Suzuki Splash – Diagnosing Intermittent Misfires With No DTC’s
What’s the problem?
This Suzuki came into the workshop with an intermittent misfire. It wasn’t evident on tickover or when I’d snap the throttle while stationary. But when I drove it any distance, I could feel it. Okay, so that’s the customer’s complaint confirmed. The next step was to connect a scan tool and check for diagnostic trouble codes (DTCs).
No codes?
Despite the misfire, no DTCs were stored. With no codes, a different diagnostic approach will be required to prove the fault. I’m suspicious of an ignition coil but I want to prove it to be sure.
What can cause a misfire?
Ok, so what scenarios are running through my head right now? What else other than a failing (COP) coil-on-plug could be causing this misfire? I need to consider the basics that make this engine work, Spark, Air, Fuel, Ignition.
- Failing fuel injector
- Ignition coil (COP)
- Failing O2 sensors
- Coolant temperature sensor
- Mass air flow sensor
- Fuel pressure
- Old/failing spark plug
- Low compression
Sometimes luck is a great diagnostic tool
While im deciding what to do the engine starts to misfire while at idle. I’ve already got the Picoscope set up so it’s the perfect time to use the Rotkee coil-on-plug probe. It takes time to set it up. Some double sided tape is needed to attach it to each coil but it works well. I should make some clips to hold them in place.
After setting up the probes, the engine continued to misfire, and I captured the perfect misfire waveform. The image clearly shows a misfire on Cylinder 2. To confirm I move the coil to cylinder 4 and the misfire follows it. I am now confident that a new ignition coil will fix the problem.
Conclusion
I had diagnosed this car over the phone from the description the customer gave. I had a gut feeling it was a failing COP. I could have ordered a set of coils and plugs. Then fitted them to the car without even plugging it in or driving it. And I would have been right. But without proof I would have possibly cost the customer a lot of money unnecessarily. I like to test before I buy anything. Even it that means holding on to the car for a longer period.
Also I got lucky here. This car wasn’t initially misfiring while stationary. But sometimes luck is your best friend when diagnosing a problem. Below is a copy of the waveform after the fix.
Simon Manley
Owner and mechanic at S.M. Auto. See the contact page to get in touch. noun: mechanic ; a skilled worker who repairs and maintains vehicle engines and other machinery --- "a car mechanic"