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

Testing the head gasket on the 4.2L Oldsmobile Bravada (Buick Rainier) is not difficult.

In this tutorial, I'll explain how to perform the four most common blown head gasket tests step-by-step.

With your test results, you'll quickly determine if a blown head gasket issue is causing an engine no-start problem or an overheating problem.

APPLIES TO: This tutorial applies to the following vehicles:

• 4.2L L6 Buick Rainier: 2004, 2005, 2006, 2007.
• 4.2L L6 Oldsmobile Bravada: 2002, 2003, 2004.

Important Tips And Suggestions

TIP 1: Do not remove the spark plugs from a hot engine.

If the engine has been running for any length of time, let it cool down for at least an hour before attempting to remove the spark plugs.

Attempting to remove the spark plugs from a hot engine can damage the threads in the spark plug holes. This is a nightmare you want to avoid.

TIP 2: If you find the spark plugs are swimming in engine oil, as you're removing them from their spark plug tubes, you'll need to replace the valve cover gasket.

If the spark plugs are soaked in engine oil, it's important to replace them with new ones.

If the spark plug boots (that connect ignition coils to the spark plugs) are soaked in engine oil, it's important you replace them too.

TIP 3: Take all safety precautions since you'll be working around a cranking engine.

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 some recommendations to you:

TEST 1: Dry Compression Test

I recommend you test the compression of all six cylinders. Once you obtain your compression test results, you'll interpret them in the next section.

If you don't have a compression tester, you can borrow one from your local auto parts store (AutoZone or O'Reilly's Auto Parts) or you can buy one.

You can check out my compression tester recommendations here: Which Compression Tester Should I Buy?

Let's get testing:

1. 1

   Remove the ignition coils.

2. 2

   Remove the spark plugs.
   
   When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plug's porcelain insulator crack (causing a misfire problem).

3. 3

   Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder.
   
   Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

4. 4

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

5. 5

   Record the value at which the needle stopped and the number of the engine cylinder on a piece of paper.

6. 6

   Repeat steps 3-5 on the remaining cylinders.

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

CASE 1: You got 0 PSI in the majority of the cylinders. This tells you your Chevy Bravada or Buick Rainier's engine has serious internal problems. This is usually due to:

• Busted timing chain.
• Engine threw a rod.

Any compression value below 100 PSI (even if it's not 0 PSI) means internal mechanical engine trouble.

CASE 2: All cylinders have compression, but their values are not the same. It's normal for each cylinder's compression value to vary slightly from one another. But if they vary too much, you'll have a bonafide misfire or rough idle condition on your hands.

To find out, the next step is to go to: Interpreting The Compression Test Results.

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

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. 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. 2

   Install the compression gauge on the cylinder and hand tighten it.

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.

5. 5

   Write the compression value down.

6. 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.

More 4.2L Oldsmobile Bravada And Buick Rainier Tutorials

You can find a complete list of 4.2L Oldsmobile Bravada and Buick Rainier tutorials and wiring diagrams here:

• GM 4.2L Index of Articles.

Here's a list of articles you'll find there:

• Blown Head Gasket Tests (2002-2004 4.2L Oldsmobile Bravada, 2004-2007 4.2L Buick Rainier).
• Electronic Throttle Body Tests (2002-2004 4.2L Oldsmobile Bravada, 2004-2007 4.2L Buick Rainier).
• TAC System Wiring Diagram (2002-2004 4.2L Oldsmobile Bravada, 2004-2005 4.2L Buick Rainier).
• How To Test The Ignition Coils (2002-2004 4.2L Oldsmobile Bravada, 2004-2007 4.2L Buick Rainier) .