18 Apr 2024

Setting the bar high

I have always attempted to present topics that vary in subject and technical challenge. This month’s subject delivers on all counts. A unique problem which challenges over vehicle knowledge, process and patience. In fact, it was so complex, this month’s topic almost ran in real time with publication.   
A story from my many years of cycling in Europe will, I hope, illustrate my involvement with this diagnosis process. Several years ago, cycling from Paris to Pisa we had to divert from Grenoble to Marseille by train due to the Alps closed by heavy snow. Why should I have been caught out in this way? Well, it was May! Our onward journey took us to Ventimiglia. On this journey we watched in amusement as some people kept boarding and leaving the train playing cat and mouse with the ticket collectors. This very accurately describes my involvement in the diagnostic process with an AUDI S3, it also reminds me of an old expression; ‘Two many cooks in the kitchen’. It also reminded me of an old army adage; ‘Never share command!’

The vehicle presented many potential causes for what initially seemed a straightforward problem. It was booked in for a catalyst efficiency error, where a failed and partially restricted catalyst was discovered. This model variant utilises the brilliant EN888 engine which produces around 300BHP out of the box. This is a power plant I know very well due to my previous research and authoring in past technical topics. It is also fitted in my SEAT Cupra. Like the song says, “when the going gets tough, the tough get going,” and I was the go-to tough guy. Where’s Billy Ocean when you need him?
Having removed the catalyst substrate, temporarily, It was noted that an intermittent misfire count was present on #1 cylinder. At this point I’m going to sound like a parrot; Misfire can and should be described more accurately as a combustion anomaly, the cause of which can be one of three possibilities; Fuelling, ignition, or mechanical malfunction. Somewhere in the mix of the repair process, responsibilities were split between three techs. This is something I do not agree with, but accept it can occur due to staff holidays which I think was the case here. Attention was first paid to the ignition, new spark plugs, and coils were exchanged. Result, no change. The intermittent combustion continued both on and off-load with a prevalence for #1 cylinder.

I was not involved with the diagnostic process at this point, but a decision was taken to remove both sets of injectors for ASNU test bench assessment. I did witness the results found by Peter B, which convinced me the fault lay elsewhere. The intake swirl flaps were cleaned and tested for smooth movement transition. David M decided to replace #1 high pressure injector which also muddied the waters. With the fault still present and apparently getting worse, the vehicle would start promptly then descend into a severe combustion malfunction this lasted for several minutes, then apparently smoothing out. However, under dynamic road test a combustion count was predominant on #1 cylinder but did display similar events on multiple cylinders. A serial data logs clearly identified a cylinder misfire count synchronised with a drop in high fuel pressure. Nominal fuel pressure during warm up is around 60 bar, this was reduced to around 35 bar with the immediate effect of increasing the misfire count. So, the problem was fuel supply related. David M took the decision to replace the high-pressure pump, believing the fault was a high-pressure pump problem. This did not have any effect whatsoever.

At this point I was asked to review the diagnostic process and provide advice, this is where I recalled jumping on and off the train with no fare in my pocket. My reputation was very much pinned on my passion for the application of oscilloscope evaluation, and still is. However, serial data is essential for capturing information. It is quick and provides the actual sensor values at the PCM and any correction values. I was updated as to the previous tests carried out to the priming system by David G and the issues with high-pressure control during cold start and warm up strategies.  Low pressure was confirmed normal at 4.5 bar with no cavitation. At this point, I need to explain how the EN888 engine utilises the dual injection system. From cold and during the warm-up phase, it employs only the high-pressure injectors at approximately 60 bar pressure, with three injection events during crank start. This is reduced to two injection events per cycle until the low-pressure manifold injectors take over. More about camshaft timing later – it’s going to get quite complicated.
Once started it continues with high pressure injectors for the entire warm-up period with two injection events in what “I” call homogenous and stratified delivery. Let me explain. Two thirds of the fuel required is on the intake cycle, homogenous mixture, with the final event on compression stroke, stratified delivery. It then switches over to manifold injection at low-pressure, approximately 5 bar for the entire low to mid-range load strategies. The high-pressure system is only used for high load and engine RPM strategies. The reason for this is quite revealing! Direct injection strategies can produce higher particulates and NOx emission levels than diesel during lean fuelling and high load strategies. Now, I’ve been a bit in the back-seat so far, what will all the sheer number of cooks in the car, looking to find the kitchen. Part two is where I take a more direct involvement in assessing the previous tests using the Pico scope and cross-referencing serial values and pcm correction. I promise some remarkably interesting results. Join us next issue for the continuation. In the meantime consider how you think the PCM should respond to sensor input, fuel trim, injector period and high rail pressure.