How To Test Engine Compression (3.3L V6 Buick, Oldsmobile)

In this tutorial, I'm going to explain how to perform an engine compression test and more importantly, how to interpret your test results on the 3.3L V6 Buick and Oldsmobile vehicles.

If you're trying to diagnose an engine performance problem, this tutorial will help you to either eliminate or confirm an engine compression problem as its source.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1989-1993 3.3L V6 Buick, Oldsmobile) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 3.3L V6 Buick Century: 1989, 1990, 1991, 1992, 1993.
• 3.3L V6 Buick Skylark: 1989, 1990, 1991, 1992, 1993.
• 3.3L V6 Oldsmobile Achieva: 1992, 1993.
• 3.3L V6 Oldsmobile Calais: 1989, 1990, 1991.
• 3.3L V6 Oldsmobile Cutlass Ciera: 1989, 1990, 1991, 1992, 1993.

Symptoms Of Low Or No Engine Compression

In a nutshell, engine compression problems usually fall into one of two categories.

Category 1: The engine compression problem causes the engine to not start.

Category 2: The engine starts and runs, but runs with a misfire problem.

Here's a basic breakdown of the symptoms you'll see, when the engine starts but is suffering an engine compression problem:

• Bad gas mileage.
• A heavier exhaust smell coming out of its tailpipe.
• Engine is not as peppy as it was once.
• Rough idle that goes away as soon as you accelerate the 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' Engine Compression Test

To get the most accurate test result, from your engine compression test, it's important that you test the compression of all 6 cylinders.

To perform an engine compression test, you're going to need an engine compression tester.

If you don't have one, you can run down to your local auto parts store and borrow one for a small deposit (AutoZone, O'Reilly Auto Parts). They'll return that deposit back to you once you return the tool.

If you want to buy your own compression tester, then check my recommendations here: Which Compression Tester Should I Buy?

IMPORTANT: Do not remove the spark plugs if the engine is hot. If the engine has been running for any amount of time, let it cool down completely before removing the spark plugs.

CAUTION: Take all necessary safety precautions. The engine has to be cranked to perform the engine compression test. Be careful and think safety all the time!

Okay, to get this show on the road, this is what you need to do:

1. 1

   Disable the fuel system by removing the fuel pump relay.
   
   This will prevent fuel from being injected into the cylinders as you crank the engine.

2. 2

   Disable the ignition system by disconnecting the ignition coil/ignition module assembly from its electrical connector(s).
   
   Don't overlook this step, since disabling the ignition system will prevent the ignition coil pack from firing spark during the test.

3. 3

   Remove all 6 spark plug wires and remove all 6 spark plugs.
   
   I recommend labeling the spark plug wires before removing them so you'll know where they go when you put them back on.
   
   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!

4. 4

   Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt).
   
   IMPORTANT: Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

5. 5

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

6. 6

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

7. 7

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

8. 8

   Repeat steps 4 thru' 7 on the remaining cylinders.

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

CASE 1: Low or no compression in 2 or all 6 cylinders. This test result indicates a serious internal problem.

The most common issues would be:

• Blown head gasket.
• Broken timing chain or timing gear.
• Engine threw a rod.

CASE 2: Low compression in one or more cylinders. Up 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

If you're reading this section, then your engine compression test results indicate one or more cylinders with a low compression value (compared to the others).

Up to a certain point, a low compression value will not cause an engine performance problem nor a drivability problem.

But if the compression value is too low, then you'll definitely notice a rough idle condition or a misfire problem.

The cool thing is that we can easily find out if the low compression values are causing an engine performance problem or not.

To find out, we need to figure out if the low compression values are lower than 15% of the highest compression value you got.

You can do this (figuring out the 15%) 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 (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 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 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

A low or 0 PSI cylinder compression test result is usually caused by one of two things.

Either that affected cylinder has severely worn or damaged cylinder head valves.

Or the affected cylinder's piston rings are severely worn or damaged.

Thankfully we can find out by doing a wet compression test.

This test simply involves adding about one or two tablespoons of engine oil to the cylinder with low or zero PSI compression and then retesting its compression.

If the compression value increases, then you now know for certain that the affected cylinder's piston rings are severely worn or damaged.

If the compression value does not increase, then you now know for certain that the problem is due to worn or damaged intake or exhaust valves (of the affected cylinder).

These are the test steps:

1. 1

   Add a small amount of engine oil to the cylinder that reported low compression or no compression in the 'dry' compression test.
   
   You don't have to add a lot of oil. 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 tighten 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.

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

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 3.3L V6 Buick (Oldsmobile).

CASE 2: The compression value stayed the same. This confirms that the low compression problem of the affected cylinder is due to worn or damaged cylinder head valves.

More 3.3L V6 Buick, Oldsmobile Tutorials

You can find a complete list of 3.3L V6 Buick and Oldsmobile tutorials in this index:

• GM 3.3L Index Of Articles.

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

• What Does The MAF Sensor Do? (3.3L V6 Buick, Oldsmobile) (3.3L V6 Buick, Oldsmobile).
• How To Test A Blown Head Gasket (3.3L V6 Buick, Oldsmobile).
• How To Test The Throttle Position Sensor (3.3L V6 Buick, Oldsmobile).
• How To Test The MAF Sensor (3.3L V6 Buick, Oldsmobile).