How To Test Engine Compression (1991-2011 4.0L Ford Ranger And Mazda B4000)

Interpreting The Compression Test Results

If in TEST 1 your compression test results indicate that one or more cylinders have a lower compression value than the rest, this section will explain how to find out what's causing them.

As the engine wears, it's cylinders will not wear out evenly.

This means that the compression values the cylinders produce won't be the same.

Depending on how small or great the variation is between them, the engine won't suffer any performance problems.

What will cause a problem an engine performance problem is if those values very too much out of a specific range.

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 Pontiac Grand Am's 3.3L V6 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

How To Do And Interpret A Wet Engine Compression Test (1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 4.0L V6 Ford Ranger And Mazda B4000).

To find out if the low compression or 0 PSI compression reading you got in TEST 1 is due to bad piston rings or bad cylinder head valves, you need to perform a wet compression test on the affected cylinders.

This test simply involves adding a tablespoon or two of motor oil to the low compression cylinder.

Once the oil has been added, its compression is tested again. If the compression reading increases, you can conclude that the piston rings are worn and causing the low compression reading.

If the compression value does not increase, the problem are worn cylinder head valves.

These are the test steps:

  1. 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. 2

    Install the compression gauge on the cylinder you just added oil to.

  3. 3

    Have your helper crank the engine till the needle stops climbing on the compression gauge.

  4. 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. 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.

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