In my quest to figure out the misfire of 5 and 6 on my L6 I got a leak down tester.

Unfortunately, it seems I got one of those cheapos that need zeroing every time and max out at around 10 PSI before the needle goes off the scale.
It appears that proper tests should be done at 80psi.
If I give it 60 - 80 psi the engine rotates which is understandable and starts opening a valve ..... so I back off on the lashing adjustment till the push rod spins. I hear air escaping best I can tell into the block past the rings.
Compression test was all within 10%

I have another gauge on the way with PSI on both gauges hopefully will give me better insight.
Am I spinning my wheels checking leak down when compression is good?
Is backing off the lash adjustment sufficient to test that cylinder properly?
Will the leak down test detect a leaky intake gasket?(tried the carb cleaner rpm test with no effect)

What are the proper steps in a leak down test am I missing something?

Thanks

Tim

 

What vehicle are you working on and how many miles does the engine have on it? Any number of things may be going on. All of the cylinders could have some leakage. The leak down tester is suppose to tell you if a cylinder is holding pressure but you say you can hear air leaking by them. Is it leaking on all of them or just one. I had a similar issue last year and the problem wasn't with the cylinders, the timing chain needed to be replaced. You can tell if an exhaust valve is bad by loosely holding a dollar bill over the exhaust pipe while idling. If the bill flutters then the exhaust system should be alright. If it tries to suck the bill into the pipe then you need a valve job.

 

Full discussion here

 

Do you have a tool to check to see if #5 & #6 plugs are firing?. Assuming you are past vacuum leaks my guess would be an issue with the distributor if you are getting no spark or a weak spark on those two. . .

 

I have a spark tester and both #5 and #6 show spark.

 

You have to have the cylinder you are testing exactly at top dead center. If it is either way from top dead center the air you put in will make engine turn- either forward or backward. How do you get it exactly at TDC?? I don't know!!! Sort of easy to do on #1, just use the timing mark, but the other 5???? I have never done a leak down test. Good luck on it, I know you are really trying.

 

I am thinking(hoping)by backing off on the lash setting will close any valves that may have opened in the slight rotation.

 

You can rotate the engine to top dead center on #1 and insert a wooden dowel rod to the top of the piston and mark the depth. Then use the dowel rod to measure the rest.

 

Sigh.

DO NOT bother to dick with backing-off the lash adjustment. It's a waste of time for leakdown testing.

I've heard of Hazzard Fraught selling junk leakdown testers that "test" at 10 or 15 psi. They're complete crap.

The FAA demands a leakdown test at 90 psi. Most guys test at 100 psi on automobiles because the math for figuring leakdown percentage is easier. (100 psi input pressure, 80 psi cylinder pressure = 20 psi leakage, which is also 20% leakage.)

Finding TDC-Compression is easy. Set your leakdown tester to some random low pressure--10-ish psi, let's say, and plug it into the cylinder you want to test. Put a socket and long-handle ratchet on the torsional damper bolt, or grab the alternator belt and make like the Hulk. You're trying to turn the crank in the normal direction of rotation. If either valve is open, your gauge will show "0" psi, or close. Keep turning. At some point, both valves will be closed and your needle jumps to whatever low pressure you set originally. At that point, turn the crank in short, sharp "bumps". Each time you bump the crank, the needle jumps UP, and then leaks back to your set pressure. When you get near TDC, the needle doesn't jump UP very much at all. When you go past TDC, and bump the crank, the needle jumps DOWN, and then recovers. When the needle jumps down, you've gone too far. Back the crank up a little and try again. You should be able to "feel" TDC with ten or fifteen PSI in the cylinder. That'll get you close enough to TDC to test leakdown. Disconnect your gauge hose, re-set the pressure to 90 or 100 psi--or whatever your gauge system uses--and plug the hose back in.

Whatever reading you get--5% leakage, 40% leakage, whatever it turns out to be--had better be fairly even for all six cylinders. Ideally, you'd test a "known-good" engine of the same bore size as yours, so you have some idea how YOUR leakdown tester reacts. They're NOT all the same. If someone tries to tell you that some certain percentage is "good", you're being misled. That percentage may be "good" on HIS leakdown tester, YOURS could be different.



You've proven on your other thread that those cylinders have compression, and you've said that the plug wires are installed in correct order. You have NOT proven that you have spark ACROSS THE PLUG GAP on those two cylinders. You have proven you have spark across your in-line spark tester, but you could still have defective or fouled spark plugs; or an air/fuel issue. I'm leaning toward spark plug problems-fouled, or cracked porcelains.

And I still think your PCV and brake booster hoses are reversed.

 

The plugs in 5 and 6 look like they never have been fired (see pic in other thread)
Not sure what the chances of 3 spark plugs being bad, but in number 6 I tried the original plug and two different new plugs (I haven't yet tried a plug from a working cylinder)
I did see spark in number 5 when the borescope was inserted while turning the engine over.

Gonna take a closer look at the valve movement on 5 and 6, at quick glance it looks no different than the working cylinders.

Thanks for the help.
​​​​​​​

 

Get the cylinder you want to test at TDC, then clamp some vise grips the the flywheel on either side of the engine. Position then as close as possible to something solid. Then, when the engine tries to rotate, it will rotate only as far as the vise grips allow.

You could probably do the same thing at the front of the engine on the balancer.

 

The leak test checked out without issues.
After setting the valves to zero lash all cylinders are now firing and the misfire is gone.