Interpreting The Compression Test Results
Depending on the age and wear and tear of your Ford pickup's engine, more than likely you're going to see a variation in the compression test values you got in TEST 1.
Up to a certain point, this variation will not cause any engine performance or driveability issues.
But if the compression value variations are too great, then you're definitely going to see the engine suffering a misfire problem or a rough idle problem.
Finding out if the low compression values (you got from TEST 1) 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 F150 (F250, F350)'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 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.
- Cylinder #5 160 PSI.
- Cylinder #6 170 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
It's not difficult to figure out what's causing the low or 0 PSI compression reading you got in TEST 1.
In most cases, the low or 0 PSI compression test result is caused by the piston rings or cylinder head valves of the affected cylinder.
Thankfully, we don't need to disassemble the engine to find out where the problem lies. Since performing a simple wet compression test (on the affected cylinder) to find out where the problem lies.
In case you're wondering, a wet compression test simply involves adding 1 to 2 tablespoons of engine oil to the cylinder that has low or 0 PSI compression and then re-testing its compression.
If the compression value increases, then you can conclude that the problem lies in the affected cylinder's piston rings.
If the compression value remains the same, then you can conclude that the affected cylinder's intake/exhaust valves are worn or damaged.
OK, let's get started:
- 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.
More Ford F150, F250, F350 Diagnostic Tutorials
You can find more tutorials Ford F150, F250, and F350 in the following index:
Here's a sample of the tutorials you'll find in the index:
- How To Test The Ford Ignition Control Module (Distributor Mounted).
- How To Test The Ford Ignition Control Module.
- Throttle Position Sensor (TPS) Test (Ford 5.0L, 5.8L).
- How To Test The Ford Mass Air Flow (MAF) Sensor.
If this info saved the day, buy me a beer!
