Interpreting The Compression Test Results
As the engine ages, up to a certain point it's normal for the cylinders to wear out unevenly.
In many cases, this variation in the compression values of each cylinder doesn't cause a problem.
But when uneven compression does cause a problem, it's usually because the lowest compression value varies more than 15% of the highest value.
How do you figure this out? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator or by calculating this 15% difference manually.
To manually find out if the lowest compression value is causing a problem, you'll need multiply the highest compression value by 0.15. You'll then subtract the result of this calculation from the highest compression value. The result of this subtraction is the lowest compression value any of the other cylinders can have. Anything lower and that cylinder will cause a misfire.
For example, let's say that my Ford Ranger gave me the following compression test results:
- Cylinder #1 175 PSI.
- Cylinder #2 165 PSI.
- Cylinder #3 160 PSI.
- Cylinder #4 120 PSI.
The next step is to do the following math:
- Multiply 0.15 (15%) by the highest value: 175 x 0.15. This gives us 26.25, but we'll round it out to 26.
- Next, we subtract 26 from 175: 175 - 26 = 144.
- So now we know that the lowest possible compression value is: 144 PSI.
This means that cylinder #4, which has a compression value of 120 PSI, is the one causing the misfire (P0304 trouble code) because it's below the 144 PSI minimum.
Once we've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: Wet Compression Test.
TEST 2: Wet Compression Test
Low or 0 PSI compression in a cylinder is usually due to one of two things:
- The dead cylinder's piston rings are severely worn out.
- The dead cylinder's cylinder head valves are severely worn-out or damaged.
The cool thing is that we can find out without having to tear the engine apart. And we can accomplish this by doing a simple wet engine compression test.
This test simply involves adding a small amount of engine oil to the dead cylinder and retesting it's compression. If the compression value shoots up, then we can conclude that the low compression value is due to worn out piston rings.
If the compression value does not shoot up, we can conclude that the problem is in the dead cylinder's cylinder head valves.
Let's get testing:
- 1
Add a tablespoon (or two) of engine oil in the cylinder you will retest. I suggest using a small and long funnel so that the oil will reach the inside of the cylinder.
- 2
Install the compression gauge and tighten it hand-tight only.
- 3
Have your helper crank the engine till the needle stops climbing on the compression gauge.
- 4
You'll see one of two results:
1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder, or...
2.) The needle will not move at all or stay at the same number you recorded earlier.
Whatever value your compression tester reads, write it down again. - 5
Repeat steps 1 thru' 4 on any other cylinder you need to retest.
Let's analyze your compression test result(s):
CASE 1: The engine compression value shot up. This test result let you know that the dead or misfiring cylinder's compression problem is being caused by worn-out piston rings.
CASE 2: The engine compression value DID NOT shoot up. This test result lets you know that the dead cylinder's compression problem is being caused by worn-out or damaged cylinder head valves.
More 2.5L Ford Ranger Tutorials
You can find a complete list of tutorials here: Ford 2.5L Index Of Articles.
Here's a sample of the tutorials you'll find there:
- Ignition System Circuit Diagram (1998-2001 2.5L Ford Ranger).
- Oxygen Sensor Circuit Diagram (1998-2001 2.5L Ford Ranger).
- How To Test The Front O2 Sensor Heater (1998-2001 2.5L Ford Ranger).
- Rear O2 Sensor Heater Test (1998-2001 2.5L Ford Ranger).
If this info saved the day, buy me a beer!
