An engine compression test can give you a wealth of information about the internal health of the engine.
But beyond that, it can also let you know if an engine compression problem is causing a driveability or engine performance problem.
In this tutorial I'll explain how to do an engine compression test and more importantly, how to interpret your test results.
You'll be able to easily find out whether an engine compression problem is causing an engine performance issue or not.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 3.4L V6 Buick Rendezvous: 2002, 2003, 2004, 2005.
- 3.4L V6 Oldsmobile Alero: 1999, 2000, 2001, 2002, 2003, 2004.
- 3.4L V6 Oldsmobile Silhouette: 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004.
Symptoms Of Low Or No Engine Compression
It's been my experience, working as an automotive technician over the years, that engine compression problems usually cause one of two things:
- The engine compression problem will cause an engine no-start problem.
- The engine compression problem will allow the engine to start and run, but it'll run with a misfire condition.
If the engine starts and runs, but is suffering and engine compression problem, you'll see one or more of the following:
- 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.
- One or more of the following misfire trouble codes:
- 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 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
Before you remove the spark plugs, it's important that you label each spark plug wire with the cylinder number it belongs to. Otherwise, you'll lose the spark plug wires' firing order and you're going to have an engine no-start problem on your hands.
If you don't have a compression tester, you can always run down to your local auto parts store (AutoZone, O'Reilly Auto Parts) and borrow one from them.
They'll ask you for a small deposit that they'll return to you once you return the tool. Now if you're interested in buying your own compression tester and saving a few bucks in the process, then take a look at 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, these are the test steps:
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 from its electrical connector.
Don't overlook this step, since disabling the ignition system will prevent the ignition coil from firing spark during the test.
Disconnect all spark plug wires from the spark plugs and remove all 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 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.