[wkenneth]

How does a leak down test work? I read about it in the pi compression thread and kinda wondered. Anyway, I gotta run a compression test on my #6 cylinder tonite, ugh. Since I was doing that and saw the leak down I was wondering how that was done. I have oil buildup in the cylinder and I hope to god my rings arent shot. I pulled the plug out and it has oil and crap deposits on it (Not metal, thank god, but crap nonetheless). So we'll see what that turns up. I think I've worked on my car more in these past 2 days than I have since I've owned it.
Maybe soon I'll be a 4.6 guru, I've almost got the haynes manual memorized.

 

[94 Daily Driven 4.6L]

Leak Down Test

A leak down test is similar to a compression test, except it will give you a much better idea of cylinder sealing. The way it works is that a set pressure is fed into the cylinder with the piston at TDC on the compression stroke (both valves closed). The tester reads say 100 psi on the inlet side of the tester and then maybe 90 psi being held in the cylinder. This would give you a 10% leakdown.

If you have excessive leakdown (>20%) you can listen to the crankcase breather, exhaust system, or intake system and you will hear the air escaping the cylinder. This will give you an idea of where your problem is (Crank = Rings; Exhaust = exhaust valve; intake = intake valve). If it is the rings, you can squirt some oil in the cylinder and manually rotate the engine a few times and test again. If the reading gets better, your rings are probably worn, if not, probably holed piston, ring lands, etc. Normal readings: 0% – 5% new engine; 5% – 12% normal reading; 12% – 20% normal for a high mileage engine; >20% worn engine. (Someone correct me on the numbers)

I have been told that only racing engines usually see 0% – 3% but I'm not sure on that. If all the cylinders are within 10% of each other in the percentage of leakage it is probably ok. If one cylinder is very low, you probably have a burnt valve or other major problem. Good luck and I hope this helped.

 

[wkenneth]

Any special tools needed to perform a leak down test? I have a good feeling #6 is almost dead. Theres a lot of oil buildup on the plug along with some crap deposits. So I'm pretty sure its a ring thing. Maybe I'm wrong and I need to try something else...

 

[94 Daily Driven 4.6L]

Leak Down Tool

You will need a leak down tester (which is basically a couple of gauges with a control valve) and a source of compressed air (air compressor). You might be able to rent/borrow the tester from Auto Zone.

Basically, all you do is screw one end into the spark plug hole and hook the other end to the compressor. Make sure the engine is warm and then make sure each cylinder you do is on TDC.

**NOTE**: When you apply the air, the piston may be rapidly forced down in the cylinder, so make sure everything is clear. And of course, always follow the instructions that come with the tool. Good luck.

 

[wkenneth]

ah hah!
Seeing as how I have an air compressor and should be able to find the rest I might do it later. Not now that its raining and lightninging (?) like a mofo....
Anyway..thanks man