"Preferred" Cylinder Leak Test Procedure . . .

[tpenfield]

Not sure if I will start a Holy war by asking this, but . . .

I am wondering what the consensus is regarding the way to perform a cylinder leak test?

It seems, by checking various publications and forums, that the popular way is to have the cylinder at TDC (top dead center) on the compression stroke.( i.e at the end of the compression stoke)

There are some who profess BDC (bottom dead center) - could be either compression of exhaust stroke. The only thing is that you have to back off the rocker arms with this method as the valve(s) is/are still open at BDC, due to 'overlap' designed into most engines. The BDC is said to be 'safer' since the piston will say at BDC.

The TDC approach runs the risk of the piston turning the engine violently.

I prefer the TDC method since it also checks the condition of valve adjustment. It just requires more precise positioning of the piston at TDC. If you do the BDC approach, then you have to perform a valve adjustment after the leak test is done.

What do others have to say?

 

[Don S]

Re: "Preferred" Cylinder Leak Test Procedure . . .

TDC is the way to go to start with. Normally a LD test is done to find out if there is a problem with compression and where the leakage is. If you un-adjust the valves to start with you could end up masking the problem and never finding out it was a tight valve adjustment that caused the low compression. To make matters worse, you end up continuing on with troubleshooting for nothing.

The TDC approach runs the risk of the piston turning the engine violently.

Instead of plugging in 100 psi of air all at once, use the regulator to slowly bring up the pressure to be sure it's on TDC.
You can also turn the engine a few degrees either way to find TDC when you only have minimum pressure on the cylinder.

If you find the valves are ok on your original LD test, then you can back the valve adjustment off and check the leak down on the bottom of the stroke if you need.