Interpreting The Compression Test Results
It's not rare for some cylinders to show a lower compression value, especially if the engine has racked up a lot of miles.
Within a certain range, this variation between cylinders doesn't lead to engine performance issues. However, if the compression in any cylinder falls more than 15% below your top compression figure, that's when problems like rough idling or misfires pop up.
In this test section, I'll break down a simple way to check if these lower compression numbers are to blame for a rough idle problem or a cylinder misfire issue.
The basic principle here is that the lowest compression reading shouldn't be more than 15% less than your highest compression (recorded in TEST 1). If it is, you're looking at a misfiring cylinder, which will make your Ford van's engine idle roughly.
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 #3 170 PSI.
- Cylinder #4 120 PSI.
- Cylinder #5 170 PSI.
- Cylinder #6 170 PSI.
- Cylinder #7 165 PSI.
- Cylinder #8 170 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
So, you've confirmed that one or more cylinders have low compression. This issue stems from either:
- Worn or damaged piston rings in the cylinder in question.
- Worn or damaged intake/exhaust valves in the same cylinder.
There's a bit of a silver lining, though. A wet compression test can clue us in on whether the problem is with the piston rings or the valves, and all this without needing to disassemble the engine.
All you have to do is add a couple of tablespoons of oil (two) to the cylinder showing low compression, then test its compression again.
If the compression goes up after the oil is added and tested again, then the culprit is worn piston rings. If the compression stays the same, then it's the intake/exhaust valves to blame.
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.
More 4.6L, 5.4L V8 Ford E150, E250 And E350 Tutorials
There are quite a few 4.6L and 5.4L Ford E-Series van 'how to' tutorials and wiring diagrams that you can check out here:
F-Series Pickups: You can find all of the available tutorials and wiring diagrams for the 4.6L And 5.4L V8 F-Series pickups in this index:
- 4.6L, 5.4L Ford F150, F250, And F350 Index of Articles (at: easyautodiagnostics.com.com).
Crown Vic And Grand Marquis: You can find all of the available tutorials and wiring diagrams for the 4.6L And 5.4L V8 F-Series pickups in this index:
If this info saved the day, buy me a beer!
