leak down cylinder test questions

Hi folks. My '75 CJ5 258 has been blowing blue/black smoke. It'll foul out all 6 new plugs uniformly and pretty quickly. Since purchasing it in March, I've changed to high ZDDP SAE 30 motor oil. I noted the PCV system was completely misrouted and was hoping that was the reason for the smoke (the PCV valve was being routed into the carb fuel bowl breather), but since rerouting the lines in the factory manner, there has been no improvement in the tailpipe emission.

Several weeks ago, I performed a compression test and got between 145-150psi per cylinder. That seemed good. So I suspected valve seals only because I figured cylinders were good and valves must be seating properly to get those readings. I realize those assumption could be way off.

Today I attempted a leak down cylinder test. I only got as far as cylinder #1 and #2. This is my first ever test, and I think I was making some rookie mistakes.

First the numbers. My input line was 90psi. On cylinder #1, the right side [cylinder] gauge read about 71psi. That's a 20% loss which by most measures is too much. I'll add that the sound of the leakage was detected via the crankcase (oil fill cap). On cylinder #2, the right side gauge read about 81psi. That's about 9% leakage.

Okay, the questions. First, my engine doesn't run at the moment so I couldn't warm it up like most all directions say to do. How much of a problem is that or how should I figure this into my results?

Secondly, while I figured putting crank timing mark at the 0° pointer for cyl#1 on the compression stroke was a good way to ensure it was at TDC, I'm completely unaware how to tell cylinders 2-6 are at tdc. I did pull the valve cover to watch the rockers. I figured if I could finger twist the pushrods, that meant those two valves for the cylinder in question were shut. I then reckoned well heck, this cylinder doesn't need to be at TDC, it just needs to have the valves shut to do a leak down test. Please confirm or deny. If you deny, please tell me a good way to determine how pistons 2-6 are at TDC (that is, if it's important to have a piston at TDC to complete a leak down test).

Thirdly, I yanked all 6 plugs to do the test. This made the engine easier to turn by hand. However, when pumping 90psi into the cylinder being tested, the crank would inevitably rotate a bit and open a valve, thus nullifying my test. I found it difficult to keep the piston from moving. I ended up putting 5 plugs back in for cyl#2's test. But this made it less enjoyable to turn the crank by hand. What is common practice here? I was thinking this evening that maybe I should use less pressure (60psi ?) without the plugs installed.

 

check it out

 

Thanks for the response.

I finished the leak down test on all 6 cylinders. For closure purposes, I'll put the results here. Before I do, I'll offer up that for my 3rd question above, the answer is indeed that the piston needs to be at TDC. Once I determined TDC for the others, 90psi input didn't move the crank, even with all plugs removed. So it takes much more force to move the piston at TDC than it does any any other point in the rotation, which makes sense if I think about the leverage angle of the rods to the crank. Also, I only needed to find TDC for three pistons. Cylinders 1 and 6 were measured at the same crank position, 2 and 5 at a second crank position, and then 3 and 4 at the third and final crank position. I used a plastic straw inserted through the plug holes with marks made from cylinder #1 at its TDC as indicated by the crank mark and timing tab on the block to gauge the TDC-ness of the rest of the pistons.

The numbers:
cylinder 1: 90psi in / 70 psi holding // loss of 22%
cylinder 2: 90/70 // loss of 22%
cylinder 3: 90/59 // loss of 34%
cylinder 4: 90/34 // loss of 62% {{<--winner}}
cylinder 5: 90/66 // loss of 27%
cylinder 6: 90/79.5 // loss of 12%

For all measurements above, I verified the leakage was coming from the crankcase (intake, exhaust, and radiator showed no evidence of hissing). Unless anybody sees something wrong with how I performed this test, it looks like my rings are shot. So it's a rebuild for me.

 

It would surely point to the Cylinders having an issue.........could be cylinder bore is now over sized, piston skirts worn allowing the piston to rock in the bore, broken or worn rings...........or all of the above.....If you have it apart may as well freshen it all up including the head. Good Luck!