This article is a true description of an AECS technical help desk problem and how it was solved. By H.P. Leijen (trainer/research).
2005 Nissan Tiida, MR18DE 1.8 petrol.
Problem presented to the help desk
This Nissan Tiida was driven into our workshop, with a complaint of lack of power. It presented itself with a terrible misfire. The workshop involved has AECS equipment and technical support.
The essence of this article is how to quickly and accurately diagnose which cylinder is misfiring or if it is a random misfire. This knowledge has a major impact on diagnostic approach and speed.
The first measurement made was crank shaft sensor vs ignition trigger while the engine was running and misfiring. Accidently the power supply of the coil was measured vs crank, with some interesting facts.
The RPM pattern live calculated from the crank shaft sensor showed clearly a misfire every fourth beat.
The coil’s power supply showed that there was a bad connection between the battery and the coils, dipping the power supply each time a coil was activated. It is easy to measure the coil activation time from that pattern, you just measure how long each dip lasts.
The outcome was that the coil just before the misfire was only activated for three milliseconds, as opposed to the working coil at 7.8 msec. Is three msec perhaps not enough energy in the coil to ignite the mixture?
Could the ECU drive the coils incorrectly as a result of a crank/cam relation fault? Some Nissan engines do have cam chain stretch issues.
A cam vs crank recording was made and no chain stretch of significance was detected, we look for more than two degree cam/crank phase variation (chain slap) as an indicator that the tensioner has ran out of stroke. We came later to the conclusion that the battery voltage variations caused the erratic coil charge durations.
Time to find out which cylinder was misfiring, and compare the coil trigger time with the findings in our first measurement. In which direction shall we go?
Even to the untrained eye it is clear that the misfire was happening on one cylinder continuously. Since the channel 2 lead was connected to ignition trigger of coil 4, it was also immediately clear that the misfire happened on cylinder 4 alone.
Let’s look at the pattern in detail just to learn a little. Zoomed-in, you can see the crank shaft speeding up after ignition of cylinder 1,3 and 2. You can also see that the compression is slowing the crank shaft down after each “speed up” (power stroke) event.
This indicates, most importantly,
that the compression of cylinder 4 is there (more about this later). You can also see that the overall engine is being dragged down by the two successive compressions of cylinders 4 and 2. The firing cylinders are making up for it (RPM line trending up), to achieve “target idle speed”. Reason for such a misfire is nine out of 10 times an ignition problem, so it makes sense to me to investigate the spark quality on cylinder number 4.
The primary spark pattern cannot be recorded on these COP coils as the coil has got an ignitor build in. We deal with the construction and how to properly measure and diagnose the spark quality in the AECS EMS 1-1 training.
The diagnostician decided to swap the coil instead of measuring the spark quality. While having the coils out he found that the spark plug gap was around 1.7 mm. He replaced the spark plugs and swapped coil 1 and 4.
He reported that the engine ran a lot better, power was back, but still a misfire at idle on cylinder 4!
Better look at crank vs Injection to see if the injection was present and to see if the duration was equal to other cylinders.
The injection duration on all cylinders was the same at idle when the engine was misfiring… Hmm, that did not help!
Back to drawing board
Better start again as we might have missed something. We wanted to see ignition quality on cylinder 4 almost right from the beginning.
We had to measure the induced ignition pattern, to see the spark quality. The coils were removed and a wrap wire was placed around the coil stalk. The following pattern was recorded – see picture captioned “Induced ignition vs crank recording on 2 channel ATS 500XM scope”.
The pattern needs a little explanation – please read in detail if you like to learn.
The crank signal is first used to transform into a Delta N (RPM) pattern, then the RPM pattern is measured for speed changes. The highest speed changes (steepest slope in the RPM line) are just after combustion and just before the end of compression.
The peaks of the slope line indicate the intensity of energy delivery after combustion and the troughs of the slope line show the energy required to achieve compression, no matter what the RPM is.
In the slope line of the above pattern, is clearly visible that the compression on each cylinder is virtually the same, so the misfire is not a compression problem. To verify this, a compression test was done by the diagnostician. They were all equal.
It is also visible that the energy delivery is not present in cylinder 4 (slope peak just after coil 4 fires).
Let’s zoom in and look at the ignition quality.
Zooming in on the spark duration in the same recording showed a spark duration of 1.12 msec. That is not a great spark but certainly not cause for misfire.
So what next?
So we had compression, we had acceptable ignition, and we had injection. The coils had already been swapped, time now to swap the injectors from #1 to #4.
Even though the engine sounded exactly the same after the injector swap, the misfire moved to cylinder 1 (with the injector). This clearly indicated an issue with injector #4. A second-hand injector was fitted, which made the engine run perfectly.
After about four days of running the car was presented back to the workshop, running on three cylinders again. A quick two minute measurement, using a scope, pre-set the previous recording and, indicated that it was cylinder 2 this time which was misfiring. Before the diagnostician ordered one more injector he advised the customer that it was better to replace all as it was very likely that the rest of the injectors would fail too.
The diagnostician, out of his own curiosity, started hunting for why the two injectors had failed, as it is not normal for injectors to play up like this. He took a fuel sample and found a large quantity of water in the tank. The customer was advised of costs of injector replacement and tank removal/refit.
The write-up of this fault took actually longer than finding the fault. It is just the thinking behind the diagnostics, we as a technical equipment provider and trainers, like to explain to our customers and prospect customers. I guess it is our passion.
Overall an efficient job, good for workshop turnover and credibility. An ATS scope, an experienced diagnostician, and a little of AECS tech support, made this job quick and profitable.
ATS 500XM scope recording of ignition trigger vs crank sensor, with a live calculated RPM trace indicating the crank shaft speed changes (Delta N) at idle.
ATS scope recording of coil power supply vs crank shaft sensor.
Scope recording of ignition trigger vs crank sensor zoomed in.
Ignition trigger 4 vs crank recording after injector 1 and 4 have been swapped.
Induced ignition vs crank recording zoomed in.
Induced ignition vs crank recording on 2 channel ATS 500XM scope.
RPM vs injection recording.