How To Test The Ignition Control Module (2.8L V6 TBI Chevrolet S10, GMC S15)

Even though the 2.8L V6 TBI fuel injected Chevy S10 (GMC S15) does not come with OBD II misfire diagnostics, you can still find out exactly which cylinder is dead (misfiring)! In this tutorial I'll show you how to do that in a step-by-step way.

From personal experience I can tell you that it's not that hard. It involves doing a very simple manual cylinder balance test.

Contents of this tutorial at a glance:

  1. The 3 Basic Causes of a Misfiring (Dead) Cylinder.
  2. TEST 1: Doing A Manual Cylinder Balance Test.
  3. TEST 2: Checking The Spark Plug Wire.
  4. TEST 3: Checking The Cylinder's Compression.
  5. TEST 4: Testing The Distributor Cap.
  6. More GM 2.8L Tutorials.

En Español Puedes encontrar este tutorial en Español aquí: Cómo Diagnósticar Una Falla En Cilindro (2.8L V6 GM) (en: autotecnico-online.com).

The 3 Basic Causes of a Misfiring (Dead) Cylinder

Every cylinder in your 2.8L v6 GM engine needs fuel, air (compression), and spark to be able to produce power. It's when one of these is missing from the cylinder that that cylinder misfires. I'm going to be referring to the misfiring cylinder as a dead cylinder throughout the tutorial

The components that are usually behind most cylinder misfires are:

  1. BAD spark plug wires.
    1. They usually don't fail all at once. For the most part, you'll have one or two spark plug wires that simply stop transmitting the spark from the coil pack to the spark plug.
  2. BAD distributor cap.
    1. You have one of the distributor cap towers not transmitting spark. The end result is a cylinder that is not getting spark does a cylinder misfire.
  3. BAD spark plug.
    1. Spark plugs don't last forever and are gonna' wear out eventually. What agravates the problem is if the engine in your car or mini-van is burning oil. If this is the case... one or several spark plugs become carbon fouled and the air gap (between the center electrode and the side electrode) closes. This results in a no spark condition for that or those specific cylinders.
  4. Low or no compression in one or several cylinders.
    1. Engine compression problems due to worn out / burned valves or worn out compression rings are another common cause of a misfire. Will cause the cylinder to misfire!

You'll notice that I did not include fuel injector problems in the list above. This is due to the fact that a fuel injector causing a specific cylinder to misfire is extremely rare in this type of throttle body fuel injection system.

TEST 1: Doing A Manual Cylinder Balance Test

Piece Of Vacuum Hose With One End Cut At A 30° Angle. How To Find a Misfiring Cylinder (2.8L V6 TBI Chevrolet S10, GMC S15) Cylinder Balance Test. How To Find a Misfiring Cylinder (2.8L V6 TBI Chevrolet S10, GMC S15)

The key to finding the dead (misfiring) cylinder is to do a manual cylinder balance test. It sounds complicated, but it's a pretty easy test to do.

In a nutshell: a small piece of vacuum hose is placed between a distributor cap tower and the spark plug wire that connects to it (the arrow in the photo above points to this small vacuum hose). Then using a 12 volt automotive test light, the spark plug wire is shorted by touching the vacuum hose with the metal pointy end of the 12 Volt automotive test light.

If the cylinder is dead to begin with, then the engine's idle will not be affected. In other words, since the cylinder is already dead, shorting out its spark plug wire will not cause the engine to shake more. But if the cylinder is ‘alive’, then shorting out its spark plug wire will make the engine idle rough (the engine will shake more).

These are the test steps:

  1. 1

    Cut 2 small pieces of vacuum hose about 1.25 inches long (no longer).

    Make a small 30 degree cut on one end. This end is going to be inserted in the spark plug wire's metal terminal (inside it's rubber boot).

  2. 2

    Disconnect spark plug wire #1 and #2 from the distributor cap. You can identify these 2 wires from the illustration below.

  3. 3

    Insert the vacuum hose into the spark plug wire's metal terminal (inside its rubber boot), then connect this spark plug wire (with the vacuum hose) back onto the distributor cap.

    Repeat the above on the other spark plug wire.

  4. 4

    Crank and start the engine once you have both spark plug wires with the piece of vacuum hose between the towers.

    If the vacuum hoses are inserted correctly, the spark will be channeled across the vacuum hose and into the spark plug wire.

  5. 5

    As the engine idles, 'short out' the spark plug wire by touching the vacuum hose with the 12 Volt test light (see the photo above).

    NOTE: The 12 V. test light has to be grounded, preferably directly on the battery negative terminal.

    This will channel the spark thru' the test light (don't worry, it's very safe) and effectively kill the cylinder. If the cylinder is dead to begin with, shorting the spark plug wire WILL HAVE NO EFFECT on engine idle. If the cylinder is NOT dead, then this will have a negative effect on engine idle... since it will get worse.

  6. 6

    Repeat this test step on the remaining 4 spark plug wires.

Let's take a look at what your test results mean

CASE 1: One or more cylinders are dead. This tells you that that cylinder is dead. It's dead because it is either missing spark or it is missing compression. Your next step is to go to: TEST 2: Checking The Spark Plug Wire.

CASE 2: All cylinders are alive. This tells you that all of the six cylinders are getting spark, fuel, and have compression.

Now if the engines idle is still rough, meaning that the engine shakes too much in idle, then you've a condition that's affecting all cylinders. Vacuum leaks, low fuel pressure, uneven compression across all 6 cylinders.

How To Test The Ignition Control Module (2.8L V6 TBI Chevrolet S10, GMC S15)