdown numbers are big.
On a freshly built motor on 40 thou test, I get 1-2%. At 10% I would consider some things are needing some attention at some time, but no reason to teardown - assuming all same. At 20% you are getting into wear territory. Probably still no reason to tear down - but it is looming. At 40% you have a poor performing motor - either bad valves, or lots of crankcase blow by, or some of both probably. Time to pull it down. 20% I would think is a reasonably well worn motor entering the end of a useful life and approaching a rebuild. But not urgent. Assuming that is all the issues (broken studs etc).
Alan

[Jonny042]

In this case we could probably consider the spread between the leakdown numbers is more important than the actual values. They are pretty consistent.

If you ignore the leakdown numbers and focus on the compression, those look pretty good, again almost identical cylinder to cylinder.

I'd call it "good" and go looking for rust....

[stownsen914]

Makes we wonder under what conditions and with which tool the test was done if 20% is "good."

The consistency across cylinders is notable.

['78 SC]

The Lang specialized tester is reading percent, but percent of what?

Most testers use two gauges both of which indicate PSIG, and we do the math to express the difference in the two readings as a percent of the supply pressure. The rightmost gauge on the Lang tester may have a narrow range such that the 40% marking is actually 90 PSIG (or some similar pressure that would be generally considered a "good" leakdown value).

At best this injects unknown assumptions about what is acceptable leakdown while masking the real readings.

[Alan L]

It is just a re calculation of the gauge face - working backward in %.

Agree with above - the even leakdown numbers suggest just normal wear and tear and nothing drastic anywhere.
Alan

[pkabush]

Interesting, I wasn’t aware of the differences in the LD test equipment.
It did seam odd to me that the numbers were very even. And, as Johnny pointed out, the compression #s are good. I’d want more information.

[Alan L]

The compression numbers support the even leakdown numbers. That is a positive sign. The leakdown is just normal wear and tear for the mileage the engine has done.
The fact they are so even I think just suggests there are no nasty surprises lurking in there.
Alan