How To Test A Misfire / No Spark-No Start Condition (4.3L, 5.0L, 5.7L 96-04)

TEST 7: Testing The Triggering Signal From PCM

In TEST 6 you checked and confirmed that the ignition coil IS NOT receiving a switching signal.

As you may already know, this activation signal (the switching signal) comes from the ignition control module (ICM).

In order for the ignition control module to activate the ignition coil, it requires power and must receive its own activation (trigger) signal from the Powertrain Control Module (PCM).

So far, you have checked and confirmed that the module is supplied with power (TEST 5). In this test, you'll verify that the PCM is sending the ignition control module the trigger signal it needs to activate the ignition coil.

IMPORTANT: Both the ignition coil and ignition control module must remain connected to their electrical connectors for this test to work. You'll need to use a back probe on the connector or a wire piercing probe on the wire to test for this signal. You can see an example of this tool here: Wire Piercing Probe.

These are the test steps:

1. 1

   Connect the black wire of the LED to the wire labeled with the letter B of the ignition control module's connector (see photo above).
   
   NOTE: The ignition coil and the ICM must remain connected to their electrical connectors for this test to work.

2. 2

   Connect red lead of LED to the battery positive (+) terminal.
   
   NOTE: It is important that it be connected at the battery positive (+) terminal.

3. 3

   Have an assistant crank the engine.

4. 4

   The LED test tool should blink ON and OFF while the engine cranks.

Let's examine your test results:

CASE 1: The LED light flashed ON and OFF as the engine cranked. This is the correct test result and it confirms that the PCM is sending the ICM an activation signal.

This test result confirms that the ignition control module (ICM) is fried and needs to be replaced if you have:

• Confirmed that none of the spark plug wires are sparking (TEST 1).
• Confirmed that the ignition coil tower is not sparking (TEST 3 and TEST 4).
• Confirmed that the ignition coil and ICM are getting power (TEST 5).
• Confirmed that the ignition coil IS NOT getting its Switching signal (TEST 6).
• Confirmed that the ignition control module is getting its Triggering signal.

CASE 2: The LED light did NOT flash ON and OFF as the engine cranked. Recheck all of your connections and repeat the test.

If the LED light is still not blinking, it indicates one of three things as possible causes for this missing signal:

1. A bad crankshaft position (CKP) sensor.
2. An open-circuit problem in the wire between the ignition module and the PCM.
3. A bad PCM, although this is rare.

If you need to test the crankshaft position sensor, I've written a tutorial that'll help you test it and it here: How To Test The Crank Sensor (GM 4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).

Other Causes Of A Misfire

So you've done all the spark tests and found no problems, but your GM car (or pickup, or mini-van, or SUV) is still running rough (rough idle) and/or misfiring. Well, here are a couple of suggestions that might help:

1. The fuel pressure regulator (that's located inside the intake manifold's plenum) is leaking gasoline.
   • This is the most common problem on these types of GM engines, where the fuel injector assembly sits inside the plastic intake manifold plenum (GM has made a ton of money, directly or indirectly, from this common defect over the years. After all, even if you buy an after-market fuel pressure regulator, that manufacturer has to pay royalties to GM to make it —unless unless of course it's a Chinese counterfeit/knock-off).
   • You can find the ‘How To Test The Fuel Pressure Regulator’ article here: How To Test The ‘Spider’ Fuel Injector Assembly (4.3L, 5.0L, 5.7L) (at troubleshootmyvehicle.com)
2. Engine compression test
   • One of the most overlooked diagnostic tests to find the root cause of a misfire is the compression test.
   • You'll need an engine compression tester of course.
   • The engine compression readings between cylinders should not vary more 15%.
   • You can find the ‘How To Test The Engine Compression’ article here: Engine Compression Test (4.3L, 5.0L, 5.7L) (at troubleshootmyvehicle.com).
3. Carbon tracks on the spark plug(s) and in the inside of the spark plug wires.
   • The photo above shows what carbon tracks look like (the arrows point to them).
   • Replace the components as affected with carbon tracks.
4. Broken spark plugs.
   • This usually happens at tune-up time, if you have dropped one on the floor.
   • You power washed the engine. This is something that should never be done on a modern GM engine.
5. Fuel injector melted and leaking fuel.
   • This usually happens when the fuel pressure regulator starts leaking fuel and every now and then the engine will back-fire thru' the intake manifold and the resulting flame catches the fuel injector assembly on fire.
   • This very common failure has helped to put a roof over my kid's head for many years (thank-you GM, brilliant!).
   • You can find the ‘How To Test The Fuel Pressure Regulator’ article here: How To Test The ‘Spider’ Fuel Injector Assembly (4.3L, 5.0L, 5.7L) (at troubleshootmyvehicle.com)

Where To Buy The OE Parts And Save $

If you run into your local auto parts store, you'll see that you'll pay a whole lot more for the distributor ignition parts (what adds insult to injury is that they're Chinese knock off parts and they should be cheaper!!!).

Or you can buy the original AC-Delco OE parts for a lot less here:

GM, in all its infinite wisdom, decided to build the distributors out of a composite plastic material and if yours is damaged and needs to replace it, you don't have to buy an expensive one from the local auto parts store. Save money and buy it here:

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