How To Do An Engine Compression Test (1995-1999 GM 3.8L)

Testing the engine compression is the best way to find out the internal health of the engine.

In this tutorial, I'll explain how to perform the test and more importantly, how to interpret its results.

You'll be able to easily find out if a compression problem is causing a no-start problem or a misfire problem.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.8L Buick LeSabre: 1995, 1996, 1997, 1998, 1999.
• 3.8L Buick Park Avenue: 1995, 1996, 1997, 1998, 1999.
• 3.8L Buick Regal: 1995, 1996, 1997, 1998, 1999.
• 3.8L Buick Riviera: 1995, 1996, 1997, 1998, 1999.

• 3.8L Chevrolet Lumina: 1998, 1999.
• 3.8L Chevrolet Monte Carlo: 1998, 1999.

• 3.8L Oldsmobile Eighty-Eight: 1995, 1996, 1997, 1998, 1999.
• 3.8L Oldsmobile Ninety-Eight: 1995, 1996.
• 3.8L Oldsmobile Intrigue: 1998, 1999.
• 3.8L Oldsmobile LSS: 1996, 1997, 1998, 1999.
• 3.8L Oldsmobile Regency: 1997, 1998.

• 3.8L Pontiac Bonneville: 1995, 1996, 1997, 1998, 1999.
• 3.8L Pontiac Grand Prix: 1997, 1998, 1999.

Symptoms Of Low Or No Engine Compression

Engine compression problems usually cause one of two types of problems. You'll either see a misfire problem or a no-start problem.

When an engine compression problem causes a misfire, you'll usually see one or more of the following trouble codes registered in the fuel injection computer's memory (along with the check engine light illuminated):

• P0300 Random Cylinder Misfire.
• P0301 Cylinder #1 Misfire.
• P0302 Cylinder #2 Misfire.
• P0303 Cylinder #3 Misfire.
• P0304 Cylinder #4 Misfire.
• P0305 Cylinder #5 Misfire.
• P0306 Cylinder #6 Misfire.

Generally, when an engine compression problem causes a rough idle, you're only going to feel the problem when you come to a stop light. But as soon as you accelerate the engine, the engine feels fine.

You're also gonna' see:

• Bad gas mileage. This is caused by the simple fact that the engine is now running on less than all 4 cylinders.
• Engine pollutes more. This is also caused by the simple fact that the engine is now running on less than its full complement of cylinders. The live cylinders have to compensate for the 'dead' ones.

If your vehicle is NOT OBD II equipped, you won't have any codes lighting up the check engine light, but you'll definitely feel a rough idle condition.

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' Engine Compression Test

The first order of business is to remove the spark plugs. It's important to not remove them if the engine is hot or you run the risk of damaging the spark plug hole threads in the cylinder heads.

If the engine has been running for any length of time, let it cool down completely before removing the spark plugs.

IMPORTANT: You'll be working around a cranking engine, so you have to be careful and stay alert at all times. Think safety all of the time!

These are the test steps:

1. 1

   Disconnect the ignition control module from its electrical connector. This will prevent the ignition coils from firing off spark to the spark plug wires during the test.

2. 2

   Remove all 6 spark plugs.
   
   As you're taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!

3. 3

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

4. 4

   When everything is set up, have your helper to crank the engine. Your job is to keep your eye on the compression tester's gauge.

5. 5

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

6. 6

   Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to.

7. 7

   Repeat steps 4-7 on the other 3 cylinders.

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

CASE 1: No compression in 2 or more cylinders. This is not good and it tells you that the engine has serious internal mechanical problems.

The most common causes would be:

• A broken timing chain.
• A blown head gasket.
• The engine threw a rod.

CASE 2: Low compression in one or more cylinders. To a certain point, it's normal for the compression to vary a little between cylinders (as the engine accumulates thousands of miles).

But if these values vary too much, then you're gonna' have a bonafide misfire on your hands.

The next step is to do some math to find out if this low compression value is within a normal parameter or not. Go to: Interpreting Your Compression Test Results.

Interpreting Your Compression Test Results

Low engine compression in one or more cylinders can have an impact on engine performance and cause a cylinder misfire problem (especially at idle RPMs).

And sometimes, these low compression values don't cause any adverse problem at all. How can you find out? -By calculating the difference between the low and highest compression value and seeing if it varies by more than 15%.

If it varies by more than 15%, then you've got a cylinder that will cause a misfire.

Alright, finding out if the low compression value is causing a problem can be done in 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.

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 a 3.8L Buick LeSabre produced the following compression readings:

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

Once you've found the cylinder or cylinders with low or zero compression, the next step is to find out what is causing the problem.

Either the cylinder's low compression value will be caused by its piston rings or by its cylinder head valves.

Thankfully, we can find out by doing a 'wet' compression test, instead of having to tear the engine apart (at this point).

You may be asking yourself, 'what's a wet compression test?' -Well, it's a compression test but with one minor difference. You're gonna' add a small amount of engine oil to the cylinders with low or zero compression and then test their compression ability again.

Depending on the result of the wet compression test, you can conclude that the compression problem is due to bad piston rings or bad cylinder head valves.

OK, let's get this show on the road:

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 while you observe the gauge.

4. 4

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

5. 5

   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.

6. 6

   Repeat steps 1-4 on any other cylinder with a low compression test result.

Let's examine your test results:

CASE 1: The compression value shot up. This tells you that that cylinder's piston compression rings are worn out and are behind the low compression test result you got in TEST 1.

CASE 2: The compression value stayed the same. This confirms that that cylinder's valves are behind the low to 0 PSI compression result you obtained in TEST 1.

More 3.8L GM V6 Tutorials

You can find a complete list of 3.8L GM V6 tutorials in this index:

• GM 3.8L Index Of Articles.

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

• How To Test The MAF Sensor (GM 3.8L V6 1996-2005).
• How To Test A Does Not Crank Condition -Case Study (GM 3.8L).
• GM 3.8L Ignition Control Module And Crank (3X, 18X) Sensor Test.
• How To Test The Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L).