british-cars
[Top] [All Lists]

Re: leak down tests?

To: jjoy@akamai.sps.mot.com (Jennifer Joy)
Subject: Re: leak down tests?
From: phile@stpaul.gov (Philip J Ethier)
Date: Thu, 30 Dec 1993 08:44:19 -0600 (CST)
Jennifer Joy writes > 

> Could someone please email an explanation "for dummies" on what
> a leak down test does exactly, and what a test result of "50%"
> means?  I know it is bad, but how bad?
> 
> Is there any (significant) correlation between cylinder compression 
> numbers and this number?   It seemed in my case than the compression
> values weren't too bad, but the leak down test was.   I'm just not
> sure what the mechanic was telling me besides it's "wore out".  One
> was too technical, the other wasn't technical enough!

What your communicationally-disadvantaged mechanic didn't tell you is
where the leak is.  The leak-down method makes it easy to isolate valve
and ring leaks.

For those that want a bit more detail, here is Steve Soar's explanation of
it all.
-----------------
LEAKDOWN TESTER PLANS

Here it is as it appeared in the September, 1990, RIGHT LINE.  
All rights for reproduction remain with Steve Soar.

------------------------------- 

Build your own Leakdown Tester!

List of ingredients:

*A spark plug with the insides removed (arguably the hardest part of 
the whole deal)

*Air tool Whip ( an 18" length of air line with 1/4"NPT male 
fittings on both ends)

*Block of aluminum about 1" X 1" X 2 1/2"

*#60 drill (or smaller)

*1/4 NPT Tap

*0-100PSI pressure gauge

*Epoxy

*Male quick-disconnect air coupling with female 1/4" NPT threads

Assembly Instructions

Break the insides out of an old spark plug. Beat on the ceramic with 
a hammer and a drift until the pieces can be removed. You want all 
the insides out, just the threaded metal shell left. Cut off the 
ground electrode with a hacksaw. File on the non-threaded end of the 
male quick-disconnect until it fits inside the spark plug where the 
ceramic used to go. Clean it and the spark plug with a clean-
evaporating solvent like brake cleaner to get off the finger grease 
and epoxy the quick-disconect into the spark plug shell. This gives 
you a way to attach the air whip to the spark plug to get the 
pressure into your cylinders (By the way, you did use a spark plug 
that fits your car, didn't you?)

Prepare to drill your aluminum block so that it looks like the 
figure.  A drill press is handy but you could do it by hand.

                  _________________________
                 |____     ________________|
                      \   /
                       \ /
                       / \
                  ____/   \________________
                 |_________________________|

                        Cross section view

Explanation: Use the drill appropriate for 1/4" NPT tap (I forget if 
its 3/8" or whatever). Drill into one end only far enough to 
adequately tap threads.  Drill into the other end far enough to come 
near, but not break into, the hole you drilled from the other end. 
Note that these two chambers are *not* connected (yet).  NOW, take 
your #60 drill and your pin vise (Note:: a double-slotted X-acto 
knife handle will do for a pin vise in a pinch), drill the small #60 
hole to connect the two chambers.

Now use your big drill bit (for 1/4 NPT) and drill a hole in the 
side of the block. This is where the gauge will screw in.

                  ____________       ______
                 |____     ___|     |______|
                      \   /
                       \ /
                       / \
                  ____/   \________________
                 |_________________________|

                        Cross section view

Now, using the 1/4"NPT tap, tap all the big holes.

Time to assemble

Use a good sealer on all threaded connections as any leak will 
introduce an error to the tester. I used RTV silicone gasket maker, 
seemed to work OK.  Screw the gauge into the hole at the top of the 
diagram. Screw one end of the whip into the hole into the long 
chamber (at the right of the diagram). Screw the epoxied spark plug 
assembly into the other end of the whip. The short chamber at the 
left of the diagram is where your shop air will enter. Screw in a 
quick-disconnect here that is compatible with your particular shop 
setup.

Congratulations, you're the proud owner of a leakdown tester.

Theory of Operation

The tester works by measuring the pressure drop across a flow 
restriction (the #60 hole in the aluminum block). The lower the 
pressure reading on the gauge, the worse shape that cylinder is in 
(WRT leakage).  To believe that this is so, consider the limiting 
cases.

Case 1--The spark plug fitting is sitting in free air. there is no 
resistance to the flow of air and the gauge reads 0 psi. You might 
actually see this reading with a holed piston or a split valve head.

Case 2--The spark plug fitting is closed off with an extremely 
tight-fitting cap. Since there is no flow through the orifice, the 
is no pressure drop and the gauge will read whatever the inlet 
pressure is, *exactly*. You will never see this reading in real life 
but you may get close with a newly assembled racing engine built to 
close tolerance.

Caveat: Professional leakdown testers read out in "%" leakage, 
converted from psi. There must be a calibration factor used based on 
the amount of pressure drop for a given orifice size but I have been 
unable to determine what the math for this might be.  Any Fluids 
types listening out there?

Fortunately, even though the tester doesn't read out in "%", it is 
still extremely useful.

Application Hints-

Warm up the engine to be tested, then stop it and remove the spark 
plugs.  Bring piston under test to top dead center on its *firing* 
stroke (or else either intake or exhaust valves will be open, making 
any measurement useless). Be sure the piston is *really* at top dead 
center, or else when you connect the air to the cylinder the engine 
will turn over and you will have to set top dead center again.  
Screw the tester plug into the cylinder. Connect shop air to the 
input port of the tester. Be sure the inlet pressure doesn't vary or 
you will get unrepeatable results. For my setup, I just ensure that 
the compressor has just shut off from filling the tank and the 
regulator is wide open. An improvement for some additional $$ would 
be to add an inlet regulator and gauge to the tester, then you 
wouldn't have to worry about this.

Now you've got air going into the cylinder and your engine is 
hissing away.  Write down the gauge reading. On my recently rebuilt 
Datsun motor, with 90 psi in, and #60 hole, cylinders read ~77 PSI. 
On the Jetta I bought with a burnt exhaust valve valve, the bad 
cylinder read 65 PSI. Your mileage may vary.

Now run around and figure out where the hissing is loudest. If 
loudest at the carb throat, the intake valve is the culprit. If you 
hear it at the exhaust pipe, the exhaust valve is bad. If its 
loudest at the oil filler cap, then worn rings are likely. You can 
also check the head gasket by looking for bubbles in the radiator, 
though you may have to remove the thermostat for this to work.

Seeing as this tool is not absolutely calibrated, its use is mainly 
as a relative measure.  All cylinders should be within several psi 
of one another. Furthermore, if you find a low one, the listening 
test should give you a good idea of where the problem is. This is 
why leakdown is a better diagnostic aid than a simple compression 
test.

It also a good idea to record leakdown figures for your race motor 
as soon as it is broken in. For one thing, this will tell you 
whether the rings and valves have properly seated (or, whether you 
will have to take the %#$!!XX%& thing back apart). Also, if you feel 
your new motor going soft in mid-season, you can do another leakdown 
test and compare with your fresh rebuild numbers. This is perhaps 
more accurate than seat-of-the-pants feel and fading memories.

Someone else on the net suggested taking apart a compression tester 
to get the whip and the spark plug inlet.    KD tools makes what 
they call a "valve holder" an adapter from 1/4" NPT to spark plug 
designed to put pressure in the cylinder to hold the valves up while 
you remove the springs to replace stem seals without removing the 
head.

Send inquiries to: Steve Soar, 3709 NE 157 CT, Vancouver, WA   98682

ssoar@tekig5.pen.tek.com

-------------------------------

Phil Ethier, THE RIGHT LINE, 672 Orleans Street, Saint Paul, MN   55107-2676
h (612) 224-3105  w (612) 266-6244     phile@stpaul.gov
"I like things that go fast" - Huey Lewis



<Prev in Thread] Current Thread [Next in Thread>