Interpreting The Compression Test Results
If your engine compression test results indicate that one or more cylinders have low compression, then in this section, we're gonna do some math to find out if they're causing an engine performance problem.
Up to a certain point, especially on high-mileage engines, it's not uncommon to see that the compression values between the cylinders vary a bit. Usually, this doesn't cause any issues or engine performance problems.
When a low compression reading does cause an engine performance problem, it's when it varies more than 15% from the highest reading you obtained.
How do you figure this 15% out? You can use 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.
- Cylinder #5 160 PSI.
- Cylinder #6 165 PSI.
- Cylinder #7 130 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 cylinders #4 and #7, which have a compression value of 120 PSI and 130 PSI respectively, are the ones causing cylinder misfires because they're below the 144 PSI minimum.
Once we've found the 'dead' cylinder or cylinders, the next step is to find out what's causing their low compression value. For this step, go to: TEST 2: Wet Engine Compression Test.
TEST 2: Wet Engine Compression Test
Typically, a low or zero compression reading stems from one of two internal engine mechanical issues:
- The piston rings in the affected cylinder are severely worn or damaged.
- The intake/exhaust valves in the affected cylinder are severely worn or damaged.
We can easily find out which of the two issues is causing the compression problem by performing a wet compression test -yup, no need to disassemble the engine!
To perform a wet compression test, we add approximately two tablespoons of engine oil to the 'dead' cylinder and then retest its compression.
If the compression value of the cylinder increases, we can conclude that the low or zero compression issue is due to worn piston rings.
However, if the compression value remains unchanged, it indicates problems with the cylinder head valves (of that specific cylinder).
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 4.6L Crown Vic And Grand Marquis Tutorials
There are quite a few 4.6L Ford Crown Victoria (Mercury Grand Marquis) 'how to' tutorials and wiring diagrams that here:
E-Series Vans: You can find all of the available tutorials and wiring diagrams for the 4.6L and 5.4L V8 E-Series vans in this index:
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).
If this info saved the day, buy me a beer!
