Tricky sounds and signals can trip up easy ignition problem diagnostics.
Even worse, ignition system faults will masquerade as noises, transmission or engine issues, vibrations or electrical concerns, basically anything other than a problem delivering the proper spark at the proper time to the air-fuel mixture.
Spotting and diagnosing an elusive ignition system fault can challenge even the best of technicians; but there are ways to both spot a sneaky misfire and cut down on diagnostic time, and you don't need to be a genius to do either.
Here's a recent case study that demonstrates many of the classic symptoms of a misfire, and also the ways to reduce diagnostic time, because learning from other people's experience is much easier than learning from your own.
'But it's brand new'
Very recently, a 2008 Toyota Highlander Hybrid came in for an elusive "knock" through the vehicle at about 30 mph. When accelerating up to this speed, it would sometimes emit a metal-to-metal banging noise, like an engine mount or axle shaft acting up. Other than that, the vehicle seemed fine and ran well. The customer assured us that it had never been in an accident and had never been abused. And he was the original owner, too.Because it was a hybrid vehicle, we were tempted to dive right into the HV components and sophisticated circuits, but we resisted and instead started with the basics — beginning with our first suspicion, which was the engine mounts.
But the engine mounts were all fine. The bolts were re-torqued to specification just in case, but the noise persisted. The front driveshafts were removed from the hubs, greased thoroughly (including the splines on hub and driveshaft), and then the components were reassembled and installed, making sure to tighten the axle nuts to specification. It won't surprise you to learn that even as a result of our hard work, there was still no change in the noise. Next, we started strategically eliminating systems that could possibly be causing the noise.
We drove the vehicle at the same speed as when the noise occurred, then shifted to neutral and listened for the noise during coasting — no noise. Then, with the vehicle in neutral, we starting lightly braking and listened for the noise — no noise then either.
The only time the noise reoccurred was when accelerating from a stop to about 30 mph, and even then only knocking maybe one to four times in total.After eliminating these systems on the road test, we tried a different tactic (OK, we should've done this in the first place): road testing the vehicle with the scan tool hooked up, recording the engine data, including any misfire counts.
We hit pay dirt right away.
After reviewing the recording, we found a 5 to 7 percent misfire on cylinder No. 3 — a very slight misfire, but worth noting and pursuing further.
So we started to move things around — strategically, of course (we are professionals after all):
- The cylinder No. 3 spark plug went into cylinder No. 2.
- The cylinder No. 3 ignition coil assembly went to cylinder No. 4.
- And the cylinder No. 3 fuel injector went over to cylinder No. 6.
By distributing each of the possible causes of the misfire to different cylinders, we cut down on diagnostic time. The trouble-making component would reveal itself by moving the misfire to another cylinder.
Sure enough, it did.
The snapshot data indicated that, after swapping the parts around and road testing the vehicle, cylinder No. 4 was now the one misfiring. Since the parts had been strategically moved around, the misfire on cylinder No. 4 confirmed (you guessed it) a defective ignition coil.
A new ignition coil made the misfire — and the noise — disappear completely.Dissecting the Diagnosis
You'd be surprised how many people in the shop claimed to have known it was a misfire problem all along, and that they knew that was the problem as soon as they'd heard it. And while that may be just wishful bragging, there were indeed characteristic indications of a misfire that could have helped us out and cut some time off our diagnosis.
The first indication was the intermittent nature of the problem. Ignition system components often act up intermittently, and it's easy to forget about how this system can interact with other components to create weird problems.
Another was the "banging noise" described by the customer. Misfires often make their presence known by causing noises, and unfortunately those noises can be mistaken for problems in other systems. A quick way to see if a cylinder is firing on a coil-over-plug system is to touch the inductive clamp of a timing light to the coil with the engine running. If the coil fires, it'll make the timing light flash. This is a useful trick when dealing with a severe misfire or dead hole.
The third indication was that the problem only occurred under load. Quite often ignition faults only occur when the components are working hard. Road testing the vehicle, or simulating load conditions can really speed up diagnosis – as long as you know what to check.Working Smarter
We could have saved time by hooking up a scan tool during the initial road test, looking for misfires and DTCs right away. But our diagnostic strategy was to eliminate the classic mechanical causes of strange, intermittent noises first by road testing the vehicle and isolating systems, noting that the noise wasn't occurring during braking or coasting.
The classic signs of a misfire (intermittent problem, strange noise, only under load) were right there if we would have paid attention, and that would have saved us some time. And we were glad that we weren't scared off or led astray by the HV components of the hybrid vehicle. It's not always the high-tech systems that cause the faults.
As is often the case, this particular problem was from something simple and easy. Ignition systems can be sneaky, but if you know what to look for and recognize the symptoms of an ignition concern, you won't be fooled by their tricks, and your bottom line will thank you.
Vanessa Attwell is a Master Technician for two major manufacturers and has also worked on the bench of an independent shop. She has developed and delivered training for both vehicle manufacturers and independents, and helped develop government training and regulations standards.