How can you do a cylinder leak down test? What, if any, special tools are needed.

- Brett Smith

Hey Brett,
You need a special leakdown tester. It measures the amount of air leaking from the cylinder when it is at TDC.
You should be able to buy one for around $50 from most auto stores.
Here is an article about building your own.
http://www.xs11.com/tips/misc/misc3.shtml
Good luck!
Dave

Not sure what you know of them but you remove the spark plug and screw in a hose into the hole. Then you take compressed air and connect it to the guages. The piston should be at TDC and you should have the wheels blocked so the car doesn't run you over. (almost seen this happen). If you do get excessive leakage you can diagnose where the leak is. In no particular order. With the cylinder presurized listen inside the intake for escaping air. If you hear some, your intake valve is not sealing. Then listen for air at the exhaust pipe, and if you hear air your exhaust valve is not sealing. Then take the oil cap off and listen for air their, if you hear some it's your rings. There will always be some air leaking, nothing is perfect. I belive 4-8% is still in the good range but this will clearly be marked on the tester. Good luck

You do this while your engine is at operating temperature.


M

Thank you very much guys.

Ran a compression check on my motor today, because theoretically the rings are bad, but they all came up to 181 psi, so the leak down is a definite now. I was hoping that the problem was going to be quite evident, and so far, it's not.

- Brett Smith

Glad it wasn't evident in your cylinders..



M

the oil ring doesn't hold much compression but it does keep oil out of the combustion chamber. you can have high compression (and good leakdown numbers) -and- bad oil rings ...

Hmmm... That's interesting. I am trying to diagnose the problem before I spend all the money to tear the whole thing apart. Labor is not an issue, but the money for potentially wasted parts is.

Thank you though. I am hoping the problem is going to be real evident, and so far, it hasn't been.

Fucker.

- Brett Smith

well - let us help you.

what is the engine, and what is the problem ?

It's an 88 911 3.2 that's in my 914. I can't tell if it has blow by at idle, but when there is pressure on the motor, i.e. in gear driving, and I punch it hard, like passing someone narrowly, and it shoots oil out the overflow tube on the oil tank.

We are thinking that maybe, just maybe, the drain tube, since it is stock for the 2 liter 6, doesn't flow enough air. So we are going to look at doubling the size and seeing if that solves the problem, also.

The motor is not overfull of oil, I have ran it down off the dipstick and it still does it. Then of course, I put it back to midway on the dipstick.

Any idea's on what to check?

- Brett Smith

Just as an extra tidbit, you don't have to have guages to do a leakdown test. The guages just tell you how much air is leaking. You can just use a high pressure hose that has a quick disconnect on one end and a threaded connector on the other. Thread it into the spark plug hole while the piston is at TDC compression and plug in the air and listen as described above to see where things are leaking. This is really effective when you have a cylinder with low compression and you want to find out if the head is the problem or if the rings are the problem.

K-D has such a tool, with adaptors for a couple popular sizes of spark plug. found it on the pegboard of the FLAPS for about $6. it's also used to apply air pressure to a combustion chamber so you can replace valve springs without the valve falling into the open cylinder and i think the name on the blister pack is aimed at this use "Valve-Holder Tool" or somesuch. NPT fitting so you can just screw on the kind of shop air adaptor you use and rock...