Leakdown Tester: Correct Procedure

[304065]

I have the leakdown tester sold by Pelican, the one that comes in the red plastic case. The instructions are:

Connect the tester to an air supply
Turn the pressure regulator knob so the right-hand dial (the one with the % leakage on it) reads "Set" or the low end of the range.
Connect the cylinder hose and read the leakage.

Here's my quandary: when I adjust the pressure to "Set" I am only showing 35 PSI input pressure on the gauge.

Grady said that the leakdown can be done with low pressure, and should, to minimize the likelihood of the engine rotating, which will mess up the test and potentially cause injury. So I'm OK with 35 PSI input pressure.

But is that right? Is that the way the tester was designed?

[Rot 911]

35 psi should work, but makes the math of figuring out the percent of leakdown a bit hard to do in your head! Grady is right about problems with using high psi. However, I tend to just use 100 psi because I can figure out the percent of leakdown just by looking at the two gauges. But, I have had the piston move off of TDC when using the higher psi.

[304065]

Update: the leakdown tester works FINE at 35 psi. You carefully adjust the inlet pressure until the right-hand gauge, which is calibrated in %, reads zero. You then connect the air supply and the pressure builds up and you read the right-hand gauge.

A couple observations:

1) You MUST have the cylinder at TDC on the compression stroke. The trouble is, the pulley only has three marks on it (four if you count the advance mark) so you don't know whether it's at TDC on the compression or the exhaust stroke just by looking. The solution is to use a whistle that attaches to the leakdown tester-- it whistles on the compression stroke and stops when the piston is at TDC. You then look at the crank pulley and line up the mark and you're there. You could also put your thumb over the leakdown tester hose and just feel for the pressure, but I like gadgets.

2) If you get a high reading in one of the cylinders, put the plugs back in and start the engine and drive it under load-- often a piece of carbon lodges under an exhaust valve seat, giving an erroneously high reading.

I measured 3%, 4%, 6%, 80%, 5% and 3% my first time out. . . thinking a rebuild was in order, I fired the engine back up and poured some B-12 chemtool down the #4 stack. . . shut it off and retested, 5%.

3) You may want to make up a tool such as the one Grady Clay described, made from a couple old spark plugs, to make it easier to get the leakdown hose in there. I used a piece of aluminum tubing I had lying around, cut to about 6" long, that just fit around the leakdown tester's spark plug fitting-- that way you can easily screw and unscrew the leakdown tester without trying to twist the hose itself.

It's GREAT to know you don't need a top overhaul!

[aigel]

John,

Instead of the whistle, you can just look at the distributor. If it points to #1, you are in compresson on #1, then just go through the firing order. But the whistle is more sexy, all right.

George

[911pcars]

...or measure the circumference of the crank pulley, then divide by three. From the factory's TDC mark, measure in either direction to locate the other two marks. Each mark will thus be 120º apart. This represents the difference between cylinder firings. Start on cylinder #1, then rotate 120º and test the next cylinder in the firing order, and so forth.

Watching the rotor may not get you close enough to TDC. The distributor only rotates at one-half the speed of the crank.

Hope this helps,
Sherwood

[304065]

[john walker's workshop]

my snapon tester has only one gauge. set it to 0, and go. the engine should not turn by the air pressure if you carefuly line up the pulley marks to the fan housing mark. like mr.cramer says, don't freak out if you get a high reading at first. it may be a bit of carbon under the valve from removing the sparkplug. run the engine again and retest. then check for a tight valve. then freak out.