# How To Test Engine Compression (1991-2010 4.0L Ford Explorer, Aerostar, Mercury Mountaineer)

## Interpreting Your Compression Test Results

There's a good chance that the compression values you obtained in TEST 1 vary between each other.

Up to a certain point, this variation will not affect engine performance or cause a problem. But if they vary too much, you'll definitely see a rough idle condition or a misfire problem.

To find out if this low compression value or values are causing a problem, you've got to find out if they vary by more than 15% of the highest compression value you obtained from your tests.

Why? Because if the low compression value varies by more than 15%, then this cylinder is going to misfire and can be considered ‘dead’.

You can do this one of two ways: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

If you want to manually calculate the 15% difference, here's what you'll need to do:

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

Now, let me give you a more specific example: Let's say that my Ford Explorer produced the following compression readings:

Cylinder Pressure
#1 165 PSI
#2   95 PSI
#3 155 PSI
#4 175 PSI
#5 160 PSI
#6 165 PSI

My next step is to do the following calculation:

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

Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.

To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.

Once you'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

You might be asking yourself, 'what's a wet compression test?'.

It's the same compression test you did before, with the difference being that you're gonna' add about two tablespoons of oil to the engine cylinder that showed a low compression reading in the 'Dry' compression test.

The engine oil that you're gonna' add to the cylinder will help determine if the low cylinder pressure or pressures you recorded previously are due to worn piston rings or worn cylinder head valves.

Depending on whether the compression pressure rises (on your compression tester) or not, you'll be able to say that the problem lies in the piston's rings or the cylinder head valves.

OK, this is what you need to do:

1. 1

Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘Dry’ compression test.

The amount should be about 1 to 2 tablespoons of oil.

2. 2

Install the compression tester onto the cylinder.

Do not use any type of tool to tightened the compression tester. Hand tight is fine.

3. 3

When all is set up, have your helper crank the engine.

4. 4

You'll get one of two results:

1.) The compression value will go up (from the one you recorded before).

2.) The compression value will stay the same.

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 (block) of the engine in your Ford Explorer (Aerostar, Mercury Mountaineer).

CASE 2: The compression value stayed the same. This test result tells you that the low compression in this specific cylinder is due to bad cylinder head valves.

## More Ford 4.0L Ford Explorer And Aerostar Tutorials

You can find more Ford 4.0L Explorer (Aerostar and Mercury Mountaineer) test tutorials by going here: Ford 4.0L Index Of Articles.

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

If this info saved the day, buy me a beer!

Ford Vehicles:

• Aerostar 4.0L
• 1991, 1992, 1993, 1994, 1995, 1996, 1997
• Explorer 4.0L
• 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010

Mercury Vehicles:

• Mountaineer 4.0L
• 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010