Leak down

[Kevinwilliams]

Can some one tell me....

When doing a leak down, do you pressurize the cylinder then remove the pressure to take the leak down reading. In other words read with pressure in the cylinder at 100psi but not continuously supplying pressure.

Thanks

Kevin

[haycait911]

need to continuously supply air under pressure.

[Kevinwilliams]

Ok…

I am tired but having a real problem doing a leak down on my 911. I have very good compression numbers, about 155 to 160 across 6 cylinders.

On the leak down, going to top dead center and following the firing order with 100 psi constant air pressure supplied, I am getting major air hiss on all cylinders and about a 90% leak. If I let the piston turn until it settles (this seems to take the crank 120 degrees off what the firing order is), I have great leak down numbers with under 5% leak down with constant air pressure supplied. If I supply 100psi then shut off air I get under 5% leak over time…

What am I doing wrong????

[haycait911]

I usually connect to a cylinder and turn engine by hand until the reading is at it's lowest. then move to the next cylinder. seems to work fine and saves a lot of time. you have to hold the engine in place because the air pressure will want to push down the piston in the cylinder you're measuring. if you let the engine settle with pressure on one cylinder that piston will be forced to the bottom of the bore, not TDC. thinking about it, that really shouldn't affect your leakdown # because the valves aren't open. so that must be why you're getting decent #'s.

[Kevinwilliams]

OK...

I need to approach this fresh, I do very much appreciate your replys. I can make good numbers, they are just not getting there like it is suposed to. Like you say, and I have logic-ed it through I should not be able to put the piston in a place to give me phony numbers... if the valves are closed and the piston is somewhere, it should not leak.....

It is just not working like the instructions???

[2.7RACER]

Tell us what the reading is on the two gauges when you are doing the test.

[Kevinwilliams]

I was way too tired last night when I was doing this. I was getting readings at 5% to 8% and was looking at the gage wrong. I was thinking that it meant it was leaking all but 5% and that when the cylinders were in the wrong place and I was getting close to 100% that it was showing me a good reading. I woke up this morning, went back at it and realized that the 5% (Where the gage says “set”) that is the good reading. For a guy that is a pretty decent mechanic, I can sure be a bone head sometimes. For the few of you that jumped in to help, I thank you very much.

Kevin