how do you do a leak down test on the cylinders, 82 928s

 

You need an air compressor and a leakdown gauge. Remove the spark plugs. Set the air compressor to around 90-100 PSI (or whatever the gauge manufacturer recommends), then screw it into the cylinder, while the cylinder has been rotated to TDC (you will need to use the firing order and numbers on the vibration dampner to determine which cylinder is at TDC). Remove the dipstick, air cleaner, and coolant expansion tank cap and pressurize the cylinder with the gauge. Read the pressure drop and calculate the % leakdown (or if you have a gauge already designed for % leakdown, read that off). Leakdown numbers should apparently be within the 3-15% range, depending on engine condition. Any more leakdown indicates problems. If you listen at the tailpipe, the intake, the coolant tank, and the dipstick, you can hear howling sometimes. This is the air escaping past the exhaust valves, intake valves, head gasket, or piston rings, respectively. If you hear air howling out of an adjacent cylinder, that may also mean a failed head gasket, assuming that the intake or exhaust valves aren't stuck open on that adjacent cylinder as well. Some people put a squirt of oil into the cylinder to seal the rings. If you have large leakdown values, you may do this to help seal the rings and thereby find other sources of leaks. I think I've covered everything. Good luck!

EDIT: Make sure your engine is in correct time first, as an out-of-sync engine will have insane leakdown through valves that shouldn't be open

 

Howling? Like . . .. "aaaaaoooooorrrrrrlllllllllllllllllll"? OMG, my shark has a little wolf in it. Not really, that would be a bad wolf.

 

Yes, not really howling like a wolf, its much much quieter than that. Hissing might be a better word in most cases, but I would hardly call the sound that comes out of the tailpipe "hissing". It's all semantics anyway. If you hear a sound there, you will know.

 

Pretty good write up, one thing I would add is you have to have a way to hold each piston at TDC on compression for each cylinder.

 

think i will wait for son to get home before i start this

thanks
stephen

 

How do you keep the pistons from going down with 100psi? put it in gear and put on the e-brake? also, helps to know the other 3 cylinders that are tied to their common TDC. (like 1 & 6 are on the same TDC cycle, but 360degrees off for the compression stroke, etc)

mk

 

True story. I have a pdf somewhere where I worked out what cylinders were closed off at a given crank position. I'll see if I can dig it out.

 

Boy was I all WET. However killed my post , THANK YOU. Brain was stuck on COMPRESSION test not leak down. I claim old farts syndrom CRS.
Thanks for the good HOW TO.

 

If that was a question I use the flywheel lock on a 928, others a wrench, airplanes I just really grab hold of the prop.