Make a Leakdown Tester
Wed, 25 Apr 2001 01:05:08 -0400
Most of us have a compression tester . . . nice to have . .but does'nt help
much in identifying problems. Leakdown testers inject air into the cylinder
and will tell you how much leakage exists. 5% is ok . . .. 20% is bad.
Got a bad leak? Listen for it. . hissing in the crackcase breather means
bad rings . . . hissing in exhaust means bad exhaust valve . . . .hissing in
manifold means bad intake valve. Intermittant readings? . . sticky valves.
The following sites have pix, sketches and theory of operation.
Parts you buy from Grainger:
2 - 100 PSI Pressure Gauge - 4X512 - $3.18 ea
1 - 1/4" Air line Regulator - 2Z767 - $16.04
1- 1/4" Male NPT Plug - 2X169 - $.98
1- 1/4" Male NPT Coupler - 4X397 - $4.01
1 - 1/4"x 2" Pipe - - Plumbing Store - $.50
1 - 1/4"x1" Pipe - - Plumbing Store - $.50
1 - 1/4" Pipe "T" - Plumbing Store - $1.00
1 - 1/8" to 1/4" female Adapter - Plumbing Store - $.50
I did not need to make a sparkplug adapter hose. I used the one from my
compression tester. If you dont have one, one of the urls listed above show
how to make one.
Before you slap the parts together, you need to make the "Damper Valve". I
used a small wad of putty epoxy pushed down into the 1/4" x 2" pipe. Stand
the pipe on a table and ram the epoxy down with the head of a large nail.
After the epoxy cures, drill a .040" hole (#60 drill) thru it.
Assembly from left to right, following air flow, is as follows:
Use Teflon tape for sealing joints.
1/4" pipe and adapter
Male NPT Plug ........... air regulator . . ........1/4"x2" pipe(Damper
Valve) . . . . . . . 1/4" "T" . . . . . . . . Male NPT Coupler
Theory of Operation:
"The tester works by measuring the pressure drop across a flow
restriction(the #60 hole in the pipe). The lower the pressure reading on the
gauge, the worse shape that cylinder is in. 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."
How to Use:
1. cylinder at TDC
2. Attach hose to spark plug hole
3. Attach tester to air supply
4. Adjust air regulator till both gauges read 100 lbs
5. Attach spark plug hose to tester
6. Observe reading on guage #2
Interpretation of readings:
example: guage #1 reads 100, guage #2 reads 90 ------> you have 10%
Test results for my 88 5kq;
When I bought the car 2 years ago, #1 piston had a compression reading of
65. All the other cylinders were up there near 200 lbs. I tested the
cylinder 4 times using two different gauges. Talked the owner down from
$4000 to $2800. Car was from Va . . no rust . .clean . . bought the car. Car
runs fine . . .plenty of power . . just has rough idle.
cylinders 1, 2, 5 indicate 5% leakage
cylinder 3 indicates 30% leakage . . OUCH!
cylinder 4 indicates 40% leakage . . OUCH!
What the heck! What happened to cylinder 1! Miracle cure?!
Went back to test cylinder 3, now it reads 5% leakage . . what the * % $#
Test cylinder 4, reading 30% . . . I rap the block with a sledge . .
.reading falls to 5%. AHA! sticky valves.
Somebody tell me how to cure sticky valves.
I put in a can of Lubro Molly valve cleaner in the tank . . . when that gets
done . . I'll put in some Techron. Will then retest.
I would like to run about a quart of water thru the engine to clean it. Easy
to do with a carburetor, but not so easy with fuel injection. Anyone use any
of the vacuum lines for introducing the water spray?