Troubleshooting a Hard to Diagnose Misfire
Case Study (GM 3.1L, 3.4L)

Case Study: Troubleshooting a Hard to Diagnose Misfire (GM 3.1L, 3.4L).

This is a real life case study that will help you diagnose a hard to find misfire. Although this case study involves a 2000 Chevy Monte Carlo (with a 3.4L V6), you can apply this info to any 3.1L or 3.4L GM equipped vehicle (see the Applies To box on the second column).

Finding the exact cause of the misfire isn't hard... if you know where to start and how to test the component you think is bad and causing the misfire. In this case study... I'll share with you some testing and troubleshooting techniques that I think you'll find useful as you get to see/read what I did to find the cause of the misfire in my brother-in-law's 2000 Monte Carlo.

OK, here's the story...

No Misfire Diagnostic Trouble Codes

Identifying the misfiring cylinder is the very first thing that has to be done to successfully find the exact cause of the misfire. In an OBD II equipped vehicle, like my brother-in-law's Monte Carlo, this involves checking for diagnostic trouble codes.

So, that was the first order of business... but my brother-in-law's Monte Carlo had only one code and it wasn't a misfire code. This diagnostic trouble code was a P0420 Catalyst System Efficiency Below Threshold. And the catalytic converter wasn't the source of the misfire.

It's not rare for a PCM not to register a misfire code when there's an actual misfire condition affecting the engine... I have seen this a lot, so not having a specific misfire diagnostic trouble code (DTC) didn't surprise.

I now needed to road test the Monte Carlo to see/feel exactly what my brother-in-law was experiencing during his daily commute.

SIDE NOTE: If this has ever happened to you on your 3.1L or 3.4L equipped GM car or mini-van, I want to tell you that there is a way to find out which cylinder is misfiring without the aid of misfire dtc's... and it involves doing a cylinder balance test. Now, I didn't do one in this case... but knowing how to do one on a 3.1L or 3.4L GM vehicle is gonna' save you some time, money, and frustration. Here's the link:

  1. How to do a Cylinder Balance Test (GM 3.1L, 3.4L).

Road Testing the Monte Carlo
and Forming my Diagnostic Strategy

As I road tested the car with my brother-in-law... the misfire could only be felt when the Monte Carlo would idle at a stop. Once I started accelerating it... the misfire was gone.

I repeated this several times to make sure and having seen this specific symptom so many times before (where the rough idle is only present at idle and goes away when you accelerate the car)... I had the feeling that one of 2 things were causing the problem:

  1. Low compression in one cylinder (or several).
  2. -OR-
  3. Or something causing a lean condition (like a vacuum leak).

My brother-in-law suspected that one of the ignition coil packs could be bad and since most misfire conditions tend to have their root cause in the ignition system, my next step was to test for spark.

Checking for Spark at all Spark Plug Wires

As you're already aware (if you own and drive a GM vehicle with a 3.1L or 3.4L engine) the ignition system is composed of 3 ignition coils (known as coil packs) that sit on top of the ignition module. Each ignition coil has two towers that feed spark to two cylinders at the exact same time (in what's known as a waste spark ignition system).

It is possible to know if an ignition coil is bad or not without having to run down to the local auto parts store to buy one to replace and see. All it takes is a spark test... but, this spark test has to be done with an HEI spark tester (HEI stands for High Energy Ignition).

Checking for spark without a spark tester is the biggest mistake most DIY'ers make. What I have seen most folks do, to check for spark, is to pull the spark plug wire off of the coil's tower (which is an extremely bad idea... this can and does fry the ignition coil and will give a false result).

Even though I suspected low engine compression, I still needed to eliminate the ignition system because these ignition coils do go bad. So the next step was to do a spark test...

  1. Removed the first spark plug wire (to test).
  2. Connected my HEI Spark Tester to the wire (see photo above).
  3. Cranked the engine while observing the HEI spark tester for spark.
  4. Repeated this test on all of the remaining spark plug wires.

Every single spark plug wire sparked. This told me that both the spark plug wires and the the ignition coils were good.

What if I had gotten no spark?: If I had had one spark plug wire NOT spark, then this result would have told me that either the spark plug wire as bad or that the ignition coil was fried (and thus confirming my brother-in-law's suspicions).

So, if had gotten a no spark result on one wire (or even on 2 wires), the next step would have been to check for spark directly on the ignition coil pack.

CASE 1: If testing for spark directly on the ignition coil gave me a spark result, then I could say with 100% accuracy that the spark plug wire was bad.

CASE 2: If I had gotten a no spark test result while testing for spark directly on the ignition coil tower, then I could say the ignition coil pack was bad.

Now, I'm explaining the above in a very brief way... if you want more details and a step-by-step way to test the spark plug wires and ignition coil packs... I have written such a tutorial and you can find here:

  1. How to Test the Ignition Coil Packs (GM 3.1L, 3.4L) (this tutorial found at: easyautodiagnostics.com).


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