How To Test Engine Compression (1994-2005 3.8L V6 Buick LeSabre)

Checking the engine compression on the 3.8L V6 in your Buick LeSabre isn't difficult, but there are a few important steps we've got to follow to get accurate results.

In this tutorial, I'll show you exactly how to perform the compression test and, just as importantly, how to make sense of the results. This way, you can find out if a cylinder with low or no compression is behind your engine's misfire or no-start problem.

The cool thing is that you don't need a bunch of expensive diagnostic equipment. A compression tester is the only special tool you'll need, and I'll show you where you can comparison shop for one.

Everything is explained step by step, so you'll be able to confidently test the engine compression on your 1994-2005 3.8L V6 Buick LeSabre and determine whether the engine has an internal mechanical problem.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.8L V6 Buick LeSabre: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.

Symptoms Of Low Or No Engine Cylinder Compression

When one or more cylinders have low or no compression, you're usually gonna notice one of two problems.

1. The engine misfires and runs rough.
2. The engine cranks but doesn't start.

When low compression is affecting just one or two cylinders, the engine will usually still start and run. It'll just run rough because those cylinders aren't contributing their share of the engine's power. In this situation, you'll usually notice one or more of the following symptoms:

• Bad gas mileage: The engine uses more fuel than normal.
• Blue exhaust smoke: Blue smoke comes out of the tailpipe, especially during acceleration.
• Stronger exhaust odor: The exhaust has a heavier fuel smell than normal.
• Loss of power: The engine doesn't have the same power or acceleration it once had.
• Rough idle: The engine idles rough but smooths out as soon as you accelerate.
• Check engine light: The check engine light is illuminated on the instrument panel.
• OBD II misfire codes (1996-2005 models): If your Buick LeSabre is OBD II equipped, you may find one or more of the following trouble codes stored in the PCM:
  • 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.

The thing to keep in mind is that every cylinder needs three basic things to produce power: compression, spark, and fuel.

If any one of those is missing, that cylinder can't do its job and it's gonna misfire. So when a cylinder can't compress the air/fuel mixture, a misfire is the expected result.

On the other hand, if three or more cylinders have little or no compression, the engine usually won't start because it can't produce enough power to keep itself running.

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

TEST 1: Dry Compression Test

Before we get started, I recommend checking the compression on all six cylinders. This gives you a much better picture of the engine's overall mechanical condition. After all, these 3.8L V6 engines have usually accumulated quite a few miles over the years.

If you're diagnosing an engine no-start condition, then you've definitely got to test all six cylinders. The main reason for this: the engine may have several cylinders (if not all) with low or no compression, and you'll need all six compression readings to accurately pinpoint the problem.

If you're troubleshooting a cylinder misfire, you don't always have to test every cylinder. If you already have a misfire trouble code pointing to a specific cylinder, or you suspect a particular cylinder is misfiring, you can test that cylinder along with at least two cylinders that you know are firing normally.

The thing to keep in mind is that those two good cylinders give you a baseline for comparison. Once you have their compression readings, it'll be much easier to determine whether the suspect cylinder has normal compression or if a compression problem is causing the misfire.

If you don't already have a compression tester, you can borrow one from many local auto parts stores or buy one. If you'd like to comparison shop before buying one, take a look at my recommendations here: Which Compression Tester Should I Buy?

IMPORTANT: If the engine has been running, let it cool down completely before removing any of the spark plugs.

OK, these are the test steps:

1. 1

   Disable the ignition system by unplugging the ignition control module (ICM) electrical connector.
   
   This prevents the ignition coils from producing spark while the engine is being cranked during the compression test.

2. 2

   Remove all six spark plugs. Make sure the engine is completely cool before removing them.
   
   As you remove each spark plug, be careful not to drop it. A cracked porcelain insulator can cause a misfire, and that's one problem we definitely don't want to create while diagnosing another.

3. 3

   Thread the compression tester into the spark plug hole for cylinder #1.
   
   NOTE: Hand-tighten the compression tester only. Don't use a wrench or any other tool to tighten it.

4. 4

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

5. 5

   Write down the highest compression reading along with the cylinder number on a piece of paper.

6. 6

   Release the pressure from the gauge and repeat the test one more time on the same cylinder to verify the reading.

7. 7

   Repeat the compression test on the remaining five cylinders, recording each cylinder's highest reading.

Once you've finished testing all of the cylinders, take a look at what your results mean:

CASE 1: One or more cylinders registered a much lower compression reading than the others. Depending on how low the reading is, this may indicate an internal engine problem.

Our next step is to determine whether those compression readings are actually outside the normal range. Go to: Interpreting The Compression Test Results.

CASE 2: Three or more cylinders had little or no compression (0 PSI or close to it). This test result explains why the engine cranks but doesn't start.

Compression readings this low are usually caused by one of the following mechanical failures:

• Broken timing chain: The camshaft is no longer synchronized with the crankshaft.
• Blown head gasket: Compression is leaking between cylinders or into the cooling system.
• Thrown connecting rod: The affected piston can no longer compress the air/fuel mixture.

CASE 3: All six cylinders produced similar compression readings above 120 PSI. This tells you that the engine's mechanical condition is good and that low compression isn't causing the misfire or no-start condition you're troubleshooting.

Interpreting The Compression Test Results

It's perfectly normal for the compression readings to vary a little from one cylinder to another, especially on a 3.8L V6 engine that's accumulated quite a few miles over the years.

The thing to keep in mind is that there's a limit to how much variation is acceptable. Once the difference between cylinders becomes too great, that low-compression cylinder can cause a rough idle or cylinder misfire.

The cool thing is that it's pretty easy to determine if that's what's happening. All we've got to do is a little bit of math.

Once we do the calculation, we'll know whether the lowest compression reading is still within the normal range or if it's low enough to be the cause of the misfire.

As a general rule, the lowest compression reading shouldn't be more than 15% lower than the highest compression reading you recorded in TEST 1. If one cylinder is more than 15% below the highest reading, that cylinder has a compression problem that's severe enough to cause a misfire.

You can let my online calculator do the math for you here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com). Or, if you'd like to calculate it yourself, here's how it works.

Let's use the following compression readings as an example:

• Cylinder #1: 175 PSI.
• Cylinder #2: 165 PSI.
• Cylinder #3: 160 PSI.
• Cylinder #4: 120 PSI.
• Cylinder #5: 160 PSI.
• Cylinder #6: 170 PSI.

Here's the math:

• Step 1: Multiply the highest compression reading by 0.15 (15%): 175 × 0.15 = 26.25. We'll round that to 26 PSI.
• Step 2: Subtract 26 from the highest reading: 175 - 26 = 149 PSI.
• Step 3: The lowest acceptable compression reading is therefore 149 PSI.

As you can see, cylinder #4 only produced 120 PSI, which is well below the minimum acceptable value of 149 PSI. This tells us that cylinder #4 has a compression problem that's capable of causing a rough idle or cylinder misfire.

Now that we've identified the low-compression cylinder, our next step is to find out what's causing it. For that, head over to: TEST 2: Wet Engine Compression Test.

TEST 2: Wet Engine Compression Test

TEST 2: Wet Engine Compression Test

In most cases, when a cylinder has low or no compression, the problem comes down to one of two things:

• Worn or damaged piston rings: The air/fuel mixture escapes past the piston instead of being compressed.
• Leaking valves: One or both of the intake or exhaust valves are burnt, bent, or not sealing properly against the cylinder head.

The cool thing is that we can usually figure out which one is causing the compression problem without taking the engine apart.

Our next step is to perform a wet compression test. This test is pretty simple. All you've got to do is add about two tablespoons of clean engine oil to the low-compression cylinder and then repeat the compression test.

Getting the oil into the cylinder can be a little tricky. The method I've found works best is to use a hand pump oiler can with a short length of translucent vacuum hose. I insert one end of the hose into the spark plug hole and then use the oiler can to pump the oil through the hose and into the cylinder.

I prefer translucent vacuum hose because I can actually watch the oil travel through it. That way, I know the oil is going into the cylinder instead of spilling somewhere else. It's a simple trick, but it makes the wet compression test much easier to perform.

If you don't already have a hand pump oiler can, you can comparison shop for one here:

• Hand Pump Oiler Can Professional Level 8 oz (Amazon affiliate link).

Here's what you're looking for after repeating the compression test:

• If the compression reading increases significantly: The added oil temporarily helps seal the piston rings, indicating they're worn or damaged.
• If the compression reading stays about the same: The compression loss is coming from the intake or exhaust valves because the oil can't seal a leaking valve.

Let's get started:

1. 1

   Add about two tablespoons of clean engine oil to the low-compression cylinder.
   
   NOTE: The easiest way I've found to get the oil into the cylinder is with a hand pump oiler can and a length of translucent vacuum hose. Simply insert one end of the hose into the spark plug hole and pump the oil through the hose into the cylinder. I like using translucent hose because I can actually see the oil traveling through it, which lets me verify that it's going into the cylinder.

2. 2

   Install the compression tester in the same cylinder.

3. 3

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

4. 4

   Record the new compression reading.
   
   You'll usually see one of two results:
   
   1.) The compression reading is noticeably higher than it was during the dry compression test.
   
   2.) The compression reading stays the same or increases very little.
   
   Write down the new compression reading before moving on.

5. 5

   Repeat steps 1 through 4 on any other low-compression cylinders you want to verify.

Now let's see what your test results mean:

CASE 1: The compression reading increased after adding the engine oil. This tells you that the piston rings in that cylinder are worn or damaged. The oil temporarily helped seal the rings, allowing the cylinder to build more compression.

CASE 2: The compression reading stayed the same (or increased very little). This confirms that the compression loss is coming from the cylinder head valves. Since the oil can't seal a leaking intake or exhaust valve, the compression reading remains essentially unchanged.

More 3.8L V6 Buick LeSabre Diagnostic Tutorials

You can find a complete list of diagnostics tutorials for your 3.8L V6 Buick LeSabre in this index:

• 3.8L V6 Buick LeSabre Index of Articles.

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

• How To Test For A Blown Head Gasket (1994-2005 3.8L V6 Buick LeSabre).
• How To Test The TPS With A Multimeter (1996-2005 3.8L V6 Buick LeSabre).
• How To Test The CKP Sensor With A Multimeter (1994-2005 3.8L V6 Buick LeSabre).
• Common Causes Of Spark Plug Failure (1994-2005 3.8L V6 Buick LeSabre).