Trying to find out which cylinder is the one misfiring can be a little difficult if the PCM (Powertrain Control Module= Fuel Injection Computer) does not set a misfire code.
Unplugging one spark plug wire at a time (while the engine is idling) to see if the engine will stumble or not is the WORST thing you can do (to find the engine cylinder that's missing). Why? Well, not only do you run the risk of shocking yourself but this could fry the coil pack the wire is connected to.
Well, this article will show you a way to do a cylinder balance test without using expensive diagnostic equipment or diagnostic software on the 2.8L, 3.1L, or 3.4L GM V6 engine with the coil pack ignition system. I have used this test for as long as I have been working on cars and it has saved me hundreds of troubleshooting hours.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Hacer La Prueba Balance De Cilindros (3.1L, 3.4L V6 GM) (at: autotecnico-online.com).
What Tools Do I Need?
You're only gonna' need three things: 1.) A 12 Volt test light. 2.) Two small pieces of vacuum hose and 3.) A helper to help you crank the car or mini-van.
The test light HAS to be a simple (non-powered) 12 Volt automotive test light. This is important because if you use any type of electronic or self-powered test light, this cylinder balance test is not gonna' work.
How Can This Test Help Me If I Have A Misfire But No Misfire Codes!
Before I jump into the test (in the next page), I want to emphasize that this cylinder balance test will help when:
CASE 1: Your OBD II car with misfire diagnostics, the PCM (Powertrain Control Module= Fuel Injection Computer) can not figure out which cylinder is the one misfiring. Sometimes, you can drive the vehicle till its out of gas, and the PCM still will not set the appropriate misfire code to identify the specific cylinder that's missing.
CASE 2: Your car (or mini-van) does not have OBD II misfire capability because it's older than 1995 (in these older cars, the PCM does not have misfire diagnostic capabilities) and so it's definitely NOT gonna' help you to find out which cylinder is the one misfiring.
Well, my friend, this is where this test is gonna' save the day. The test I'm gonna' show you is the best way to find out which cylinder is misfiring without doing something as scary and risky as pulling a spark plug wire off of the spark plug (while the engine is idling) or buying expensive diagnostic software and equipment.
Once you find out what cylinder or cylinders are missing (‘dead’/misfiring), you can proceed to troubleshoot the cause. OK, enough of my yakking, let's turn the page and we'll get this show on the road!
The Cylinder Balance Test
To find the ‘dead’ (misfiring) cylinder, you're gonna' do is to ‘short out’ two cylinders at a time.
And you're gonna' do this with two pieces of vacuum hose and a test light. By taking some very simple precautions, this test will give you a very, very accurate result that won't shock you or fry the coil pack that you're testing.
As you already know, each coil pack has two towers, therefore, you'll be testing both towers of one coil pack (one coil pack at a time of course), and this is what you'll need to do:
Choose the coil pack that you want to start with. It doesn't matter which one you start with, since you'll test them all.
Now, disconnect the two spark plug wires connected to it. Attach one piece of vacuum hose to the inside of the spark plug wire. This has to be a small piece of vacuum hose.
Once you've got the vacuum hose inserted into the spark plug wire, connect the wire back onto the coil pack tower. See the photo in the image viewer.
The next step is to do the same thing to the other spark plug wire.
When everything is set up and you're sure that the spark plug wires are not going to come unplugged from the vacuum hose (that's now tightly connected to the coil pack tower), have your helper crank and start the engine.
The vacuum hoses will help in transmitting the spark to the spark plug wires without causing a misfire themselves.
Now, with your 12 Volt test light (and whose alligator clip MUST be grounded), touch one of the vacuum hoses (see photo 2 o f 3 in the image viewer).
What will happen is one of two things, either you will hear a drop in the RPMs or you won't. If you do have a tower that you short out and it DOES NOT produce a drop in RPMs, then this cylinder is dead (but more about this a little later).
A cylinder that is NOT dead and producing power, will cause a drop in RPMs when you short out its coil pack tower with the test light.
Repeat step 6 as many times as necessary on both coil towers, to make sure you do or you don't hear a drop in the engine's RPMs.
The next step is to do the same thing to the other spark plug wires.
Let's take a look at what your results mean:
CASE 1: The engine RPMs dropped when you shorted the coil tower with the test light: This result confirms that that cylinder is NOT dead and working. More specifically, it means that the cylinder has good Compression, has fuel and spark and that it is NOT causing the misfire.
CASE 2: The engine RPMs DID NOT drop when you shorted the coil tower with the test light: This result confirms that that cylinder is dead. This dead cylinder will cause a rough idle/ misfire condition. You have found the cylinder that's misfiring.
Now, this test does not tell you the specific reason the cylinder is dead. This test just helps you to find which of the six is dead.
What you have to do now, is to troubleshoot the coil pack, spark plugs, engine compression, fuel injectors -you get the idea.
Coil Pack Assembly Is Under Exhaust Manifold
If you're driving and/or working on an older 2.8L or 3.1L V6 equipped GM car that has the coil pack assembly under the exhaust manifold and you're wondering if this test applies, the answer is YES.
You can do this test on this type of vehicle (I have many times) but you have to be careful not to get burned by the hot exhaust manifold. The key to avoiding this is to not let the vehicle run too long while doing the testing.
Related Test Articles
OK, so now you need to test the coil packs or the spark plug wires. Well, I've written articles on how to test these components and you can find them here:
- Coil Pack Tests. Also includes spark plug wire tests (at: easyautodiagnostics.com).
You can find a complete list of 3.1L/3.4L tutorials in the this index: GM 3.1L, 3.4L Index Of Articles. Here's a sample of the articles/tutorials you'll find there:
- How To Do A Fuel Injector Resistance Test (GM 3.1L, 3.4L).
- How To Test Engine Compression (GM 3.1L, 3.4L).
If this info saved the day, buy me a beer!