How To Interpret The Engine Compression Test Results

For starters, let me tell you that you're not gonna' see the exact same compression value on each of the 6 or 8 cylinders you tested on your Ford 4.9L engine, 5.0L engine or 5.8L engine and this is normal.

What is not normal is to have one or several a compression values that are radically different than the average compression of the good cylinders.

Keeping the above in mind, this means that compression between cylinders should not vary more than 15%. If you're scratching your head and wondering how to do this, don't worry, this is how you can find out:

  1. STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
  2. STEP 2: Round off the result (for example: 25.6 would become 26).
  3. STEP 3: Subtract the result (the number that was rounded off) from the highest compression value.
  4. ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.

NOTE: You can also use my online low compression calculator if you want to avoid doing the math. You can find it here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

To make better sense of this calculation, I'll give you a specific example on how to do this 15% calculation. Let's say that my 5.0L Ford Crown Victoria gave me following compression values when I did a compression test on all 8 cylinders:

Cylinder Pressure
#1 175
#2 170
#3 170
#4 140
#5 165
#6 170
#7 155
#8 160

The next step is to do the math:

  1. STEP 1: 175 x 0.15 = 26.25.
  2. STEP 2: 26.25 = 26 (rounded to nearest one).
  3. STEP 3: 175 - 26 = 149.
  4. ANSWER: 149 PSI. Any cylinder with this compression (or lower) value will misfire.

Since cylinder #4 gave me a compression value of 140 PSI, I now know that this is the one causing the misfire I'm trying to diagnose.

The next step, after finding out that a cylinder's low engine compression value is not within range, is to do a ‘wet’ compression test on it.

TEST 2: ‘Wet’ Compression Test

How To Test Engine Compression (4.9L, 5.0L, 5.8L)

If you've gotten a very low compression reading from any one cylinder on your 5.0L engine (4.9L engine or 5.8L engine), the next step is to add a little bit of engine oil (to that cylinder) and re-check its compression to see if the compression pressure will go up or not.

Adding engine oil and checking the cylinder's compression is known as a ‘wet’ compression test. This ‘wet’ compression test will help you determine if the piston rings or cylinder head valves are the ones causing the low or no compression reading you obtained in TEST 1.

The amount of oil you're gonna' add is about 2 tablespoons. If the piston rings are worn (and the cause of the low compression reading), this oil will cause your compression tester to record a higher compression value.

Now, since oil could never help seal the cylinder head valves, if the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.

OK, this is what you need to do:

  1. 1

    Add about 2 tablespoons of engine oil to the cylinder that reported low or no compression in the ‘dry’ compression test.

  2. 2

    Install the compression tester. Once again, hand-tighten the compression tester.

  3. 3

    Crank the engine when all is ready.

  4. 4

    Once the needle on your compression tester stops climbing, have your helper stop cranking the engine.

  5. 5

    You'll get one of two results: either the compression value will go up (from the one you recorded before) or it will stay the same.

Let's take a look at what your test results mean:

CASE 1: The compression value shot up. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end of your 5.0L engine (4.9L engine or 5.8L engine).

CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.

Why An Engine Compression Test?

One of the things that can cause a misfire condition (also known as a miss or dead cylinder), is an engine cylinder or engine cylinders that produce less than normal compression.

When this happens, not matter what gets replaced (like spark plugs, spark plug wires, fuel injectors), nothing solves the miss (misfire). This is when the compression test comes in to save the day.

Over the years, I have solved many unsolvable misfire codes, rough idle, lack of power issues by doing a simple engine compression test and if you're faced with something similar, I highly recommend doing an engine compression test.

Which Compression Tester Should I Buy?

There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:

1) Which one to buy:  The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.

Engine Compression Gauge Testers

2) Where to buy:  You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!

Related Test Articles

To see all of the Ford 4.9L, 5.0L, 5.8L specific tutorials, go to: Ford 4.9L, 5.0L, 5.8L Index Of Articles

Here's a sample of the tutorials you'll find in the index:

  1. Testing A Blown Head Gasket (Ford 4.9L, 5.0L, 5.8L).
  2. Ignition Coil Test -No Spark No Start Tests (Ford 4.9L, 5.0L, 5.8L).
  3. No Start Troubleshooting Tests.
  4. Distributor Mounted Ignition Control Module Test (at: easyautodiagnostics.com).
  5. Fender Mounted Ignition Control Module Test (at: easyautodiagnostics.com).
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Ford Vehicles:

  • Bronco 4.9L, 5.0L, 5.8L
    • 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996
  • Crown Victoria 5.0L
    • 1989, 1990, 1991
  • E150, E250, E350 4.9L, 5.0L, 5.8L
    • 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996

Ford Vehicles:

  • F150, F250, F350 4.9L, 5.0L, 5.8L
    • 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997
  • Mustang 5.0L
    • 1988, 1989, 1990, 1991, 1992, 1993
  • Thunderbird 5.0L
    • 1988, 1989, 1990

Lincoln Vehicles:

  • Continental
    • 1988, 1989, 1990
  • Mark VII
    • 1988, 1989, 1990, 1991, 1992
  • Town Car
    • 1988, 1989, 1990

Mercury Vehicles:

  • Cougar 5.0L
    • 1988, 1989, 1990

Mercury Vehicles:

  • Grand Marquis 5.0L
    • 1988, 1989, 1990, 1991