When a maintenance service fixes an elusive drivability problem

When a 2008 Audi Q7 4.2L automatic transmission with 110,579 miles was brought to us, the customer had already invested a lot of money into the vehicle (Figure 1). As we like to say, “the parts cannon had been unloaded.” Although the initial checkout gave me a solid direction, selling some additional diagnostic time helped me win this battle. This vehicle was brought to us because of a misfire fault code and flashing check engine light. The repair order stated that the customer had been trying to fix a misfire on cylinder 2. I was not made aware of what the customer did in an attempt to fix the misfire. I was also not made aware of how to duplicate the misfire concern. I think that sometimes the history of a vehicle can be necessary and other times it is not needed at all. Each situation dictates the tactics used to diagnose the vehicle efficiently. How to duplicate the problem is something that always needs to be relayed from the customer to the technician. More often than not, if I interview a customer, I can get them to tell me how to drive the vehicle to duplicate the concern. That also means that if I have to interview a customer, something got lost upfront during the check-in process.

 In our shop, we have what is called an initial vehicle inspection (IVI). This inspection is the starting point for every concern with few exceptions like maintenance checks, brake inspections, etc. This IVI allots one hour to the technician to diagnose up to three concerns. In that one hour, we’re not guaranteeing to be sure what the fix is to those three problems. What we guarantee is that the technician will follow the IVI checklist for each problem. This checklist is a guideline for the technician to follow so that easy stuff doesn’t get overlooked (Figure 2). To explain how we built this checklist, I will give you a scenario that has happened to me more than one time.

I have been into a diagnosis for three hours and traced the problem back to a blown fuse. This sort of oversight cannot happen, so we added checking fuses into our initial vehicle inspection process. There is a valid reason for every bullet point on the checklist. At the end of the inspection, what we always try to provide the customer is two options. The first option is additional pinpoint testing because we couldn’t diagnose the problem within the allotted hour of the IVI. The second option would be to sell the customer a part that we concluded is faulty. When it comes to a misfire under load, there are some bullet points that I can safely overlook in the guideline, but some I still have to follow. The guidelines I followed for this diagnosis were:

• Performing a visual inspection  
• Performing a full module scan 
• Confirming the customer concern

I could see that new coils and spark plugs were present from the visual inspection. I also could see that the intake manifold had been recently removed. I concluded this was likely to perform cleaning of the intake valves, because this engine is a gasoline direct injection system. Coils, plugs and carbon build-up on the intake valves are some of the most common causes of misfires on these engines. When I work on VW/Audis, I always perform an auto-scan using VCDS (A popular VW/Audi scan tool). I then save the entire file to my computer for later access. These log files are impressive for drivability concerns, because they offer a freeze-frame attached to each fault code (Figure 3). Using VCDS for VW/Audi can provide some insight on how to drive the car to duplicate the problem, in the event the service advisor was not able to relay that information from the customer to the technician.

In the log file, there were three misfire fault codes: a P0300, P0302 and P0304. There was also a fault code that I was not familiar with — P2231, “Air-Fuel Signal Shorted To The Heater Circuit.” What I noticed between the misfire faults and the P2231 was that they were all setting under heavy load. I now knew how I needed to drive the vehicle to duplicate the concern. I tried to cheat by doing a heavy brake-torque in reverse, but I could not duplicate the misfire in the bay. Before I went on the road test, I thought that based on the calculated load results in the freeze frame and my personal experience, I was going to be condemning an ignition component (I have seen plenty of cheap aftermarket coils bad right out of the box). It had been my personal experience with VW/Audi that I could frequently reproduce the symptoms of  a bad coil or plug right in the bay. It’s very rare for me to need to drive the car to observe misfire activity data on the scan tool. 

As I prepared myself for a road test I started thinking about what PIDs may be valuable if an ignition component was NOT the cause. After all, I only got an hour to diagnose a concern that is only exhibited while driving. Based on any heavily loaded misfire, my top three suspect-areas were: 

• Ignition components
• Fuel restrictions 
• Airflow restrictions 

What I have noticed on CAN (Controller Area Network) controllers for VW/Audi is that they typically don’t turn the injector off for a misfire. This is particularly important when monitoring fuel trims during a misfire. If the PCM shuts the driver off for an injector, the exhaust gasses will be lean once that occurs. As I started to work on more UDS (Unified Diagnostic Services) controllers, I saw this exact situation very frequently. I typically find a P130a associated with a misfire fault code. This fault code indicates to the technician that an injector has been shut off to protect the catalyst. In my experience, once the PCM detects a misfire, the injector is shut off until the next key cycle or the codes are cleared. The controller in this Q7 is a CAN controller. For me, it is safe to say that if I feel a misfire, I can trust that the fuel trims are a valid diagnostic indicator, because the engine controller did not stop pulsing the injector for that applicable cylinder. As a result of the injector still functioning, I should not experience a lean exhaust scenario (one that is induced by the engine controller).  

I did a total of four test drives with this vehicle. During the first, I monitored all eight cylinders’ misfire counters. I found that under heavy load,  cylinder 4 was counting misfires rapidly. This brought my attention to a different cylinder than the customer has been trying to fix. During the second road test I monitored:

• RPM
• Load
• Lambda bank 1
• Downstream oxygen sensor voltage b1
• Misfire counts for cylinder 4
• Fuel adaptions for partial load and idle

In the VW/AUDI world, Lambda-Control is another term that replaces short term fuel trim. This is the PCM’s response primarily to the air-fuel sensor input or upstream oxygen sensor input. I was unsure that I could trust the air-fuel sensor due to the circuit fault DTC set, so I used the downstream sensor as a backup. What I found was that as load increased, the Lambda-Control would climb positive, the downstream oxygen sensor was indicating lean and the misfire counts started racking up (Figure 4). 

This information tells us a lot about the agreement between the upstream air-fuel sensor and the downstream oxygen sensor, but there was a small problem that I’ll explain in a moment. Short term fuel trim or Lambda-Control  in the VW/AUDI world is the addition or subtraction of fuel to keep the exhaust in a stoichiometric condition. Lambda went positive during the second road test, showing an addition in fuel, likely because the air-fuel sensor input indicated lean (or above 1.0). The downstream sensor also went lean. This suggests that the air-fuel sensor agrees with the downstream oxygen sensor output. 

I was still convinced that I had a problem with the air-fuel sensor, because VW/AUDI can also use the downstream oxygen sensor to influence fuel trim. When I did my road test, I never monitored the pumping current or actual output value of the air-fuel sensor. This meant I was taking some liberties concluding that the two sensors agree with one another. What if the PCM used the downstream values for Lambda-Control? A lean downstream HO₂ ’s value can cause the PCM to add fuel to that bank. The next fastest way for me to rule out the air-fuel sensor was to just unplug it.

My third road test included me unplugging the air-fuel sensor and duplicating the problem one more time in an open-loop fault strategy (Figure 5). Since the air-fuel sensor is the primary input for fuel control verification, that bank would now go into open loop fault because it was not reporting any data. This effectively puts the PCM strategy in base fuel control, based on load. My thoughts were if the engine cannot run well in base fuel control, the misfire must not be caused by an air-fuel sensor problem. This technique was not necessary for older vehicles. Wide-open throttle typically forced open-loop drive/ full-throttle enrichment. This effectively puts the engine into base fuel control, with no oxygen sensor input.

Now, with modern vehicles, the engine remains in total fuel control under almost all conditions, including wide-open throttle. The road test results with the air-fuel sensor unplugged revealed cylinder 4 misfires were still counting up under heavy load, even while in open-loop/no trim correction. I thought to myself that I could safely rule out an air-fuel sensor fault. I then started to focus on the added fuel trim to compensate for a lean exhaust gasses. 

For more great information on fuel trim, reference “Increased use of turboochargers brings new lessons to using fuel trims for diagnosis” (September 2017) by Scott Shotton. He addressed some in-depth fuel trim analysis and how it can apply to misfire diagnostics. You can check it out, as it backs up what I’m saying at MotorAge.com/fueltrimdiagnosis. 

Before we say “this looks like an injector issue,” let’s start with what can we deductively conclude this is NOT:

I could likely rule out an ignition issue. In my experience with VW/Audi, I have never seen high trims due to an ignition misfire. On a Ford, that’s a different story. Even a lot of older Ford products would disable the injector for a misfire, and this would cause a lean exhaust condition.

I could likely rule out an airflow restriction on the intake side. If the valves were loaded with carbon or the valves couldn’t fully open, the result would be a rich combustion event. Because the injector is in the combustion chamber on this engine, fuel is going to get into the chamber, unlike other injection designs (such as port injection where the fuel is delivered into the intake manifold).

I couldn’t definitively rule out an exhaust path restriction for that cylinder, so I took some liberties with my hypothesis. Based on my knowledge and experience, and since the engine is quiet, I was comfortable with saying that an exhaust path restriction on this engine was unlikely. When I started to look at the position of cylinder 4 in the engine layout, I noticed something that stood out to me. It was at the back of the fuel rail on bank 1 (I will elaborate on the significance of this shortly).

So what was the next step? Should I scope the injectors to make sure my current flow is equal to that of the known-good injectors? High resistance in the injector circuit may cause an issue with fuel delivery. I’m usually all in for scoping the injectors, but I thought that if there was a circuit problem with this injector, the PCM would likely set a DTC. Could I AC-couple the high-pressure fuel rail sensor, sync off an injector and look at the injection pulses? That test can be done, but in my experience, it’s highly unreliable. Since the fuel rail pressure sensor covers a wide range of pressures, the pressure pulses can be hard to detect. Simply put, the signal is not sensitive enough to do so accurately. To view these pulses well, there has to be a lot of vertical enhancement done with the scope and the result is a bad quality image. Another problem with this test is the rail pressure sensor can be dampened heavily with all of the fuel rail tubing. The dampening absorbs the pressure drops related to the injector firing event. What about the location of the injector? On a GDI system, the high-pressure side is returnless. On a lot of vehicles, I have encountered clogged injectors and the injector that is clogged is at the end of the rail where contaminants cannot exit from. As I mentioned earlier, cylinder 4 is at the back of the rail. The location of the injector influenced my hypothesis that a restricted injector was probable. So my options at that point were to replace the injector, swap the injectors or try and clean them. 

 Swapping a GDI injector is a lot of work with additional parts for an outcome that may not reveal any results. The same goes for putting a brand new injector in the vehicle. I decided to run a can of BG 44k (a popular fuel injection system cleaner) through the rail on this vehicle. I have a lot of luck cleaning GDI injectors that are restricted with BG fuel cleaning products. This is maintenance that really should have been done a long time ago. Even if it didn’t fix this vehicle, I wasn’t going to beat myself up for selling an overdue maintenance item. But to no surprise, the vehicle  was fixed. My fourth road test included all the same PIDs as described in the previous road tests — but without misfires (Figure 6). Lambda-Control is in a good range and the downstream exhaust is reporting “rich” (meaning the CAT is functioning well by storing and using the oxygen).

The right scan tool can be a very powerful tool when coupled with some knowledge and  experience. Sometimes performing maintenance fixes broken cars. For me, it is a rare situation, but when it happens it’s usually the best-case scenario for the customer!