Troubleshooting A Hard To Diagnose Misfire Case Study (GM 3.1L, 3.4L)

Engine Compression Test ‘Nails Down’ The Cause Of The Misfire

Having eliminated the most common cause of a misfire condition (ignition system issues), the next step was to check the compression of all six cylinders.

This would also afford me the opportunity to check that none of the spark plugs were broken. Specifically, that all of the spark plugs porcelain insulators weren't broken (since they had recently been replaced).

Here were my compression test results:

1. Rear Bank (by firewall)
   1. Cylinder #1: 145 PSI.
   2. Cylinder #3: 185 PSI.
   3. Cylinder #5: 190 PSI.
2. Front Bank (by radiator)
   1. Cylinder #2: 150 PSI.
   2. Cylinder #4: 150 PSI.
   3. Cylinder #6: 165 PSI.

I started with the front bank, since it's easier to remove the spark plugs from this one than the rear one and the compression test results looked normal.

It wasn't till I got to the rear bank that I knew that I had found the source of the rough idle condition at idle: engine compression varied by more than 15%.

How To Interpret An Engine Compression Test

Knowing that the engine compression for cylinder 1 was low and causing the rough idle/misfire condition saved my brother-in-law from having to spend money on parts that aren't gonna' solve the problem.

The key here, is knowing how to interpret the results of the compression test and how to figure out the lowest compression any one cylinder can have before it starts to affect idle quality.

This is the math you're gonna' have to do:

1. Multiply the highest compression reading you get by 0.15 (that's point 15).
   • This is 15% as a fraction.
2. In the Monte Carlo, this is the math: 190 X 0.15 = 28.5
   • We can round off the number and just call it 28.
3. The next step is to subtract 28 from 190 (190 - 28), which gives us 162.
4. 162 now becomes the lowest compression value any one cylinder can have before it starts to misfire due to low compression.

So, in my brother-in-law's Monte Carlo, this formula told me that cylinders #1, #2, and #4 are the ones causing the rough idle/misfire condition when the car rolls to a complete stop.

Additional Resources To Help Troubleshoot A Misfire Condition

From personal experience, I can tell you that solving a rough idle or a misfire condition that doesn't leave codes (or does leave misfire codes) can sometimes be a challenge.

What's gonna' help you is to remember that 1 of 3 things are missing and causing the issue (fuel, spark, or air [compression]) and:

1. Identifying the misfiring cylinder (if possible).
2. Eliminating the ignition system from the get-go by testing for spark with a spark tester.
3. Checking engine compression.

I realize that there isn't a ‘one size fits all’ or a ‘cookie cutter’ method to troubleshoot a misfire condition. So, below you'll find a few of the articles I've written that might help you solve the problem.

1. How To Do A Cylinder Balance Test (GM 3.1L, 3.4L).
2. How To Test The Ignition Coil Packs (GM 3.1L, 3.4L) (at: easyautodiagnostics.com).
3. Troubleshooting Misfire Codes (GM 3.1L, 3.4L).
4. Fuel Injector Resistance Test GM 3.1L, 3.4L.
5. Engine Compression Test GM 3.1L, 3.4L.