Ralph77

So I am trying to get a handle on how to do a leak down test on a 2000 4.0 XJ.

Steps appear to be:

Take the radiator cap off.
Get her up to operating temperature.
Pull all the plugs, air cleaner, oil cap, and dip stick.
Find top dead center on the compression stroke.
Set the regulator on the air compressor for 10 lbs higher then what you are going to put through the leak down tester.

So being that it is an '00 there is a coil bar and not a distributor.

So I should feel air coming up out of the spark plug hole meaning it is on the compression stroke when turning the engine clockwise.
With a extension in the spark plug hole find the highest point it sticks out.
Check that cylinder.

Can you turn the engine counterclockwise if you over shoot top dead center by a bit?

I found this:

The firing order helps identify the cylinders, and in the Jeep inline six, the order is 1-5-3-6-2-4, making #1 and #6 pistons reach TDC at the same time. Remember, too, that we're working with four-stroke engines, and the piston must be at TDC on the compression stroke, not the exhaust stroke!

So it appears that #1 and #6 are at top dead center at the same time on the compression stroke.

Is that true? If so does that mean other cylinders are too? Figure it would save some time and trouble being able to test 2 cylinders at a time.

Now a little embarrassed by this question. When setting the regulator on the air compressor, will use 110 1lbs at the compressor side and 100 lbs for the leak down tester, is it set once the compressor filled and shut off? No air running to the tester?

Any insights would be appreciated. Hoping to run a battery of tests this weekend to see if I can get a handle on the condition of the engine in my Jeep. Not having any problems of any sorts as best I can tell but would like to know.

5-Speed

Ralph, you are over thinking this! It is honestly simple to do but there are a few different routes you can take. A few people wont agree with how I do it but there is a reason I do it this way and I will get to explaining it later. Here is how I normally do it.

- Run engine up to temp
- Remove coil rail and spark plugs
- Remove valve cover
- Remove rockers

At this point, its time to plug in the leak down tester. Now you might have noticed I said nothing about TDC. The reason is, 99% of the time when you fill the cylinder with air, it pushes the piston all the way to the bottom on its own, even if you get it at TDC. On top of that, I would rather the piston be all the way down cause thats where the cylinder wall wears most.

What kind of leak down tester are you using? You normally need to zero it out based on what the input psi is from the compressor. I typically leave my compressor around 130 psi and adjust the tester accordingly.

Ralph77

OTC 5609. Meant psi not lbs as I wrote above. 100 psi with the leak down tester makes the math to get the percentage easier. My Jeep Service manual and the instructions for the tester themselves are where I am getting the instructions from which seem to match most instructions I find on the internet.