How to build and use a leakdown tester

[gboezio]

Since I had to explain to many people in the past weeks how to build and use a leakdown tester, I thought that it may be a good idea to make an official leakdown testing thread.
Leakdown is measured by applying air pressure to a cylinder set at the fire stroke top dead center (TDC)
First, you need a compressor and enough air hose to get to your car.
Don't tear your engine apart yet because you need to go to the hardware/tool store
Here is a picture of the one I built
From the left, Regulated air pressure comes in from the left male fitting to the first block, the gauge reads the base pressure.
In the pipe between the two gauge there is a reference restriction, this is just a small hole drilled in a plug that measure .040 inch or 1mm. The plug can be a bit of anything, some people are using epoxy glue, some tap threads and screw an allen plug, I soldered a piece of copper with lead. Then you have to drill it with a .040 drill bit (tool shop)
Past this restrictor, comes the next junction block with the second gauge that reads cylinder pressure.
Then comes a female and male fitting that separate the nest part of the tester, this is not required but I find practical not to have to rotate the whole thing while screwing it in.
Then a 1/4 NPT to 1/8 NPT reducer, a grease gun hose and at last an old spark plug socket. To make this socket, cut the plug just above the hexagonal part with a zipcut or saw, all the way around to the ceramic, then drill and tap it 1/8 NPT to fit the hose and take off the crush ring and replace it by a nice o-ring. You can use pieces of wood not to damage the threads.

Now the test, take off spark plugs valley covers, unscrew the coil packs bolts, remove the wires from the ECU, pull the coils, remove the five rear spark plugs leaving only the front one, make room to access the crank bolt. On my M60 engine I had to remove the radiator fan. Put some cardboard to avoid breaking your radiator with a 400 $ bad move.
Time to calibrate the tester, while air is blowing out of the end of the tester bring the first gauge to 100 psi with the regulator knob, then take the grease gun hose off to stop the air flow.
Now you need to bring piston #1 at TDC (the point where it's not going up or down) of the compression stroke, take a wrench and turn the crank clockwise until you feel some resistance (this the compression on cyl #1), then remove the last spark plug. Insert a small piece of wood or plastic that is clean, at least 10 inch long, in cylinder #1 plug hole until it touch the piston. Then turn the crank until it go down, then back up until it's at the highest point. REMOVE THE WRENCH, (the engine could rotate and it can damage something) and the piece of wood and screw the tester hose in the plug hole, it should go easily by hands, it don't need to be tight with the rubber o ring, then apply air pressure by connecting the other half to the hose.
The difference between the 2 dials is your engine leakdown.
There should be no more than 5 psi.
Now if there is more than 5 psi open your oil cap, air filter box, coolant tank and listen to your exhaust pipe for air escaping.
Intake or exhaust means bad valve sealing.
Bubbles in the coolant means blown headgasket.
Some crankcase blow by is normal so you should be able to hear something from the oil cap, but excessive blow by means poor ring sealing.
If there is a lot going out of intake or exhaust, the engine may have moved back (intake) or past TDC (exhaust). It's also possible that you have the wrong cycle (exhaust TDC instead of intake TDC)
Firing order for inline six is 1 5 3 6 2 4
V8 1-5-4-8-6-3-7-2 (1 to 4 from the front passenger side to the back and 5 to 8 from the front driver side to the back)
Now you have to repeat this for the next cylinder on the firing order, find TDC and test, hear, then turn crank for the next cyl and so on... On a six you will have to turn 1/3 turn (1/4 on a V8).
Furthermore, it's a great time to change your spark plugs and maybe even test the primary resistance of the coils with a multimeter. (less than 2 ohms on the outer pins)


My comments,
there is a but, the measurement that we are taking is from a reference leak, so the results are percentage of the air that can flow trough the reference hole, the smaller the hole the higher will be the pressure difference. Lets say you measure 50 % of the first dial, this means that you measure 100 % leakdown. It act like serial electric resistors, you are measuring between two equal resistances, so you get half voltage (pressure). The .040 hole seem to be the reference used by most people that are building such thing.
So when we say I have 5 % leakdown, 5 % of what ?
What I suggest is to try the tester on a healthy engine to see how a good engine test as a reference for you car, in that case the reference hole could be smaller and the pressure difference will be higher increasing precision.
This is all I know on leakdown testing, hope this helps some people, feel free to post if you have a comment or question or if I made a mistake somewhere.

[MackinE46]

thanks for taking the time to write this up

[Antrieb]

Nice write up I'll have to do this to my new 4.0 v8.

[belligerent]

whoa crrazy write up ! Awesome though

thanks ! I will definitely try it sometime .

On a similar tester what is the 'good' range for leakdown ????

[gboezio]

For my 170k miles M60 it ranged to 1 to 4 psi @ 90 psi (bad compressor) as I said a smaller reference hole will broaden the range and could be easier to read, but it's hard to find a drill bit small enough, I thought of using a carburator jet to increase precision. As you can see on the pics, the opened valve gave a 15 psi reading, I would rather see pressures close to zero but this setup still give good results if you can compare to a known healty engine. I will definitely try the ref reduction when I'm done with my garage.