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.
Brandon Steckler
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.
Brandon Steckler
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?
- Re-time the exhaust camshaft
- Re-time the intake camshaft
- Inspect for cam lobe out of phase/disassociation from camshaft
- 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.
