Setting the bar high
Part Three
By Frank Massey |
Published: 22 April, 2021
Before I begin part three I have somewhat of an important admission, right up to the closing paragraph of this instalment; I still don’t know the actual cause of the incorrect fuel pressure during warm up.
I hope that part two showed a methodical approach to data acquisition to determine how the fault occurred and a clear path towards further evaluation. I also need you to accept that a great deal more testing behind the scenes had been carried out, but for the flow and purity of the topic I have cherry-picked the more interesting elements within the logic timeline. In other words, I have not cheated you with the facts, as presented to me.
So, what do we know? GDI fuel pressure is reducing in a predictable, non-random manner under PCM control. We have not yet discussed Lambda feedback. We did monitor this much earlier in our evaluation, but I decided to introduce it within the topic, in a way that is logical, allowing me to explain fully, and in detail, the diagnostic process with component functionality.
Understanding
It is not possible to accurately diagnose any fault without fully understanding how the system responds to data input. With any fuelling fault evaluation, you must observe request and corrected data in order to understand if the PCM is responding in closed loop or attempting to correct a fault condition. Our PCM is in closed loop but appears to be causing not correcting the fault.
Most sensors in Europe tend to be 5-wire Bosch, the remainder fall into the 4-wire DENSO type. The 5-wire ID is as follows: Grey, NBV; White, pulsed heater ground; Yellow, reference low 2.2v; Black ref high, 2.8v; Red signal milli/amp, voltage.
The early Bosch variant carries a zero current on the signal wire there will also be a voltage transition between 2.2v and 2.8v, if AFR = Lambda 1. An excessive oxygen condition will cause current to go high of zero and oxygen deficiency would cause current to go low of zero (+/- 5ma). Voltage response on red is similar, lean above 2.8v, rich below 2.2v. The two reference voltages, black/yellow do not change.
I am mindful to avoid the rich/lean description as it can lead to incorrect diagnosis especially without noting fuel trim characteristics, air leaks and dribbly injectors for example, as our problem vehicle clearly demonstrates.
Pressure
Now look in your pocket. I previously mentioned the GDI system storing pressure, unlike common rail diesel. If you rev the engine hard and cut the ignition at peak RPM, you should reach approximately 180 bars. Cycle the ignition back on and observe for any pressure decay. Pressure will hold semi-indefinitely over time.
The later Bosch broadband sensor as fitted to our 1.8 engine is somewhat different in circuit response. Both high and low reference voltages are a little higher, the red signal wire does respond to current in a similar way, however both reference voltages do change in the opposite direction. Sorry if this is confusing, I did warn you.
We need to confirm what the Lambda current is doing on the Audi A3 at the instant of the fuelling anomaly, i.e., when the fuel pressure drops, and more to the point what the PCM is doing about it.
To be sure of our findings, we obtained an identical engine management system fitted to a SEAT LEON FR. Please refer to Fig.1, our first Pico image, which shows current dropping below zero red trace, with both reference voltages rising symmetrically in response to low exhaust oxygen content {rich} From left to right, red trace, initial current at zero, open throttle, load enrichment, overrun fuel off, high exhaust oxygen, repeat test. All normal responses.
Now, please refer to Part two Fig.2 in the March issue, which is conveniently also Fig.2 here. This shows serial data during warm up, taken at a similar time as in the previous data from the faulty Audi A3.
It is obvious the fuel pressure taken from the FR is dramatically higher than the AUDI A3, so what would cause the PCM to adopt a lower GDI pressure? Answer that in the privacy of your own mind. This is the moment that defines the essence of a diagnostic technician. Assess data, do not guess, measurements are essential, prediction is the mother of all mistakes.
Pressure
Back to the PICO scope, take a close look at the Lambda current at the point the fuel pressure is reduced (please refer to Fig.3). Red trace, lambda current, cursor set at zero = Lambda 1. Blue trace, rail pressure, cursor set at 45bar, which is too low? Green/ black = reference voltages, normal response to current change. Bank 1 sensor 1, red trace, suddenly outputs a negative current which theoretically represents a rich condition. The PCM obliges by reducing fuel pressure still further from 45bar to 38bar. This is the essence of the problem. The pressure was already too low. Looks like a faulty sensor. However, replacing the sensor had no effect on the fault condition.
So, we went back to look at fuel trim characteristics, when 38 bar pressure was set, the pcm adopted between -25-32% trim. With a reduction of mean injector quantity from 2.5m/s at 19mg/s to 1.6m/s at 12mg/s. remember the mean fuelling value is taken from two injection events per cycle.
At this point, and based on the absence of any obvious sensor deviation or cross-reference variation, I suggested that cloning the PCM from the LEON FR would confirm or exclude any internal PCM error. My thoughts here were based on the PCM adopting a rich fuelling correction without any input request from a sensor, Lambda error accepted. Diagnostics can be defined by a series of negative results leading to the eventual successful conclusion. So long as it has discipline and a logical process, coding the donor PCM from the LEON FR did not solve the problem. That was a big positive for me. We now know for certain that the error is within the engine fuelling system or an obscure sensor input deviation.
Endgame or Infinity War?
Endgame, we hoped, arrived at ADS Preston, with David G, and me. Earlier interruptions did not help continuity of thought! Today David G and I were given uninterrupted time and access. I suggested we limited the scope observation to lambda current observation only, as this was the critical instant of the fault occurrence. Concentrating on focused blocks of serial data using VCDS.
Fuel trim correction was selected with all the following group data:
- All temperature sensors
- App and throttle angle
- Camshaft angle and MAP values
- Mean fuel injection value
- High, low fuel pressure actual and corrected.
There were no obvious changes with sensor inputs, except the puzzling rich Lambda response. However, we did note that bk1 Lambda did respond first, in other words appears to be driving the fault. There is a clue here. We then went back to Pico very carefully checking voltage and ground at the PCM, looking for any current surges, voltage spikes or background noise at the instant of Lambda reaction. No errors found.
The fountain of knowledge was running dry, but my determination was growing stronger, although we could not see the wood for the trees. The vehicle must be lean? Yet the Lambda current indicates rich. It’s right in front of our eyes, yet we cannot see it. So, we cut the red Lambda signal wire at both ends, spliced a direct temporary circuit and started the vehicle. The Lambda current instantly indicated lean, i.e. positive current. This is good, as the PCM trim was previously in rich trim. The fuel trim very quickly corrected to 0% and the vehicle ran clean.
Not daring to claim success David G went for lunch while I continued to monitor near perfect trim and perfect fuel pressures, 5bar priming, 160bar GDI, a figure not previously reached.
My conclusion was that current appeared to be decaying between sensor and PCM, thus driving the fuel trim in an ever-spiralling rich correction. Yes, mistakes were made in the process and in my opinion due entirely to interruption and workload constraints. The real lesson here is an incredibly old and well understood one, you must separate diagnostics from the general workshop demands and environment, in both space and time.
Remember, I am writing this in real time, and all of this happened yesterday. Since then, despite our efforts the fault has returned. David G has opted to remove the GDI injectors for the second time. If you want to guess how it ends, answers on a postcard please, and see you here next month for the exciting conclusion.
- Setting the bar high
Where were we? I’m wondering that myself, so I will begin with a recap of part one, along with an honest critique of what has gone up to this point. So far, the following parts have been replaced; Four spark plugs, four ignition coils, high pressure fuel pump, and #1 high-pressure injector.
The phrases ‘dirty washing’ and ‘public’ come to mind. Despite what I always tell you, these parts were replaced as a result of a reaction to the symptoms and not as a result of thorough data analysis.
We understand, with confidence, that the fault is due to a lean fuelling condition, but we do not understand the cause. I do, however, have a high degree of confidence it is not a hydraulic-mechanical injector fault, following the ASNU bench test.
David G and I took a step back to review our approach and plan a way forward. Using VCDS, we elected to monitor critical events from crank start through to hot idle. Referring to Fig.1, please note there were no initial issues during at first, then quite suddenly after 30-50 seconds, you will see what happened, coinciding with the onset of combustion error. High pressure is a touch low though.
Moving onto Fig.2, please note the drop in high fuel pressure. At this point it is sitting at 45bar. This is not correct, so why do request and actual match? Has the PCM in error calculated this as the correct value? Or is it an incorrect load value from a sensor, wiring or environment? Maybe it is a PCM internal fault? Experience generally convinces me it is not the PCM however.
Evidence
Let’s discuss the evidence, while also keeping an eye on the camshaft timing which I alluded to in part one last month.
From cold, the exhaust camshaft increases its lift by approx. 0.6mm and adopts an advance angle of 35°. The inlet remains at zero and does not have any lift function. As a point of interest, you should hear a distinct click from the cam housing when full exhaust lift ends together with a sudden reduction in open angle. Consult data frames to see what I mean. This occurs normally after approximately two minutes. Please also note the change in exhaust cam timing to 2.8° actual 4.0° specified. The inlet now adopts an angle of 15° actual and specified.
Moving onto Fig.3, the data displayed shows values from the engine mid-way through the warm-up cycle. The engine is still fuelling from the high-pressure system. The high pressure has now deteriorated to a mere 35bar, and 50% of the nominal expected value. The lean combustion problem is now extreme with misfire count increasing dramatically.
We now reach Fig.4. Finally, after approximately 10 minutes, the PCM reverts to port injection. This can vary dependant on environmental temperatures and engine speed and is accompanied by a more prominent click from the exposed port injectors. The engine now recovers its combustion composure, with the useful visual evidence, high pressure increases to 90bar. The reason for this is to prepare the high-pressure system in readiness for any instant high load demand. Keep this information in your pocket until later.
Assessment
With all this information available, what is my assessment? It is a fact that the only route for fuel to enter the engine combustion chamber is via the lateral feed injectors. The only explanation for incorrect fuelling quantity is a control deviation due to a circuit fault, physical hydraulic-mechanical injector fault, or a PCM calculation error.
Having previously expressed confidence in the hydraulic-mechanical injector function focus transfers towards the PCM fuelling feed back system, the Lambda sensors should theoretically provide all the critical answers we need.
Just to fill in a few gaps before you all go dashing to the internet blog sites, we did conduct exactly accurate injector current profile analysis. The ultimate PCM injector control is fuel pulse time and current path. Using Pico scope and a Hall Effect current clamp, we monitored the injector function together with high rail pressure. We noted no discernible change in injector control when witnessing a rail pressure drop.
Please refer to Fig.5 for this. Blue/black trace represents the injector current path across two injector circuits, with both homogenous and stratified events visible. Green trace represents the PWM control for the high-pressure actuator. We continued monitoring current and rail pressure until the moment port injection took over. Looking to Fig.6, blue/ black trace in this instance represents the current path to the direct injectors, while the red trace shows the seamless transition to the port injectors.
Coming up
Keeping up so far? Well, it’s not over yet. Part three will discuss the response of Bank1 Sensor1 function and response. This will be conducted through direct current measurement, with Pico and serial data via VCDS, paying particular attention to fuelling correction.
Now things are going to get very interesting. What you are expecting is not going to happen. Exciting isn’t it? Good enough for a direct-to-Netflix action movie, or even a mini-series? See you next month.
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The phrase ‘does what it says on the tin’ expresses that a product or service offers exactly what is advertised, and required. On that basis, Premier MOT Training, based on two sites in Hockley, Essex and near Derby, would seem to be pretty explicit in the services you would expect it provides to the motor sector. Sometimes though, a name will only tell you a small part of what is being offered as Director at Premier MOT Training Donna Chapman observed: “We are an automotive training company that specialises in MOT training, but we also do F-Gas training, and hybrid training as well, and more.”
The Essex site, based on a industrial estate in the small town outside Southend-on-Sea is the company’s H.Q, and fields 10 staff, with three in the Midlands. “This is where all the bookings go through. Every student or garage that contacts us will be dealt with in the office upstairs. We are the busier centre, being next to London, and with the M25 nearby, running about 30% more courses here than we do in our Midlands site every week.
“We do the MOT Tester entry qualification every other week and then we do a MOT Site Manager course every two weeks. We do a motorcycle course every two weeks as well. Those are the core courses, but like I said, we do run other courses and a hybrid course as well.” There are three rooms on-site for the class-based training. “We have one smaller room upstairs, and two big classrooms downstairs where we can have up to eight students, two laptops per student, with a projector.”
Relationships
Obviously, with the training being undertaken, it is not all theoretical, and the site includes a large workshop area, including four MOT bays. The site is a real working MOT station, and is also used for commercial purposes in the evenings and weekends, as Donna explained: “We work with two local businesses who serve contracts for the County Council, taxi fleets etc. We've got a good relationship with them because rather than investing £50,000 to £100,000 setting up a MOT station, they rent out our bays.” It also helps Premier MOT Training to have real MOTs being performed on-site: “In order to keep compliant, we have to have a live MOT bay. All my trainers have to keep their live testing status in order to be compliant. You've got to do a quality check on each of your testers too. This means we need a live MOT bay, so it is a win-win. When one of our trainers is doing an MOT, we can QC them, or we can use that as part of a course to Quality Control check somebody else, and then it goes onto the system.”
Donna continued: “Our busiest courses are always MOT, hence why we have run them every week since the pandemic. We used to run groups of nine, but since COVID-19 we keep them below eight just because then they get a desk each and a laptop each, without banging elbows.”
While MOT training is the core offering, the business is keen to promote its wider training, and is looking to push in this direction in the next few months. “We are currently building a new website. We currently have two websites. our MOT Annual Assessment site, which is www.annualtraining.co.uk, and we're currently merging that into our Premier MOT website.”
There are also new courses on the horizon:“We're about to do a pilot in December with Ian Gillgrass, which is going to be launching us as becoming accredited with the IMI. We're going to launch a IMI-accredited diagnostic IMI accredited course for Level 3. Obviously we do offer the hybrid and the F-Gas as well. We are trying to put it out there more about hybrid and F-Gas.”
Surprising
That’s not all: “We had a first-generation Toyota Prius here, and we kept it as a historical talking-point, because people still don't believe the first hybrid vehicle ,the Toyota Prius, was first released in 1998. People were just still shocked that. Types of vehicles that in our sector we tend to think of as new have been out on our road for 25 years. We've still got 70% in the market to train up in this area, which is quite surprising. We try and keep our cars to pretty much what our students see. We could go out and buy brand new cars, but none of our students are ever going to do a test on that or ever do any servicing on it. We are not keeping a brand-new EV though, because that's not relatable to our students. They won't be seeing them. They won't need an MOT or a service in an independent for some time. We try and find something around seven years old because that's what they're going to be getting in their workshop.”
Growth
Looking at what Premier MOT Training will be looking to achieve during 2023, Donna added: “We've really put ourselves about lately, and will continue to do so. We were at the UK Garage and Bodyshop Event at the NEC in June, we were reception drinks sponsor at the Top Technician and Top Garage Awards Evening in October, and we will be at Automechanika Birmingham in 2023. We've gone for a bigger stand, and we are going to try and bring two bikes along because we noticed there was no bikes there last year.” They will also look to grow the team: “We're going to look at taking on another trainer and another member of staff in the office. We don't have sales staff – They are Training Consultants. They are not selling a product, they are supporting garages around their trading needs.”
Donna added: “If we don't feel like it's the right course for someone, we won't book them on it. We are all about matching a candidate for the right kind of course.”
For more information, visit: www.premiermot.com
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