Setting the bar high

Part two

Published:  15 April, 2021

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.

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.

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.

Related Articles


©DFA Media Group
Terms and Conditions