How To Test Engine Compression (1993-1996 2.2L Buick Century)

Checking the engine compression on the 2.2L engine in your Buick Century is a pretty easy test since the spark plugs are easy to get to.

In this tutorial, I'll walk you through the entire compression test procedure and, just as importantly, explain how to make sense of the compression readings. By the end of the test, you'll know whether a compression problem is behind your Century's 2.2L engine misfire or no-start issue.

The only special tool you'll need is an engine compression tester. You don't have to spend a fortune on one either, and I'll point you to a few that'll help you get the test done.

APPLIES TO: This tutorial applies to the following vehicles:

• 2.2L Buick Century: 1993, 1994, 1995, 1996.

Symptoms Of Low Or No Engine Cylinder Compression

When your Buick Century's 2.2L engine has a compression problem, it's usually gonna show up in one of two ways:

1. The engine starts and runs, but it has a cylinder misfire or rough idle.
2. The engine cranks but doesn't start.

If you're dealing with a cylinder misfire or rough idle that's being caused by a compression problem, you'll usually notice one or more of the following symptoms:

• Bad gas mileage.
• Blue smoke coming out of the tailpipe.
• 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.
• The check engine light is illuminated on the instrument panel.
• On the 1996+ OBD II equipped Century, you'll see misfire trouble codes illuminating the check engine light. You many see one or more of the following:
  • P0300: Random Cylinder Misfire.
  • P0301: Cylinder #1 Misfire.
  • P0302: Cylinder #2 Misfire.
  • P0303: Cylinder #3 Misfire.
  • P0304: Cylinder #4 Misfire.

You might be wondering why a compression problem can cause a misfire or even keep the engine from starting. The reason is simple: every cylinder needs three things to produce power: air (compression), spark, and fuel.

If even one of those three ingredients is missing, that cylinder isn't gonna contribute any power. So if the cylinder can't compress the air/fuel mixture, it's gonna misfire. And if two or more cylinders have little or no compression, the engine usually isn't gonna start.

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: Finding The Dead Cylinders

Before you get started, I recommend checking the compression on all four cylinders. Doing so will give you a good overall picture of the engine's mechanical condition.

Here are my suggestions:

1. If you're trying to diagnose an engine no-start condition, go ahead and test all four cylinders.
2. If you're diagnosing a cylinder misfire, you don't have to check all four cylinders. In most cases, you can test the suspect cylinder and two others that you know are firing correctly.
   
   Those two good cylinders will give you a baseline to compare against. Once you have their compression readings, it'll be much easier to determine whether the suspect cylinder has normal compression or if it's low enough to be causing the misfire.

If you don't already have a compression tester, you can either borrow one from a local auto parts store or buy one of your own. If you're thinking about buying one, check out my recommendations here: Which Compression Tester Should I Buy?

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

OK, these are the test steps:

1. 1

   Disable the ignition system by disconnecting the ignition coil pack from its electrical connector.
   
   This will prevent the ignition coil from sparking during the test.

2. 2

   Remove the spark plugs. Remember, the engine can not be hot!
   
   When removing the spark plugs, be careful not to drop any on the floor, or you run the risk of having the spark plug's porcelain insulator crack and then you'll have a misfire on your hands.

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

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

5. 5

   Record the value at which the needle stopped and the number of the engine cylinder on a piece of paper.

6. 6

   Release the pressure on the gauge and repeat step 4 and 5 one more time.

7. 7

   Repeat test steps 3 - 5 on the remaining cylinders.

Let's examine your test results:

CASE 1: One cylinder had a noticeably lower compression reading than the others. A small difference between cylinders is normal, but if one cylinder is much lower than the rest, you'll need to determine if it's low enough to cause a misfire.

To find out, head over to: Interpreting The Compression Test Results.

CASE 2: Several or all four cylinders had little or no compression. When most of the cylinders have extremely low compression (or 0 PSI), the engine will crank but won't start.

In most cases, this type of test result points to one of the following problems:

• A broken timing chain.
• A blown head gasket.
• A thrown connecting rod.

CASE 3: All four cylinders produced similar compression readings of 120 PSI or higher. This tells you that the engine's mechanical condition is good and that a compression problem isn't causing the misfire or no-start condition you're diagnosing.

Interpreting The Compression Test Results

Don't be surprised if the compression readings aren't exactly the same across all four cylinders. That's completely normal, especially on a Buick Century that's accumulated a lot of miles over the years.

A slight difference between cylinders isn't a problem. But when one cylinder's compression drops too far below the others, that's when it can start causing a rough idle or cylinder misfire.

The good news is that it's easy to figure out if the lower compression reading is actually a problem. All it takes is a simple calculation.

Once we do the math, we'll know if the low compression reading is still within an acceptable range or if it's low enough to explain the misfire you're trying to diagnose.

Here's the rule of thumb: the lowest compression reading shouldn't be more than 15% lower than the highest compression reading you recorded in TEST 1. If it is, then that cylinder has a compression problem that's severe enough to cause a misfire, which will usually show up as a rough idle or engine miss.

How do you figure this out? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com) or by calculating this 15% difference manually.

To understand how to figure out this 15% thing manually, I'll use the following compression test results:

• Cylinder #1 → 175 PSI.
• Cylinder #2 → 165 PSI.
• Cylinder #3 → 160 PSI.
• Cylinder #4 → 120 PSI.

The next step is to do the following math:

• Multiply .15 (15%) by the highest value: 175 x 0.15. This gives us 26.25, but we'll round it out to 26.
• Next, we subtract 26 from 175: 175 - 26 = 144.
• So now we know that the lowest possible compression value is: 144 PSI.

This means that cylinder #4, which has a compression value of 120 PSI, is the one causing the misfire because it's below the 144 PSI minimum.

Once we'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

When a cylinder has very low compression or no compression at all, the problem usually comes down to one of two things:

• The piston rings in that cylinder are badly worn or damaged.
• The intake or exhaust valves aren't sealing because they're burnt, bent, or otherwise damaged.

The nice thing is that you can usually narrow down which one is causing the compression loss without taking the engine apart.

The next step is to perform a wet compression test. To do this, add about two tablespoons worth 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 use is a hand pump oiler can and a long piece of translucent vacuum hose. I insert the hose through the spark plug hole and then pump the oil directly into the cylinder via the vacuum hose.

I prefer using a translucent vacuum hose because I can actually see the oil travel through the hose and into the cylinder. It's a super simple setup and it makes performing a wet compression test so much easier.

If you don't already have a hand pump oiler can, you can check out the one I recommend here:

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

This is what we're looking for with the wet compression test:

• If the compression reading goes up, you can conclude that the piston rings are worn or damaged.
• If the compression reading stays the same (or increases very little), then the compression loss is coming from the intake or exhaust valves in that cylinder.

These are the test steps:

1. 1

   Add about two squirts of clean engine oil to the cylinder you're going to retest.
   
   NOTE: The easiest way I've found to get the oil into the cylinder is by using a hand pump oiler can with a long piece of translucent vacuum hose attached to its spout. Simply insert the hose into the spark plug hole and pump the oil through the hose and into the cylinder.

2. 2

   Install the compression gauge on the cylinder you just added oil to.

3. 3

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

4. 4

   You'll see one of two results:
   
   1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder.
   
   2.) The needle will not move at all or stay at the same number you recorded earlier.
   
   What ever value your compression tester reads, write it down again.

5. 5

   Repeat steps 1 thru' 4 on any other cylinder you need to check.

Let's examine your test results:

CASE 1: The compression value of the cylinder you added oil to increased. This test result tells you that the low or near 0 PSI compression value is due to worn out piston rings of that specific cylinder.

CASE 2: The compression value of the cylinder you added oil to DID NOT increase. This test result confirms that the cylinder head valves of that cylinder are worn-out or damaged.

More 2.2L Buick Century Diagnostic Tutorials

You can find a complete list of diagnostics tutorials and wiring diagrams for your 2.2L Buick Century in this index:

• 2.2L Buick Century Index of Articles.

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

• Common Causes Of Spark Plug Failure (1993-1996 2.2L Buick Century).
• How To Test For A Blown Head Gasket (1993-1996 2.2L Buick Century).
• How To Test The TPS With A Multimeter (1993-1996 2.2L Buick Century).
• How To Test The MAP Sensor (1993-1996 2.2L Buick Century).