Compression test

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Seems to me that I read once that while doing a compression test you should have the throttle open while turning the engine over. True ?

Bob
 
As well as the other spark plugs removed. I also like to repeat each cylinder a couple times; the readings will vary from test to test so I sort of average them for each cylinder.
 
warm. So you don't burn the hell out of your hands while pulling plugs and screwing in the compression gauge. If carbed, make sure the choke is fully open.
 
You might also want to check the gauge, particularly if it has not been used in a while. After getting somewhat erratic and low readings when I was testing, I hit the check valve at the end of the gauge with a little WD40 and let it sit overnight. After that, the gauge was very repeatable and the readings were higher. I think the check valve sticking was causing the problem. I also checked it against another pressure gauge and they were very close (they could be both off I suppose).
 
dllubin said:
they could be both off I suppose
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Actually it is important (in my opinion) that you get fairly CONSISTENT readings, rather than extremely ACCURATE ones. The differences between cylinders is as important as the actual numbers. A little low, but even across the board is better than higher but very uneven.
 
Dr.Jeff said:
Actually it is important (in my opinion) that you get fairly CONSISTENT readings, rather than extremely ACCURATE ones. The differences between cylinders is as important as the actual numbers. A little low, but even across the board is better than higher but very uneven.
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I agree. If I recall, within 10% of each other is within reason if all is well. I also do each cylinder (warm engine, WOT) twice as Jeff suggested. I just did mine, before pulling the motor so I would have a record of the values:

#1 186
#2 190
#3 200
#4 202
 
Dr.Jeff said:
Actually it is important (in my opinion) that you get fairly CONSISTENT readings, rather than extremely ACCURATE ones. The differences between cylinders is as important as the actual numbers. A little low, but even across the board is better than higher but very uneven.
Click to expand...
All the more reason to make sure the gauge has repeatable readings.
 
One way to help with repeatability is with repetition. If you are not 100% confident that your compression tester is giving readings that are quite as consistent as you like, then you can repeat the process several times and average the findings. Randomization is a key method in scientific research and to some extent may help in this case. The more times you repeat the test, the less likely an erroneous reading will influence the outcome. I've found that doing compression tests can be a little awkward at times. Depending on the situation it might be difficult to get the fitting well seated into the spark plug hole, or the starter might not be cranking the engine at the same speed every time, or there might be some oil in a cylinder or two (as seen on the end of the tester fitting when removed), engine temp might change significantly during the process, not being to clearly see the gauge in certain positions, etc, etc. So repeating the process will help rule out these issues. Perhaps try starting from cylinder #1 one time, then from #4 the next. Anything to make it more random with additional attempts.

Also make sure the battery is fully charged and working properly, there are no obstructions in the intake or exhaust, and everything is clear and safe before cranking the engine.

Another thing I like to do is observe how the needle jumps with each stroke (pump) of the cylinder as the engine cranks over. The first jump might differer a lot depending on the piston's location in the cylinder when the initial rotation occurs, but after that all cylinders should build pressure at about the same rate (pounds increase per stroke and number of revolutions until it reaches the max). The maximum reading is not the only thing a compression test tells. How it gets there tells you something about the health of that cylinder. If you suspect something funky is going on, consider how it progressed. For example the maximum reading might look a bit low, but it built up pressure strongly in the process. As opposed to having good max pressure but a lack luster build to reach it.
 
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