Some Diagnostics Hurt Your Brain... Fixing Difficult and Perplexing Problems

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Some Diagnostics Hurt Your Brain... Fixing Difficult and Perplexing Problems

Jeff Taylor boasts a 31-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, he is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.

I enjoy working through diagnostics, with my favorite being driveability concerns. The most common diagnostics that I perform relate to check engine lights, and lately some of those have been very challenging.

I still get pattern failures that are for the most part easy to solve and diagnose, but I don’t shy away from the more demanding problems that other shops or techs claim can’t be fixed. I have never been afraid to take on these tasks, maybe because I have a different attitude toward these repairs. I want to be the guy who can say, “I fixed it.”

I have had many of these issues come into my bay for diagnostics over the last 12 months. A couple really stood out as difficult and perplexing, and both had had lots of time spent on them by other shops or other techs.

2007 VW Beetle

The first vehicle was a 2007 VW Beetle 2.5 (BPS engine code). The problem: The check engine light (CEL) is on, and the car is due for its emission test. This car had been at another local shop and the customer tells a story of many dollars being spent, the CEL still being on, but also adds that the vehicle runs and drives just fine. He just wants the CEL light off so that it will pass its emission test.

The first thing I did was look over the invoices from the previous repairs to gain some insight into what’s been done; in this case the vehicle kept setting a P0171 lean code. The other shop had replaced the MAF sensor, changed the fuel pump, fuel filter, plenum gasket, air intake snorkel and the A/F sensor. But after all these repairs the P0171 would return within a day with the CEL on.

After a thorough visual inspection that didn’t expose anything obvious, I plugged in my scanner to see what codes were set, and it was a P0171. I looked quickly at the freeze frame data and it definitely showed that the vehicle was adding fuel (I also noted that the code set at a low rpm). I wrote down the MAF sensor’s part number to have our dealer verify that it was the correct model for this engine combination (Bosch made a number of MAF’s that look the same and fit just fine, but have different calibrations). After a quick call, I confirmed it was the correct MAF sensor, so I needed to look elsewhere.

I traditionally check for CEL codes using OBD-II: it’s the system that turns on the CEL light, so I trust what it tells me. But I wasn’t getting anywhere quickly with the data OBD-II was giving me, so I installed my VAG COM scanner to see if there were any specific VW codes that would help me in any way.

There were two VW-specific codes, 16555 and 17559. The 16555 is the VW equivalent of an OBD-II P0171 and the 17559 translates to an OBD-II P1151. Both these codes pointed to the fact that this engine was experiencing a lean condition, but the second VW code 17559 also told me that the lean condition was only at idle.

Most European cars express fuel trim values in two terms: additive or Idle O2 Adapt (the additive value is the fuel trim variation at idle either adding or removing fuel) and multiplicative or Run O2 Adapt (the multiplicative value is the fuel trim variation at all other engine speeds but idle either adding or removing fuel).

So using the scanner I went to the VW measuring block that would show me the additive and multiplicative (fuel trims): group 32 in this case.

With the car at idle the additive number was high at +7.9%, and the multiplicative was OK at 0%. Everything was pointing to a vacuum leak, but these engines have very few vacuum lines, and the ones it did have were connected and made of hard plastic with good connectors that don’t often fail. I was starting to think an injector O-ring or the lower intake gasket could be the concern, so I attached my smoke machine to the intake snorkel. To my surprise there was no smoke at all under the hood. The engine was tight and sealed.

The only spot that revealed any smoke was the tailpipe and the right rear wheel area. Yes, the right wheel well had smoke coming out of it! This didn’t make a lot of sense, but after putting the car up on the hoist and removing the wheel and inner fender the actual issue was revealed. The plastic vacuum supply pipe to operate the EVAP leak detection pump was damaged, it was chewed through by something. The car did have a vacuum leak… it just wasn’t in the engine compartment.

After repairing the chewed line and clearing the codes, both the additive and multiplicative were good at 0% and 0%, and after an extended city road test (lots of stop and go) with no CEL light the issue was fixed and the vehicle returned to the customer.

2009 Ford F-150

Another memorable diagnostic was on a 2009 Ford F-150 with 176,000 miles and a 5.4 V8 Triton engine. The complaint: CEL is on, sometimes the CEL will flash and it had lacked power under load and when driving hard at highway speeds. These issues had been ongoing for almost nine months and it always had the same code: P0302.

Ignition coils, plugs, and even an injector had been changed over a nine-month period, but the problem always came back and with the same P0302 code. The original tech was convinced that the truck needed a timing chain or cam phaser because of the mileage and the fact the engine was noisy. I was asked to get involved and help solve the problem.

I wasn’t so sure about a cam timing or phaser concern that was expressed by the previous tech. It had never set a cam code or had any other misfire codes on the same bank, something I would expect if there was a cam timing or phasing issue.

I took the truck for a road test to verify the complaint and it definitely had a power concern, the CEL did flash and you could feel the truck missing. If you continued accelerating hard, the truck actually started to slow down.

My initial road test feeling was that this truck had a partially plugged exhaust. I returned to the shop, scanned it and, yes, it had a P0302. I looked at the freeze frame data and fuel trims. Fuel pressure and temps were all in the normal range.

My initial suspicion was still a plugged exhaust, so I checked the back pressure by removing the pre-cat O2 sensors and it had a reading of less than 1/4 of a psi on both the left and right. That value for back pressure is well within spec, so that blew up my theory of a plugged exhaust.

Next I hooked up my Ford scan tool and went to the power balance display during the road test. It verified that it was only cylinder Number 2 that was causing the misfire.

I went through all the basics, testing cylinder compression both running and static, a cylinder leak down and all with normal results. We all know that a Ford misfire can be a bit of a bear to diagnose, due to the fact that the misfire monitor may initiate failure mode effects management (FMEM) to prevent catalyst damage. When a Ford enters FMEM, it will shut off the fuel injector to prevent raw fuel in the misfiring cylinder form entering the catalytic convertor and overheating it. The FMEM will keep that injector shut off for 30 seconds on a 2009 model, and after those 30 seconds it looks to see if the misfire is still occurring. If it is, it repeats this cycle until the misfire stops.

Knowing this I hooked up my small two-channel scope to the Number 2 injector and Number 5 injector on the opposite engine bank as a reference. Performing a road test quickly verified that the engine was going into FMEM and shutting off the control circuit to the Number 2 fuel injector. The next step was to scope the cam and crank sensors to see if there were any anomalies that could cause a miss, and the patterns showed me nothing out of the ordinary (I was grasping but didn’t want to miss anything). I knew that the injector was OK; it was new and had even been moved to another cylinder just to verify it was OK. The PCM was only detecting a misfire on the Number 2 cylinder; at this point I was confidant in saying that the issue wasn’t mechanical or a fueling issue: it had to be ignition.

But how could it be ignition? All the parts of the cylinder Number 2’s ignition system had been changed and were now new genuine Ford parts. But the same problem persisted. In town this truck never misfired, only during hard acceleration or under heavy load did it act up.

I went back to something that I hadn’t looked at for a while. I visited the Motorcraft website and started to look for the OBD-II information that Ford provides there. Remember that it is the OBD-II system that is turning on the CEL light. I found the information that I was looking for under the header “OBD-II Theory & Operation” (Ford and most manufactures provide this information free on their diagnostic websites). Looking through the document I found the section on misfire monitor, and after reading it carefully, I saw something that I had missed: that an ignition coil’s primary circuit failure can also cause fuel injector deactivation (FMEM).

The primary ignition circuit is in the PCM, so I hooked up my current probe to the Number 2 ignition coil and went for a road test and finally saw the problem, it was a bad driver circuit in the PCM. The PCM was not able to supply the ignition coil’s primary side the 5.5 amps of current required to properly fire the coil after the rpms got above about 2,500. The result was the PCM activated FMEM and shut off the injector.

I checked all the powers and grounds one more time before installing a new PCM. The next road test provided a great feeling as the truck smoothly accelerated with lots of power, no misfires and no CEL. It was fixed.

Not all repairs are easy or straight forward. We are at a point in our industry of great change, in the way that things are repaired and diagnosed. There are occasional silver bullets on the Internet, and there are repair information databases devoted to collecting common repairs and fixes.

But these things are just tools; you have to understand how to use them and when. You need to have a vast understanding of how systems work, or how they were designed to work, but even then you need to have never-ending persistence to overcome all the road blocks that can prevent you from unravelling the most difficult repairs. ‚óŹ

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