1999 Honda Civic, engine D16Y7, experiencing random misfires caused by something quite simple.
A friend of mine called me in hopes I'd help him with his Honda. It had the check engine light on and lacked power off idle.
First Visit
My friend only was able to stay about an hour. Upon his arrival, he let me drive the car. When I drove it, it felt like his clutch was slipping. So, I suggested that he have his clutch looked at. He spent the next day researching a good place to get his clutch replaced, but he indicated his off idle acceleration was getting much worse. In addition, now at higher RPMs the engine was demonstrating a lack of power up to around 2500 RPM. After 2500 RPM, the engine’s power returned like it had no issues.
Second Visit
On the second visit, his car had its check engine light on. He indicated it was on once before but cleared it by disconnecting the battery. So, up this point,
I did not know that a check engine light was ever on. The codes being reported were P0301 and P0303, indicating cylinders one and three were encountering misfires.
At this point, I knew for sure the clutch was fine.
We tried to attack things that were easy to test. To perform a cylinder contribution test, with the car running at idle, I systematically disconnected each cylinder’s plug wire and immediately allowed the spark to jump to a grounded test light. At the same time, I was ensuring that each cylinder had strong spark. This test did not show any weak cylinders and each cylinder had strong spark.
Using a scopemeter, I then captured the secondary and injector wave forms of each cylinder. All checked out. Yet, the car still had the issue.
While idling the car, I used a test light connected to ground to test if any of the spark plug wires had insulation issues, by running the test light up and down each plug wire slowly. I was looking for a spark. No sparks were encountered, meaning the plug wires were good.
Unfortunately, it was dark and my friend had to limp this car home. I was not happy that I did not help my friend out.
Later that night he called me, and we used FaceTime so we could work together as he inspected each spark plug. They looked ok but were a little on the black side. He also removed the rotor cap and noticed lots of black dust. He cleaned all the plugs, cleaned the rotor cap, and reassembled everything. Unfortunately, the problem persisted.
Third Visit
My friend the very next day called me and told me the car was really acting up.
So, he planned to get off work early so that we could have ample time to diagnose and fix his car. So, in preparation, I found a shop manual on line and found a very good ignition diagnostic procedure on line. So, I felt ready this time.
When he arrived, I quickly followed the ignition test procedure, yielding no issues found. I then scoped out the secondary ignition and injector wave forms for each cylinder again and noticed no issues. I tried to use the scan tool to see if it would tell me the misfiring cylinder, but his PCM did not support this feature.
I also performed an EGR test by applying vacuum to the EGR valve, and of course, the engine stalled, meaning the EGR valve was working.
While scratching my head at this mystery, my friend insisted that we change out the cap, rotor, wires, and plugs. I honestly didn’t see any issues with his current ignition system other than his plugs kept getting fouled.
Being a complete failure up to this point since none of my ideas did not show any component failures, I reluctantly agreed. Yup. I became a parts swapper; a very sad day indeed.
I was thoroughly disgusted with myself in that I have become a parts changer, and I felt I was letting down my friend big time. My friend took off the rotor cap while I strung up the new cap with new plug wires. I laid down the new cap and wires and started to remove the rotor. And, there it was. I could easy pull off the rotor. That is it! There was supposed to be a bolt that firmly attaches the rotor to the distributor shaft, keeping it from changing it indexing in relation to the plugs. So, the loose rotor was moving causing the spark timing to change unexpectantly.
So, I found a screw that would temporarily work and reassembled everything using the new parts. We started the car, and it had no issues. It's a fix.