Interpreting The Compression Test Results
It's pretty common to see the compression values vary a bit, especially on high mileage engines. Up to a certain point, these variations don't affect engine performance.
It's when one or more values a too low that you're gonna have an engine performance issue causing a rough idle/misfire problem and lighting the check engine light on your Ford Fiesta.
Finding out if the low compression values you got from your test are causing a problem is not difficult. In this section, I'll explain how to do just that.
The rule of thumb is that the lowest compression value can not vary more than 15% of the highest value (that you wrote down in TEST 1). If any value is lower by more than 15%, then that engine cylinder is going to misfire. This misfire will cause your Ford Fiesta's engine to idle rough.
How do you figure this out? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com) or by calculating this 15% difference manually.
To understand how to figure out this 15% thing manually, I'll use 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 .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 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 Engine Compression Test.
TEST 2: Wet Engine Compression Test
One of two problems is usually what's behind a low or zero compression value:
- The affected cylinder's piston rings are severely worn or damaged.
- The affected cylinder's intake/exhaust valves are severely worn or damaged.
The cool thing is we can do a wet compression test that'll tell us where the problem lies without having to tear the engine apart.
The wet compression test simply involves adding about two tablespoons of engine oil to the 'dead' cylinder and then retesting its compression.
If the cylinder's compression values goes up, you can conclude that the low/zero compression issue is due to worn piston rings.
If the cylinder's compression value does not go up, you've got cylinder head valve problems in that affected cylinder.
These are the test steps:
- 1
Add 1 or 2 tablespoon of engine oil in the cylinder you need to 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 on the cylinder you just added oil to.
- 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.
2.) The needle will not move at all or stay at the same number you recorded earlier.
What ever value your compression tester reads, write it down again. - 5
Repeat steps 1 thru' 4 on any other cylinder you need to check.
Let's examine your test results:
CASE 1: The compression value of the cylinder you added oil to increased. This test result tells you that the low or near 0 PSI compression value is due to worn out piston rings of that specific cylinder.
CASE 2: The compression value of the cylinder you added oil to DID NOT increase. This test result confirms that the cylinder head valves of that cylinder are worn-out or damaged.
If this info saved the day, buy me a beer!
