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Old 12-13-2015
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Compression Test Readings

So I did a compression test on my 94 3.0 ranger 235,200 mi. During the static compression test with fuel pump disabled I got a 125 psi on all cylinders after cranking for about 4-5 seconds. During a running compression test I got a steady reading of 75 psi on each cylinder after resetting gauge 3 x's after 3 cycles at TDC. I thought these readings were great since the engine has so many miles on it...Am I correct with this assumption ???
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Old 12-13-2015
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Running compression test???

Point of compression test is to see if any cylinders are reading more than 10% different than the others.

3.0l engine runs 9.3:1 compression ratio, at sea level to 2,000ft that would mean you would expect 165-170psi
18 x (compression ratio) 9.3 = 167.4
18 = 15psi air pressure + mechanical compression of piston and rings, some used 18.3, I find 18 to be better in real world, lol.

But you also have gauges that are not calibrated and batteries or starter motors that can't get max crank speed so lower numbers come up.
You are just looking at an average psi then any cylinders that are more than 10% lower.

Compression test
Cold engine
ALL spark plugs removed, MUST have all spark plugs removed or don't bother doing the test
Screw in compression gauge, rubber plug gauges don't work well in this application
Hold gas pedal down all the way, this turns off fuel injectors and allows maximum air into engine.
Crank starter motor
Listen for 4 or 5 compression "hits", you will hear them.
Write down result and move to next cylinder.

Once all 6 are done, add up the middle 4, so don't use lowest or highest.
Then divide by 4, that will be your average
so 160 + 162 + 158 + 168 = 648 divided by 4 = 162

10% of 162 = 16.2
162 - 16.2 = 145.8
So any cylinder at 150 or lower would be suspect

If there is one, add a teaspoon of oil to that cylinder and repeat test, it will go up but by how much will tell a tale
If it goes up close to average(above 155) then rings are the problem
If it only goes up a little then valves are problem.

All cylinders at 125psi would be impossible, between 120 and 130psi possible.
Compression test is not about highest numbers possible, you do need to be cylinder specific to get any usable information

Then you can add up the middle 4 and divide by 4, then get your 10%, and in this case 10% would be 12, 125-12=118, so a cylinder with 118 or lower would be suspect.

Last edited by RonD; 12-13-2015 at 07:47 PM.
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