This job came about after the vehicle was booked in for an MOT and to look into the warning light for VSA. (Vehicle Stability Assist).
When the technician tried to move the vehicle into the workshop, to carry out the MOT, he found that the engine had a misfire condition, with a severe lack of driveability. (The vehicle, a Honda Civic VTEC 1.8l petrol, is equipped with an auto-shift manual gearbox or ASM).
Their initial contact with the owner after discovering it’s poor performance, was met with a denial of any running fault. However after some further digging and probing, the owner admitted that the car did have some sort of driving fault.
In fact, the VSA warning was due to the performance fault from the engine. The traction control system will take priority for warning when there is an engine torque related problem.
The garage looked at the spark plugs and discovered they were in a shockingly bad condition. They had a general look at the state of service and found that the car had not been serviced for a considerable amount of time. They mentioned that finding to the owner who responded that; “it was a Honda” and suggested they require very little attention!!
So they replaced the spark plugs, after reading DTC’s for misfires on cylinders No.1 and 4. They also swapped the coils from cylinder to cylinder and found no difference. (This engine uses individual coil on plugs or COPs).
After that, they asked me to look at it. I was already on site for another vehicle, and so I started to look into the fault. I noted that I could get the misfire to occur whilst revving the engine, but the idling was fine. I confirmed, whilst monitoring serial data misfire count, that the misfiring was most pronounced on cylinder No.4.
I then carried out a quick relative compression test and found no anomalies. (I didn’t record the capture). I did back that up by completing a full set of compression tests, using my Pico WPS500. All cylinders producing 13 to 14 bar WOT.
I carried out a secondary ignition profile analysis on all cylinders, but only saved a trace for cylinder 4, as this would produce a misfire under load. The illustrations show the ignition profile both loaded during wide open throttle and no load during idle. You can see the disruption in the loaded signal, with some potential to represent fuel starvation. I say potential, as I find analysing the detail of secondary ignition under load, very difficult. I always prefer to see primary signals wherever possible, but this can’t be done with these transistorised coil on plug (COP) coils.
This outcome did push me to test the fuel pressure, which I had been avoiding, as I had no adaptor for the coupling on this car and there is no test port. So I ended up purchasing the necessary parts to make up an adaptor, so that I could measure the fuel pressure and inspect the fuel injectors hydraulic performance. I did this together with an intake pressure analysis, once again utilising my WPS, and that test certainly raised some questions! I took the capture at higher engine revs, as this is where the faults occurred.
The injectors all showed uniform hydraulic performance, but the intake pressure showed an anomaly with cylinders 1 and 4. I couldn’t say it was a fault, but it was a flag for me to consider, and the engine management did keep reporting cylinders 1 and 4 for misfire. I also captured the exhaust pressure at the tailpipe, for analysis, but found it disappointing from a diagnostic purpose.
I have included a reference of the intake and exhaust signals taken at idle, with no load. It shows symmetry between all the cycles. No faults occur here.
As you can see in the image below, the intake pressure signal illustrates an anomaly, but the exhaust pressure signal has lost any significant diagnostic ability.
I guess there is a problem with sampling via the tailpipe at high engine speeds, maybe due to exhaust baffle design.
Because of the usual time restraints for me, I asked the garage to inspect the camshaft lobes and rockers, for damage. They did a visual inspection as best as they could, but as I later discovered for myself, this was more difficult than first thought. They couldn’t see any obvious problems, but visibility was not good.
I returned to the job, for a fresher investigation. This time I wanted to concentrate on the possibility of valve train operation, based on the previous findings. So I set up my scope to measure ignition trigger per cylinder, intake manifold pressure, via the WPS, in-cylinder pressure, also via WPS and mass airflow, via the MAF. I started with cylinder No.1 and repeated the tests for all 4 cylinders.
I set the scope to a single shot trigger on the in-cylinder pressure and used a 20 second screen, enough to capture a start-up event, followed by a wide open throttle held for a short period, (engine speed restricted by the ECM, to approximately 3500 rpm), followed by a return to idle.
On completion of the full set of captures, I found the evidence I needed.
During the WOT stage, I recorded the MAF voltage at 2.6 volts, engine speed at 3,600rpm and the manifold pressure at approximately -550mb. Cylinders 1, 2 and 3 had in-cylinder pressures of 10 bar +- 0.5 bar. But cylinder 4 had an in-cylinder pressure of only 4 bar.
This information was the killer. This fault had to be down to the VTEC system, because the cylinder pressures were all even and good under normal test conditions. So I checked for oil pressure at the VTEC assembly. I used serial data to measure pressure at the VTEC head, with engine idling. I actuated the VTEC solenoid and found an instant oil pressure of over 6.5 bars. That seemed good enough for me to say there was engine oil pressure available. I would have carried out a physical oil pressure check at the switch, but it was a problem to reach. The oil pressure light would go out and 6.5bars at the VTEC head, suggests the pressure was ok.
Unfortunately that is where the story ends, as the owner decided not to proceed any further and managed to part exchange the car for another. I had hoped the garage would be able to strip the valve train assembly for further inspection, so that we could see the root cause of the problem, but that was not to be.. That is so often the case with my job, rarely seeing the job to a close..