Leak-down Test: Best Tester and method?

[Gunter]

Title says it.

What do the Experts recommend?

Thanks.

[afterburn 549]

Not sure on the expert part...
But a quality compression gauge will give you enough knowledge if you should move on to a "proper" leak dwn test or not.
As with most anything there are correct and incorrect ways of applying this .
I have seen people either on purpose or not screw up a leak dwn test.
If you own a good comp. gauge and it comes in close to a few % of highest and lowest that would be good enough for me. (engine warm throttle open, spark and fuel disengaged , batt fully charged....maybe a battery charge on for good measure.)
If you do a leak dwn...and you get a low number...wind your engine through and try again..as any little particle can deceive you.
Personally I am not a big fan of leak dwn test as they are time consuming and deceiving to just to try to do cylinder balance.
If you have a problem.....rings or valves...it will help isolate it for you..but so will compressed air ... just listen for the hint.

[Gunter]

So, back to the original question:

What is considered the best tester and method to do a leak-down?

[Steve@Rennsport]

Gunter,

Leakdowns are all we do anymore to guage engine health. I heven't used my compression tester in 15+ years since its doesn't provide the same information as a properly done leakdown test.

I have a Snap-On one that I've had since 1978 and its served me well. I use shop air regulated to 100 psi to perform the test. One must be VERY sure that each piston is at TDC and your wrench is not left parked on the crank pully bolt; remove the tool each time.

[304065]

Just to add to the comments from the Gentleman from the Beaver State, when you use 100PSI air, you don't want to inadvertently spin the engine off tdc, this can be the source of personal injury.

I use a setup as follows: M14 spark plug fitting with a piece of rubber hose attached to it, male compressed air fitting on the end. Hose is about 14" long, it's the one that comes with the leakdown tester. OVER the fitting, up to where the threads begin, is a piece of approximately 1" diameter aluminum tubing that is about six inches long. This allows you to position the spark plug fitting in the plug hole reliably without the hose flopping around on you. It also allows you to tighten down the fitting-- finger tight is all that is necessary because there's an o-ring on the threaded portion that seals. (Don't lose it down the hole!)

There's an inexpensive device called a "TDC Whistle" which goes in the spark plug hole - it's got an M14 spark plug fitting on one end, a piece of hose, then a whistle on the other. You turn the crank and when the whistling stops you are very close to TDC on the compression stroke. (It won't whistle up to TDC overlap because one or both valves are open).

I cut the whistle portion off the tester with about 6" of hose attached-- you can slip the hose over the male compressed air fitting on the leakdown tester-- so the procedure is as follows--

remove plug,

insert tester

tighten

put whistle on tester

turn crank till whistle stops

verify Z1 is aligned with the notch in the blower housing. That is TDC #1 on the

Compression stroke.

Remove whistle.

Double-check that your 22mm wrench is off the alternator nut !

Connect air supply to gauge. DO NOT CONNECT GAUGE TO HOSE!

Set gauge to 0% leakdown. Bump gauge, note how reading changes. Marvel at inaccuracy of gauge. Reset to 0%

VERIFY nothing on the alternator nut!

Connect air, note reading.

Turn crank 120 degrees and do it all over again.

[911pcars]

What's the best? If you stick with a name brand automotive tool company, you probably can't go wrong. However, the tool isn't a unit of rocket science. A DIY tool has fewer requirements than one used everyday in a commercial application.

I would add to the procedure.
Since highly compressed air is always hazardous, I would slowly increase the supply pressure to the cylinder up to 100 psi. If the test cylinder isn't exactly at TDC, the engine will rotate. There's much less chance of that happening if pressure is slowly applied, but the cylinder has to be pretty much at TDC (on the compression stroke).

Sherwood

[cashflyer]

Please note some of the terminology is specific, and some is generic.
Know what you want to do before you buy a tool.

This is a simple compression gauge:




I use a differential compression tester.

[euro911sc]

There are some products out there that have only 1 gauge. I prefer the twin gauge models. I got mine off Ebay and its a Longacre brand. Harbor freight also sells one. The key to a good gauge is the machining of the center section to the correct orifice size so you may want to err on the side of brand name here. You can always replace the gauges and regulator with good quality parts if needed, but the middle part is the key element in the tool.

Make sure that the tool connections them selves do not leak!! That gave me some erroneous results once.

I really don't think its that much harder to do then a compression test.

-Michael

[Grady Clay]

Leakdown tester

What does "% leak down" really mean?

Here is the Sun Electric I use.

I think you can still find these used.


I made a useful tool from an old sparkplug and some other pieces.


Best,
Grady

[Gunter]

Thank you all; very informative.

Compression tests are not reliable to really show condition; leak-down is preferred.
Sounds like the air supply has to be a steady, reliable source indicating correct pressure going in.
TDC is imperative or else.
Remove the wrench used to turn or else.
Wear safety goggles when using compressed air.
Take notes and repeat to confirm results.

Air used in SoCal may contain particles that can lodge under the exhaust valves, ah so........

[BK911]

In my opinion, spending lots of $ to get a real accurate reading is not that necessary. 1 - 2 psi off is not that big of deal. You are really looking for any disparity between cylinders.

I do mine cold with a single pressure gauge. The setup cost me around $10 and gives me a good enough result to know what I am dealing with.

[jeff91C2T]

Hi Gunter,

Lots of good advice here...thought I'd add another item from some past leak downs;

- If a high leak down number is found (say significantly higher then others) and it's possibly valve related, pull the valve cover off and tap the valve with a rubber mallet.

The idea here is make sure it's not a valve seating issue.

And a picture of what a setup looks like...

[Charlie V]

Originally Posted by Gunter

Thank you all; very informative.

TDC is imperative or else.

Did a leakdown a couple of years back and the piston was not exactly TDC. 100 psi compressed air drove the engine backwards. Not good. I now go with 80 psi and haven't had the surprise again.......I am also much more careful with getting TDC for each piston.

[Gunter]

Search indicates that "TAVIA" is discontinued? NLA?

And the dual-gauge from Harbor Freight sure looks cheap.

[euro911sc]

Try eBay for a decent used snap-on or longacre or similar... I think I paid $50 shipped for my used one. Not bad.

-Michael

[tabasco]

A leakdown test is a bit like a carfax.
If it tells you there's something wrong, you can be sure there's something wrong (unless you did it wrong).
But if it tells you it's good, it may not be. It just tells you the valves and rings are sealing properly, at TDC.

We're currently rebuilding my engine. I had between 2 and 5 % leak numbers, all leaks from the rings. Pretty good, huh ?
Yesterday my mechanic called me after he measured everything, and we won't be able to reuse the pistons and cylinders.
The nikasil is in bad shape, it's got marks and pockets in some places.
Everything in the engine is tired (I kinda knew it), still, it would produce good leak down numbers.

Now maybe 5% on my tester would show up as 10% on Steve's tester, I always wonder if all the testers have the same, calibrated "restrictor" between the 2 gauges.
I doubt it, especially for the cheap ones.
The point is, if you want to hear where it's leaking from, or just compare your cylinders, a cheap one will work. Or just build your own.
If you want numbers that you can compare with other engines, buy a good brand.


I bought the "Stomski compression gauge adapter" that our host sells, and even though it's a bit expensive for what it is, I'm glad I did.
It takes a few seconds to get to the spark plug hole, and you're less likely to pick up dirt as you try.
Screwing a curvy rubber hose directly in there is impossible.
The Stomski thing makes it real easy and quick, with no leaks. I highly recommend it.

-Guillaume

[jwasbury]

I bought my leakdown tester from Summit Racing. Not too expensive, and twin gauge model with an adjustable pressure regulator. Its probably similar to a cheapo made in China model from Harbor Freight, but since I don't do leakdown tests everyday (maybe once every few years) the tool should be reliable for my long term needs.

I got 44% percent leakdown on #6...when I pulled the engine apart, I found a cracked piston ring and also signs that the piston had become hot enough to deform slightly. So the tool worked fine. Test was easy to perform in my opinion. No more difficult than a compression test. I'd add that #6 showed a slightly lower number vs. the others (-10%) in a compression test I performed months before the leakdown, but the compression number was still pretty good. I also found broken rings on two other cylinders, the leakdown % on those were 6% and 10%. Others were <5%.

Melty piston anyone? Note the gap in the top ring land...and the second ring was captive in the land, no longer able to rotate:

[James Brown]

The one I use for aircraft is specific for each engine. there is a orifice that regulates the pressure into the cylinder and I don't know what Porsche recommends. If you know that, you can get a tester from US tool & supply co. Part Number TP104-9B for 14mm plug size. It uses a .040 orifice @ 80 PSI.

[304065]

James, too bad our engines don't use REM40E plugs!

I used to own a Citabria and would do leakdowns with 80-- this is peculiar to the aviation industry where one often sees logbook entries like 76, 74 etc., with the implication it's out of 80 psi. Using 100 makes the math easier for single gauge.

I have the "los cheapos" Harbor Freight version-- when you set the gauge for 0% leakdown, you are actually using about 35 or 40 psi on the inlet side. Remember the operating principle behind an automatic reserve SCUBA tank, that the flow through a calibrated orifice is directly proportional to the diameter of the orifice and the pressure? Same thing with leakdown testers-- you get more flow with more pressure, so the higher the pressure, the more likely you will reveal defects in the cylinder. Which makes sense. That said, the higher pressure can also mask things like poor ring tension.

On my 901/05 engine I measured 45% leakdown, scratched my head and couldn't believe the results, so I repeated the test over and over. Tore it down and found FIVE broken top compression rings. DPO just re-ringed even though the ring grooves were worn beyond spec, they probably broke when he started the engine.

Leakdowns definitely work, I do one after every race as part of the routine.

[Charlie V]

Originally Posted by tabasco

A leakdown test is a bit like a carfax.
If it tells you there's something wrong, you can be sure there's something wrong (unless you did it wrong).
But if it tells you it's good, it may not be. It just tells you the valves and rings are sealing properly, at TDC.

The point is, if you want to hear where it's leaking from, or just compare your cylinders, a cheap one will work. Or just build your own.

-Guillaume

I do a leakdown each year to compare values. I agree that it is just an indication of how the engine is doing. I have noticed over the last 42,000 kms that the numbers are creeping upwards as it should be. Eventually, I can justify a rebuild and a few mods for more HP. Until then, drive it like you stole it!!