How To Test The Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L)

This article presents very simple and effective on-car tests of the ignition coil packs on the GM 3.8L V6 equipped engines (Buick, Chevy, Oldsmobile, Pontiac).

You'll be able to troubleshoot and diagnose a bad ignition coil, or bad spark plug wires, or a bad ignition control module as the cause of a misfire condition on your 3.8L V6 equipped GM vehicle.

Now if your car CRANKS but DOES NOT START, you may want to check the ignition control module (ICM) and 3X/18X crankshaft position sensor tests article, by clicking here: Testing The Ignition Control Module (ICM) Buick, Chevy, Pontiac, Oldsmobile 3.8L V6 Engines since this article (this one you're reading right now) concentrates on tests when the complaint is that of a vehicle that starts and runs but runs with a misfire condition.

If you're looking for the article to test the ignition coil packs on the 3.1L and 3.4L V6 GM engines, go here: GM Coil Pack Test (3.1 & 3.4 V-6 Engines).

You can find this tutorial in Spanish here: Cómo Probar Las Bobinas De Encendido (GM 3.8L) (at: autotecnico-online.com).

Common Symptoms Of A Bad Ignition Coil Pack And/Or Spark Plug Wires

The most common complaint is that your car (or mini-van) will start and run, but run with a misfire. Here are few others symptoms that your car or mini-van may be experiencing:

1. The check engine light (CEL) will be illuminated on your instrument cluster but not always.
2. Engine misfire codes:
   • P0300, P0301, P0302, P0303, P0304, P0305, P0306.
3. Your car will idle rough.
4. A 'rotten egg' smell may come out of the tailpipe when you drive your car (or mini-van).
5. Really bad gas mileage.
6. Your vehicle does not pass the emissions test.

Basic Working Theory Of The 3.8L Ignition Coil Pack

In a nutshell when you turn the key to crank and start your car:

1. 12 Volts flow into the ignition control module.
2. This voltage is distributed by the module to the 3 ignition coils sitting on top of it and to the crankshaft position sensor assembly.
3. Once the engine starts to crank, the crankshaft position sensor starts producing its signals which are sent to the ignition control module (ICM).
4. When the ignition control module (ICM) receives these crank sensor signals, it starts to create a switching signal for each individual ignition coil pack.
5. It's this switching signal in combination with 12 Volts that make the ignition coil packs start sparking away.

Each ignition coil sitting on top of the ignition control module (ICM) has two towers. Both towers fire off spark to two different cylinders at the exact same time. It's very important for you to note that the cylinders that get spark from the same ignition coil are called 'paired cylinders' in this article.

In case you're wondering what the heck is the switching signal, this term describes the switching ON and OFF of the Primary Current (12 Volts) that flows thru' the ignition coil by interrupting its path to Ground by the ignition control module.

As already mentioned, if both power and the switching signal are present, then the ignition coil has to fire off spark, unless its fried. Testing for these two signals is easy and doesn't require any expensive testing equipment, you don't even need a scan tool. I'm gonna' walk you thru' the whole testing process in detail.

What Tools Do I Need?

Using the right tool will help you to avoid complications and/or confusing test results. The best way to waste your time and money, is using the wrong tool for the job. Akin to eating a bowl of soup with a fork. Below is a list of tools that are used in the test instructions in this article:

1. An HEI spark tester
   • This inexpensive spark tester is a MUST have tool to be able to correctly diagnose the coil packs on your 3.8L GM vehicle with the info and tests in this article (don't have an HEI spark tester? Need to buy one? You can buy it here: OTC 6589 Electronic Ignition Spark Tester).
2. An LED light (to see what this tool looks like, go here: Abe's LED Light Tool).
3. Battery jump start cables.
4. A small piece of vacuum hose.
5. A digital multimeter (don't have a digital multimeter? Need to buy one? Click here to see my recommendations: Buying A Digital Multimeter For Automotive Diagnostic Testing).
6. A 12 Volt test light.
7. A helper.

I can not stress enough the importance of using an HEI spark tester for all of your spark tests described in this article. Any other spark tester or method of testing for spark might have you chasing up the wrong tree and spending your hard earned money on the wrong diagnostic conclusion.

Important Precautions, Do's And Don'ts

The tests you're about to learn are very simple and easy but require that you do them with safety as your foremost concern and using the proper tools to avoid complications and/or wasting time and money. Since most of the testing will be done with the engine cranking, you have to be alert and take all necessary safety precautions so that you won't get hurt. Here are some more suggestions:

1. If you don't have an HEI spark tester, buy one.
2. Do not use a normal spark plug instead of an HEI spark tester.
   • You may have read somewhere that using a regular spark plug to test for spark is acceptable. Nothing could be further from the truth.
   • If you love to waste time and money, then use a regular spark plug instead of an HEI spark tester.
3. Do not remove the spark plug wire from the spark plug while the engine is cranking to test for spark.
   • Testing for spark this way can damage the ignition coil that that spark plug wire is attached to.
4. Have your helper (who's helping you crank the car) wait outside of the vehicle till you're clear of the engine before giving the order to crank the engine for you. After your done with the test, have him (or her) step out of the car again.
5. Some of the tests require that you crank the engine to observe some test result. The engine will start as the test is being done, so it goes without saying that you need to be alert and very careful. Take all necessary safety precautions.
6. Do not use a test light where an LED light is called for, this is important.

Safety is a matter that can't be taken for granted, so be careful, stay alert and use common sense when performing the tests in this article.

Where To Buy The Ignition Coil And Save