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

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).