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.
Contents of this tutorial:
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 two recommendations to you:
1) Which one to buy: The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.
Engine Compression Gauge Testers
2) Where to buy: You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!
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:
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.
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.
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!
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.
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.
Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
Write down the compression value on a piece of paper.
Include the number of the cylinder this reading belongs to.
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.