Interpreting Cylinder Compression Readings

Question

So I've been having some engine trouble with my 98 Mazda 626 GF 2L, specifically a low, rough idle and lack of power on acceleration. One of the things the WSM says to check for both these problems is compression.

So I did both dry and wet compression tests and came up with the following numbers. I turned the engine over once each time, then went around to the engine to record the reading ( real pita and took awhile ):

While I'm inclined to think compression is not the source of my problem there are some things going on here which I don't understand on cylinder #2. The first time I cranked it both times I got a really low reading, so I released the pressure and started over, I may have done too short a crank. Both times, after finishing the fourth cylinder I went back to the second one to take an additional set of readings. On the wet test I even did a third test. I'm wondering if maybe there is something going on with the spark plug threads in that cylinder, maybe there's a crack there or some other problem with the threads.

Answer 1

From the looks of it, you're going about your compression test incorrectly. You should rotate the engine, using the starter, for the same number of revolutions with each cylinder. This will give you a truer tale of what's going on. Usually 4-5 revolutions gets the job done. The reason you want the same amount of revolutions for each cylinder is so the tests will be even. You can do this for both dry and wet. There really should only be one reading per cylinder for each of the wet and dry cycles.

As for your numbers, it's really hard to tell what's going on with all of the numbers you have present. I'm not understanding #2 either. From the looks of it, the wet was lower than the dry on the first go around.

Looking over the numbers, if we take a look at your wet numbers and assume the second column on the second cylinder is good, your numbers are spot on. Your #4 has a little higher reading, but everything else looks good (if not a little high). According to what I saw, your compression for the Mazda 4-cyl should be between 120 to 180 psi.

There are two main things you are looking for when doing a compression test. First, you are checking to ensure the numbers are within range of where they are supposed to be (the 120-180 psi numbers), which your numbers are. The second thing to look for is that the numbers are all within a certain percentage of each other. I don't know specifically what that should be for your Mazda, but as it sits, it's at about 14% difference between cylinders 3 and 4 on the dry tests. Considering the wet tests appear to be right in line with each other (181, 180, 181, 185), I'd actually wonder about the validity of the dry test on cylinder #3 and redo it at some point. But the numbers on the wet side looks right were they should be. If the engine has a few miles on it, you'd expect there to be a little wear, which is what the tests are showing me.

If you are wondering about a good procedure for doing your tests, I found this one by Eric the Car Guy which is pretty good. He's a little long winded, but he gets the job done.

Answer 2

Not sure why you have so many numbers. You should just keep cranking the engine until you get a steady reading on the compression gauge. From the numbers you have, it looks like No3 could be a little low due to piston ring wear, since it seems to improve the most in the wet test, but it doesn't seem too bad.