I suspect a bad head gasket on my 1997 L (2.2 litre engine) and would like to do a leakdown test. My concern is that some people say to have the piston at TDC, while other people say to have it at BDC.

For those who say to do the test at TDC, how do you ensure that the air pressure doesn't push the piston down? I am particularly concerned about the motor spinning backwards, which seems like a bad thing.

For those who do the test at BDC, how do you ensure that the valves remain closed during the test? Can you back off the valve adjusters enough to keep the valves closed?

Every leakdown I have ever done has been with the piston at top dead center, if you are spinning the motor with air pressure you using to much or applying it to fast.

slowly apply air pressure to the cylinder till you see the motor starts to turn, then test it just below that pressure

Jay

The reason you test at TDC is to ensure that all the valves are closed.

If you get it exactly at TDC, the crankshaft won't turn. If you need to, you can apply a extra holding force by putting the transmission in 5th gear (if it's a 5MT) or by jamming something in the flexplate teeth through the inspection hole (if it's a 4EAT).

I don't see why turning the motor backwards a little would hurt anything.

I dunno Ive turned over Lincoln Navigator motors with air pressure with the piston at TDC, but most of the time as long as your paying attention and applying pressure slowly, and not to much it wont turn over.

The reason the motor turns over is because the valves are closed and the piston is at tdc, the pressure has to have some affect somewhere, thats why you use air to pressurize the cylinder, because it will escape through worn/bent valves or a damaged cylinder, rings etc.

Jay

Yeah, I guess if the engine's healthy you have a higher chance of pushing the piston down. :)

...I don't see why turning the motor backwards a little would hurt anything.I'm not sure about that, either. Motors with timing chains aren't supposed to be turned backwards. I don't know whether that same rule applies to motors with timing belts.