Most of us have witnessed the arrival of new automotive technologies, and most of us have learned to master them under sometimes difficult conditions. Lately, however, new technologies are introduced at such a rapid pace that we don’t always realise that what was new yesterday, had become the norm today.
Direct Petrol Injection is a good case in point. Today, most of us can diagnose and fix most of the problems that come with extremely high fuel pressures, excessive oil consumption, and carbon build-up on intake valves. With these kinds of issues, we know what to expect in terms of driveability problems, but what happens when we encounter a problem that does not seem to fit any of the patterns we have come to expect?
For instance, just how likely is that an HPFP (High-Pressure Fuel Pump) on a direct injection engine will develop too much fuel pressure, instead of too little, which is a common enough issue on direct injection petrol engines? In this article, we discuss the curious case of an overachieving HPFP on a 2014 2.0L Audi A4. The root cause of the problem might surprise you, but let us begin by stating-
When the Audi arrived at this writer's workshop on the back of a recovery truck, it was accompanied by its owner, who was not pleased with the service he had received at a competing establishment. In the interest of fairness to the other workshop, though, it must be stated that we knew them to be consummate professionals, but sadly, they had no, or limited experience with direct petrol injection because they specialised in diesel repairs.
Somehow, we never learned why the car’s owner had allowed the diesel specialists to do as much on the fuel system as they did. Nor did we ever learn why the diesel specialists had not referred the car to someone that had some knowledge of direct petrol injection systems in general, and of direct petrol injection systems on Audi engines, in particular, sooner than they did.
None of those details was relevant to the problem at hand, but we did eventually manage to persuade the Audi’s owner to tell us what issues he was experiencing with his car. Here is what he told us-
According to the owner, the problem started a few months ago when he noticed a marked increase in the car's fuel consumption. He also noticed that the car was not as "zippy" as before and that the idling quality was deteriorating, and that it got progressively worse as time went on. Then, one day, a few weeks ago, the MIL light came on, and a workshop close to where he lived diagnosed a defective catalytic converter, which they replaced.
However, replacing the catalytic converter did nothing to fix the problem. The idling quality continued to deteriorate, the car’s fuel consumption continued t increase, and the loss of power became more noticeable. Three weeks ago, the car started spewing black smoke from the exhaust, and it became very difficult to start the car when it was cold. This continued for a few days until he could not start the car at all, regardless of the engine temperature.
The customer also stated that the MIL warning light came back on directly after the catalytic converter replacement. Asked about previous repairs, the customer rather pointedly said the HPFP had been replaced, not once, but twice, on the recommendation of the diesel repair shop. He explained that this workshop had found a trouble code relating to excessive pressure in the fuel rail, but when neither the first nor the second HPFP replacement fixed the problem, the workshop replaced the fuel pressure regulator.
When the fuel pressure regulator replacement also did not work, the diesel repair shop admitted defeat and recommended that the owner take his car to a workshop with more experience of petrol direct injection. They recommended us, and here he was with his problem Audi, which brings us to-
The circumstances of this tale seemed a little strange, to say the least, but sure enough, when we scanned the Audi, we found the result shown above. There was a clear history of fuel pressure issues, but the thing was this; none of us had ever seen a case of excessive fuel pressure on a direct petrol injection engine before.
On the other hand, we had seen many cases of insufficient fuel pressure. In the majority of these cases, either the fuel pump leaked fuel into the oil, or more commonly, the pump plunger or cam lobes driving it wore out or broke, thus limiting the pump’s stroke.
In any event, the Audi would not start, so we decided to scope the fuel pressure regulator’s output to get a better sense of the actual situation. Here is what we found-
This trace showed that the pressure in the fuel rail increased upon the pump’s first stroke, but we did not have service information ready at hand to see what the desired fuel pressure should be. Nonetheless, we now knew that the fuel pump was working, and for the moment, we accepted the OBD system’s word that the actual fuel pressure was higher than what was desired.
We also noticed that the pressure in the fuel rail remained relatively constant when we cranked the engine for a few seconds, which told us two things. The first thing was the fact that there was no way two new fuel pumps could be defective in the same way. The second thing was that we needed to obtain service information on the high-pressure fuel system before we did anything else.
So, while we waited for a service advisor to find, buy, and download the information we needed, we reviewed the situation in some detail so we could formulate a proper diagnostic strategy. Based on what we knew up to that point, we could exclude a few things as probable causes. These included-
Thus, based on what we could reasonably exclude as causes or contributing factors, we could reduce the issue to its simplest form, which was the fact that the HPFP was developing too much pressure. We already knew that, so that did not help us much, but still.
Since we were still waiting for the service information, we decided to remove the tappet cover to see if we could find anything wrong with how the pump was driven. Consider the image below, which was obtained separately from an official Audi source-
Image source: VW/AUDI
As with all VAG direct petrol injection engines in this class, the fuel pump is driven by four lobes on the back-end of the exhaust camshaft. The schematic shown here shows the fuel pressure regulator that is mounted on top of the fuel pump, as indicated by the blue arrow. The tabs indicated by the green arrow on the intake camshaft act on the camshaft position sensor to monitor the camshafts’ correlation with the position of the crankshaft.
There was nothing strange about what we saw. The four drive lobes on the exhaust camshaft showed no signs of wear, and the pump's plunger seemed to be in perfect condition. Therefore, based on what we saw there was no obvious cause for the excessive fuel pressure, which set us to thinking about diagnostic procedures in a general sort of way.
We knew that reducing a problem to its simplest form is usually the best starting point in a diagnostic procedure. We also knew that the basic principles of diagnostics have not changed much over the years if they changed at all. For instance, and regardless of the complexity of a system or component, the same laws of electricity and physics apply to all systems and components, meaning that a logical approach to diagnostics will (eventually) solve all problems.
The only thing that does change is how the laws of electricity and physics are implemented between applications. For instance, for the fuel pump on this engine to build pressure, the fuel it displaces must be contained. In this case, the fuel had to be trapped between the injectors and the fuel pressure regulator. If the fuel pressure regulator works as designed, and only one injector is open at any one time, the fuel will be sufficiently contained for the pump to build and maintain the system’s rated pressure.
If on the other hand, the fuel pressure regulator does not work as designed, the fuel will follow the path of least resistance. Thus, whatever pressure the pump does manage to build up will simply escape through the defective pressure regulator, and return to the fuel tank through the low-pressure fuel return line.
Just then, though, the service advisor announced the arrival of the service information we needed. Here is-
It turned out that this particular engine could be fitted with one of two different types of fuel pressure regulator. The one type is "normally open", meaning that it needs an electrical signal to close to contain the fuel pressure. The other type is "normally closed", meaning that it needs an electrical signal to open to vent fuel pressure from the fuel rail.
Therefore, the approach one takes to diagnose fuel pressure problems on these engines depends entirely on which type of fuel pressure regulator is fitted to the engine. The service information also recommended a way to check which type of pressure regulator is present on the engine. This involved connecting a scan tool to the vehicle and then cranking the engine for at least three seconds.
If the fuel pressure PID showed an almost immediate increase in the fuel pressure, the pressure regulator is normally closed. Conversely, if the fuel pressure PID drops from a residual value, or remains at a low value, the pressure regulator is normally open. In this case, our initial scoping of the fuel pressure showed that our pressure regulator was of the normally closed variety because the fuel pressure increased dramatically within one engine revolution and remained relatively constant.
The service information also told us that the fuel control module maintains the fuel rail pressure in a narrow range by cycling the pressure regulator "ON" and "OFF" as required. Therefore, if there was something amiss in the regulator's monitoring and/or signal circuits, the fuel pump could perhaps generate excessive fuel pressure if the pressure regulator could not vent excess pressure. However, this was an unlikely scenario simply because there were no pending or active fuel pressure regulator trouble codes present in the car’s fault memory.
Essentially, the service information was not very helpful, apart from telling us that there were two types of fuel pressure regulators available for this engine. Clearly, it was time to rethink this problem in general, and the relationships between the various parts that made the high-pressure fuel system work as designed, in particular. As a result, we decided to-
Sticking to the principle of reducing any problem to its simplest form, we were beginning to realise that we were overlooking some basic principle of this system's operation. For instance, we assumed that the loss of power and misfires were the direct result of an over fuelling issue, but what if it was, in fact, caused by something else?
One candidate issue was a miscorrelation between the positions of camshafts and the crankshaft. This would explain all the symptoms except the excessive fuel pressure, but the more we thought about it, the more it seemed like a viable diagnostic avenue to pursue. However, there were neither active nor pending camshaft/crankshaft correlation codes stored in the fault memory, which complicated matters somewhat.
Nonetheless, the more we thought about a possible miscorrelation issue, the clearer it became to us that the fuel control module might be using this correlation to control the fuel pressure. Thus, to test this theory, we decided to scope the correlation between the camshafts and the crankshaft. This is what we found-
While we suspected a miscorrelation, we did not expect a miscorrelation of six degrees, as shown here. Such a large offset would certainly cause misfires and a loss of power, but the strange thing was that it did not set a trouble code. However, we could only work with what we had, so we set about working out the details of the problem by using simple logic. Here is-
Our point of departure was the fact that the fuel pressure regulator had to be closed during each injection event to ensure sufficient pressure for each injector to produce a proper spray pattern. The best way to achieve this would be to use four cam lobes to drive the pump, which would ensure that maximum fuel pressure is always achieved just before each injection event on each cylinder.
Therefore, if the position of the drive lobe on the exhaust camshaft did not correlate with the position of a piston in any cylinder because of a miscorrelation between the crankshaft and the camshaft, the ECU will energise/de-energise the fuel pressure regulator either too soon, or too late. However, since there was no wear on the drive lobes, each stroke of the pump would still deliver the proper fuel pressure, but at the wrong time.
At this point, it dawned on us that we had been looking at the problem in the wrong way. It became obvious to us then that the problem was not so much one of a discrepancy between the actual and desired fuel pressures, as it was one of an undesirable fuel pressure being present at the wrong time, because the fuel control module uses the cam/crank correlation as an input to control the fuel pressure regulator.
Despite our belated insight into the root cause of the problem, we were still faced with an absence of cam/crank correlation codes. Six-degree offsets are a big deal on any engine, but to save time, we bought some more service information to clarify the issue.
This information duly arrived, and it told us that a cam/crank miscorrelation on this engine needed to reach, or exceed, a massive 11 degrees before the ECU will set a trouble code. Therefore, no miscorrelation codes were present because we were 5 degrees from the critical threshold. Nonetheless, a six-degree offset was big enough to upset the relationships between the various parts of the high-pressure fuel system.
While we had largely solved the mystery, we still had to find the proverbial smoking gun. However, from what we could see of the timing chain and camshaft sprockets, it was not clear what the actual problem was. It could be that the wear lining on a timing chain guide had come adrift, which might prevent the hydraulic chain tensioner from achieving the proper tension on the chain.
It could also be that the rollers between the chain links had become worn, which would have the effect of "stretching" the chain, thereby creating a huge miscorrelation between the crankshaft and the camshafts.
We could not be sure about the exact nature of the problem with the timing chain without first removing it from the engine, but we presented our findings to the Audi's owner, nevertheless. Although he was still hurting from the two fuel pump replacements, he agreed with our diagnosis and authorised us to replace all the timing components on the engine.
As a point of interest, when we compared the old timing chain with the new chain, it turned out that the old chain was so worn that it was longer than the new chain by almost two links, which was positive proof that the miscorrelation was caused by the worn timing chain.
The result of replacing the chain, all sprockets, guides, and the tensioner was as expected. The engine fired up immediately after we cleared all stored trouble codes, and no new trouble codes set during an extensive test drive, which leaves us with this-
Although we derived a great deal of satisfaction from this experience, it was by no means an easy diagnosis to make for two reasons. The first reason is that we did not have proper service information ready at hand, and the second reason is that it took us a long time to realise that the laws of physics do not change just because we had not seen this particular problem before.
This experience also reminded us that automotive technology will always improve, but more importantly, that it will improve at an always-accelerating pace. More to the point, though, this experience again taught us that a), the basic principles of diagnostics do not change and, b), that the most important diagnostic skill one can have is the ability to reduce a problem to its simplest form.
This ability served us well in this instance, although it took us perhaps longer than it should have to first define the problem so that we could reduce it to its simplest form. Oh, well, lesson learned, (again), which is the most important thing, after all.
