A Chevy Spark's intermittent misfire

May 17, 2024
Intermittent misfire complaints can be some of the most difficult to solve simply because they can be the most troublesome to replicate and test for. However, having a structured game plan and understanding how to test and interpret the results keeps you ahead of the game.

Welcome back to another edition of “The data doesn’t lie,” a regular feature in which I pose a puzzling case study, followed by the answers to the previous issue’s puzzle.  

Intermittent misfire complaints can be some of the most difficult to solve simply because they can be the most troublesome to replicate and test for. However, having a structured game plan and understanding how to test and interpret the results keeps you ahead of the game.

Today’s contestant

My good friend Rick was challenged with a 2014 Chevy Spark exhibiting a misfire complaint that comes and goes around 1,000 rpm. Otherwise, it seems to run pretty well. Rick is a knowledgeable and experienced technician. With that, using his Pico scope 4425, he captured the intermittent misfire through the eyes of the ignition scope. Although this is only a single capture, this pattern is quite repetitive for cylinder #3 (Figure 1).

At first glance, Rick assumed what he was seeing in the ignition pattern was the cause of a lean condition. As a result, he tested for vacuum leaks at the intake manifold runner for cylinder #3 and swapped the fuel injectors, with no change to the pattern or cylinder it correlated to. This is where Rick reached out to me for a second opinion.

Preliminary analysis

According to the capture above I do agree with Rick that the telltale sign of a lean condition (indicated by the upward slope of the burn line) is evident. I’m more concerned with the turbulence present. This is a clear indicator of a variation in conductivity within the cylinder. This problem can be caused by several reasons (two of which have already been accounted for). These include injector spray pattern, EGR dilution, and lack of cylinder integrity.

The vehicle configuration doesn’t utilize an EGR valve. EGR is accomplished via variable cam timing. Logic will tell you a fault here would affect all four cylinders equally. Because this is a port injection fueling system (not GDI) it’s unlikely that a poor injector spray pattern will affect the operation at only 1,000 rpm. This pushes a mechanical fault to the top of the list.

Diagnostic stepping stones

Because of the likely mechanical fault and the ease of the test procedure, I persuaded Rick to send me a capture of a cranking intake vacuum with an ignition sync. He captured this using his pressure pulse sensor and a piston chart from Driveability Guys. This will allow me to see how each of the four cylinders is breathing, relative to one another. It serves a similar purpose as a relative compression test (The relative compression test was captured but not displayed as it shows no real variation).

As can be seen, of the four intake pulls, only cylinder #3 is different from the rest; it's another clue that we are on the right track (Figure 2). So, continuing with the pursuit of an engine mechanical fault, logic told me it was time to commit to in-cylinder pressure transducer testing as it offers not only compression values but insight into how the cylinder is breathing.

I advised Rick to capture the in-cylinder pressure waveforms for both suspect cylinder #3 and known good cylinder #2 (Figure 3). He acquired this data using his Pico WPS500 pressure transducer. To the left of each capture is an orange circle surrounding the exhaust ramp. They demonstrate a difference from cylinder to cylinder. Towards the right of the captures and using the vertical cursors I denote the degrees of offset for the deepest portion of the intake pocket from the 360-degree marker. This variation from cylinder to cylinder pinpoints the issue.

The data doesn’t lie

With all the information in front of us, and the desired information not yet obtained, we are faced with deciding how to proceed. Here are some bullet points of what we know to be factual, and I will ask all of you, diligent readers, for your input:   

  • Misfire on cylinder #3 at 1000 rpm
  • Ignition waveform demonstrates turbulence
  • No other cylinder is affected
  • The fault appears to be mechanical in nature

Given this information, what would you do next?   

  1. Re-time the exhaust camshaft
  2. Re-time the intake camshaft
  3. Inspect for cam lobe out of phase/disassociation from camshaft
  4. Inspect the camshafts and lifters for wear

Be sure to keep reading Motor Age to find the answer to this month's challenge, and future tests down the road.

About the Author

Brandon Steckler | Motor Age Technical Editor

Brandon began his career in Northampton County Community College in Bethlehem, Pennsylvania, where he was a student of GM’s Automotive Service Educational program. In 2001, he graduated top of his class and earned the GM Leadership award for his efforts. He later began working as a technician at a Saturn dealership in Reading, Pennsylvania, where he quickly attained Master Technician status. He later transitioned to working with Hondas, where he aggressively worked to attain another Master Technician status.

Always having a passion for a full understanding of system/component functionality, he rapidly earned a reputation for deciphering strange failures at an efficient pace and became known as an information specialist among the staff and peers at the dealership. In search of new challenges, he transitioned away from the dealership and to the independent world, where he specialized in diagnostics and driveability. 

Today, he is an instructor with both Carquest Technical Institute and Worldpac Training Institute. Along with beta testing for Automotive Test Solutions, he develops curriculum/submits case studies for educational purposes. Through Steckler Automotive Technical Services, LLC., Brandon also provides telephone and live technical support, as well as private training, for technicians all across the world.

Brandon holds ASE certifications A1-A9 as well as C1 (Service Consultant). He is certified as an Advanced Level Specialist in L1 (Advanced Engine Performance), L2 (Advanced Diesel Engine Performance), L3 (Hybrid/EV Specialist), L4 (ADAS) and xEV-Level 2 (Technician electrical safety).

He contributes weekly to Facebook automotive chat groups, has authored several books and classes, and truly enjoys traveling across the globe to help other technicians attain a level of understanding that will serve them well throughout their careers.  

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