Interpreting The Compression Test Results
There's a good chance that some of the compression values you got in TEST 1 are lower than the majority of the other values.
Within a certain range, differences in your engine's compression values won't cause any engine performance issues.
However, if the low compression values differ by more than 15% from the strongest cylinder, you're gonna have engine performance problems.
By doing some easy math, which I'll explain how to do in this test section, you can find out if these low compression values are causing an issue.
As a general rule, the lowest compression value shouldn't be more than 15% lower than the highest value you noted in TEST 1. If it's off by more than that, the engine cylinder will misfire. This misfire will make your Ford pickup's V8 engine 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 #3 170 PSI.
- Cylinder #4 120 PSI.
- Cylinder #5 170 PSI.
- Cylinder #6 170 PSI.
- Cylinder #7 170 PSI.
- Cylinder #8 90 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 #8, which have compression values of 120 PSI and 90 PSI respectively, are causing the misfire issue because they're below the minimum of 144 PSI.
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
If your results from TEST 1 show a cylinder or cylinders with low compression, the next step is to perform a 'wet' engine compression test.
To do this, add a small amount of motor oil to the affected cylinder and check its compression again.
If the compression value increases (compared to what you recorded in TEST 1), it tells you that the compression rings are causing the low compression issue.
If the compression value does not increase, you can conclude the problem is caused by worn/damaged cylinder head valves or valve seats in that cylinder.
Here's what you'll need to do next:
- 1
Add a tablespoon (or two) 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
Once you've added the oil, install the compression gauge.
As before just hand tighten the gauge. - 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 cylinders you need to test.
Let's take a look at what your compression test results mean:
CASE 1: The compression value shot up for the low compression cylinder. This confirms that the low compression value registered in this cylinder in the dry test is due to worn piston rings.
The reason the compression value shot up is due to the fact that the motor oil you just added helped the piston rings to create a tighter seal. This type of test result only happens when the problem is due to worn piston rings.
CASE 2: Your compression value DID NOT shoot up (stayed the same). This result tells you that the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.
More 4.6L, 5.4L Ford F150, F250, And F350 Tutorials
There are quite a few 4.6L, 5.4L V8 F-Series pickup 'how to' tutorials and wiring diagrams you can find 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:
- 4.6L, 5.4L Ford E150, E250, And E350 Index of Articles (at: troubleshootmyvehicle.com).
Crown Vic/Grand Marquis: You can find all of the available tutorials and wiring diagrams for the 4.6L V8 Ford Crown Vic and Mercury Grand Marquis in this index:
- 4.6L Crown Victoria And Grand Marquis Index of Articles (at: troubleshootmyvehicle.com).
If this info saved the day, buy me a beer!
