How To Test Engine Compression (2002-2004 4.2L Oldsmobile Bravada, 2004-2007 4.2L Buick Rainier)

By: Abraham Torres-Arredondo
Created: 10 June 2022
Updated: 19 March 2024
Articl Id: 1356

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Interpreting The Compression Test Results

It's not unusual to see compression values differ, especially on high-mileage engines.

Up to a certain point this difference does not cause any engine performance problems.

But if the difference is more than 15% from the highest value, the engine will experience a misfire problem.

You can calculate this 15% difference in one of two ways: With pen and paper or with my online low compression calculator.

If you want to use the online calculator, you can find it here: Online Low Engine Compression Calculator.

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 I got the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	175 PSI
#6	170 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 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 Compression Test.

TEST 2: Wet Compression Test

Wet Compression Test. How To Test Engine Compression (2002, 2003, 2004, 2005, 2006, 2007 4.2L Oldsmobile Bravada And Buick Rainier)

A low compression value is usually the result of a problem with the affected cylinder's intake/exhaust valves or piston rings.

Thankfully, you can find out without tearing the engine apart, since a simple wet compression test will help you determine where the problem lies.

For a wet compression test, you'll add about 1 to 2 tablespoons of engine oil to the low compression cylinder.

If the cylinder's compression value increases (after retesting it), you can conclude that the low compression value is due to worn-out or damaged piston rings.

If the compression value does not increase, you can conclude that the problem lies in its intake/exhaust valves.

These are the test steps:

1
Add a tablespoon (or two) of engine oil in the cylinder you need to retest.

Use a funnel to make sure the oil reaches the inside of the cylinder.
2
Install the compression gauge on the cylinder and hand tighten it.
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.
5
Write the compression value down.
6
If you have another cylinder to test, repeat steps 1 thru' 5 on it now.

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

CASE 1: The compression value increased. This tells you the low compression problem is due to worn piston compression rings.

CASE 2: The compression value DID NOT increase (in other words, it stayed the same). This result tells you the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.