You remove all the spark plugs first and then test one cylinder at a time with a compression gauge in spark plug hole.
All spark plugs need to be out so you can get good crank speed from starter motor and battery.
You need 4 or 5 "hits" of compression stroke to get accurate reading.
Write down results then move to next cylinder
After all 6 are tested remove highest and lowest number, then add up the other 4 numbers and then divide that by 4, that will be your Average compression number.
Divide that number by 10, that will be 10% of average, and any cylinder that is lower than 10% of average has a problem
There is no "running of the engine involved"
Say you got
#1 = 165
#2 = 130
#3 = 155
#4 = 160
#5 = 155
#6 = 135
You don't use 165 and 130 at this time, highest and lowest
Add up the rest.
155 + 160 + 155 + 135 = 605
605 divided by 4 = 151.25
10% is 15.12
151.25 - 15.12 = 136.13
So any cylinder below 136.13 has a problem
That would be #2 and #6, but thats pretty easy to see without all the math, lol.
If you have 2 or 3 cylinders testing at 160 and the others below 140 then averages won't help much, my guess would be valves at that point
Low compression cylinders should be retest, then test again "wet", wet test is when you add a teaspoon of oil to cylinder via spark plug hole, I use a straw, dip it onto oil bottle then put finger over the end and lift it out of the oil and place straw into spark plug hole and remove finger.
reinstall compression gauge and retest
compression number will go up, but how far up will tell you if it is Rings or Valves that is causing the low compression.
If it was 130, and goes up to 140 then valves are the issues, if it goes up to 150 then rings are the problem.
And compression numbers are not about the exact number, i.e. higher than 150, compression gauges are not calibrated to exact standards, lol, unless you pay to have that done, and batteries and starter motors are not all equal, so crank speed is a factor.
If you are getting 140 as the highest and 110 as the lowest that is fine, test all cylinders, the point is the comparison of the numbers, they should all be equal or within 10% of each other
Last edited by RonD; 12-03-2015 at 10:29 AM.