How To Test Engine Compression (3.1L V6 Buick, Oldsmobile)

This tutorial will help you test the engine compression on your 3.1L V6 Buick or Oldsmobile.

I'm also going to show you how to interpret your compression test results to find out if you do have a compression problem on your hands.

All of the test steps are explained in a step-by-step manner.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.1L V6 Buick Century: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.
• 3.1L V6 Buick Regal: 1994, 1995, 1996.
• 3.1L V6 Buick Skylark: 1994, 1995, 1996.
• 3.1L V6 Oldsmobile Achieva: 1994, 1995, 1996, 1997, 1998.
• 3.1L V6 Oldsmobile Cutlass: 1997, 1998, 1999.
• 3.1L V6 Oldsmobile Cutlass Ciera: 1994, 1995, 1996.
• 3.1L V6 Oldsmobile Cutlass Supreme: 1993, 1994, 1995, 1996, 1997.

Symptoms Of Low Or No Engine Compression

For the most part, engine compression problems cause two types of issues. These are:

• An engine compression issue will cause a no-start problem.
• - or -
• The engine will start and run, but it will run with a misfire or will have a rough idle.

If the engine in your 3.1L Buick or Oldsmobile starts and runs, then an engine compression problem will cause one or more of the following symptoms:

• 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.
• Misfire trouble codes (on OBD II equipped vehicles):
  • 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.

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 test the compression on your 3.1L V6 Buick (Oldsmobile) you're going to need to remove the spark plugs.

Before you remove them, it's very important that you label each spark plug wire with the cylinder number it belongs to. This will save you the headache of losing the spark plug wires' firing order.

And of course, as you're already aware, you're going to need a compression tester. If you don't have one, you can run down to your local auto parts store (AutoZone, O'Reilly Auto Parts) and borrow one from them.

If you like to buy one and save a few bucks in the process, take a look at my recommendations here:

IMPORTANT: 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 completely. Removing the spark plugs from a hot engine can damage the spark plug hole threads in the cylinder head. This is a nightmare you want to avoid!

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 pot boiling, this is what you need to do:

1. 1

   Disable the fuel system. You can do this by removing the fuel injector fuse(s).
   
   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).
   
   IMPORTANT: 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 removing the spark plugs, be careful and don't drop any of them on the floor, or you could cause its 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 3 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

It's not unusual to see a variation in the compression values from TEST 1. Especially if the engine in your vehicle has a lot of mileage and/or wear and tear.

In most cases, if the variation is within a certain specification, you're not going to see any driveability issues nor engine performance problems.

But if the variation between the compression values is too great, then you'll definitely see a misfire problem on your hands. Finding out if there is a problem isn't hard and in this section, I'll explain how you can do that.

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

If you're reading this section, then you've got some cylinders with low or 0 PSI compression.

That low value will usually be due to problems in the affected cylinder's piston rings or in its cylinder head valves.

You and I can easily find out without having to disassemble the engine. And we can do this by doing a simple 'wet' compression test on the cylinder with the low or 0 PSI compression value.

The wet compression test simply involves adding a small amount of engine oil to the affected cylinder and then retesting its compression.

If its compression value shoots up, after adding oil to it, then you can conclude that its low compression value is due to worn piston rings.

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

OK, let's get testing:

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.

5. 5

   Repeat steps 1 thru' 4 in any other cylinder you need to test.

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 3.1L 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 3.1L V6 Buick, Oldsmobile Tutorials

You can find a complete list of 3.1L V6 Buick (Oldsmobile) tutorials in this index:

• GM 3.1L Index Of Articles.

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

• How To Test Engine Compression Test (3.1L V6 Buick, Oldsmobile).
• How To Test The MAP Sensor (3.1L V6 Buick, Oldsmobile).
• How To Test The Fuel Pump (3.1L V6 Buick, Oldsmobile).
• How To Test The 24X Crankshaft Position Sensor (1995-1997 3.1L V6 Oldsmobile Cutlass Supreme).