996scott

This question shows how little I know about how engines work, but have to ask. I know it will give me a readout of the % on each cylinder and the lower the better. But what do those % numbers mean in "laymans terms"? Will it tell me if I need a top end done? thanks

MarkD

I'll try... hope this helps

100 psi is applied to a combustion chamber (via the spark plug hole) with both intake and exhaust valves closed
If the cylinder holds 95psi, it will show 5% leakdown on the gauge.

It can tell you if:
Exhaust valve is leaking (most common on these engines, if there is excessive leakage)
Intake Valve is leaking
Piston Rings are leaking

Leakage via a valve can also be caused by carbon buildup on the valve seat, most often on the exhaust valve. This could mean valve guides and seals are excessivley worn... but not always.

In general, it can give you a good picture of overall engine health.
It cannot tell you if valve guides are shot, however, if guides are badly worn then the valve will "wobble" resulting in wear to the valve and/or valve seat over time.

That is the short answer... hope it makes sense.

Quadcammer

Think of it like your house. If you were to pressurize your house, there would probably be some leakage by the doors and windows. The more leakage you have, the harder your a/c and heaters have to work to keep your house at the right temperature.

In the case of your engine, the leaks will either be into the intake (Intake valves), exhaust (exhaust valves) and the crankcase (rings). The more leakage you have, generally the less efficient your engine will be.

Will it 100% tell you if you need a top end...sometimes. Anything above say 12-14% is a pretty clear indication of a problem that you'll need to address soon.

MarkD

Good analogy

jimbo3

How long do you wait between pressurizing to 100 psi and taking the leakdown pressure reading?

Also, what does a leakdown test reveal that a compression test doesn't, and vice-versa?

Quadcammer

the leakdown test is pretty quick. Generally cylinders hold at whatever pressure almost immediately...and if you leave it too long (read: hours) the cylinders will eventually leak down much farther.

Compression and leakdown both have benefits and drawbacks. Compression gives you a quick and dirty measure of how well the cylinder is sealed, but is the number is primarily based on the mechanics of the combustion chamber (piston dish, chamber volume, etc). Therefore, the real benefit of compression is to get an idea of how consistent the cylinders are with each other. Consistency is more important than the actual number.

Leakdown pinpoints the source of the leakage. It also doesn't rely on the starter, so there are no worries about a weak battery or starter. Furthermore, the actual number itself tells you something useful and the lower the better.

race911

FIFY

Mark, you didn't live through the 2.7-3.0 eras of running a Por$che $hop....... We've got it so reliable with the 3.6L air cooleds.

race911

Oh, and in all fairness to Mark's original explanation--over half of the time you'll hear head/cylinder leakage in the form of an exhaust sounding leak which will be pretty obvious to all but the most clueless owner.

ja78911sc

On a 3.6 should you do the leak test on a warm motor?

MarkD

correct... I gave a 993-centric answer

yessir

jimbo3

That's why I was wondering. At what point is the leakdown number measured? I'm thinking that, if one mechanic waits one minute, he's going to get a significantly different number than a mechanic who waits 15 minutes, even on the same engine. Or, maybe engines don't leak as much as I'm imagining and there is a miniscule difference.

IXLR8

I think you're not understanding the way the leak-down tester works. The applied pressure is maintained, not shut off with the mechanic then waiting to see how long the cylinder holds pressure.

IXLR8

Not a heck of a lot unless a standard is specified.

If you check out the link in the above post, it explains the workings of a leak-down tester. I never really thought about it, so I assume the diagram is the principle of construction.



From that link...
Based on the above and the possibility of having leak-down testers not following a "standard", then it is quite clear that what one leak-down tester measures may be different from what another leak-down tester of another make measures.

A leak-down tester is easy enough to calibrate. If the standard orifice is 1.0 mm (0.040"), then a 1.0 mm diameter carb jet screwed into a fitting at the output end of the leak-down pressure gauge should provide a 0% indication. A 0.50 mm diameter jet should provide a 75% indication. Can someone confirm that.

MarkD

and so goes information on the interwebz...

IXLR8

Exactly, and I would have thought all leak-down testers are created equal...one would hope they are for the application. The only time I have ever used one was on an aircraft...nice having a prop to hold onto as one is moving the crank back and forth.