(Printed in the Journal of The Alliance of Automotive Service Providers, AASP) 

Have you ever dug deep into a diagnostic problem and then suddenly wondered if you should have stuck to an idea you had early on in the repair? Isn't that called Monday night quarterbacking? Well, not exactly.

This summer a customer came in with a pinging complaint on a 2000 Nissan Maxima. Our service technician drove the car several times but was never able to duplicate the pinging. The customer was instructed to use premium fuel as recommended for that model in the owners manual. The customer then decided to seek a second opinion from a local Nissan dealer. That dealer referenced a Nissan TSB that also recommended using premium fuel.  Several weeks later, the customer returned to us with the pinging complaint. The customer insisted that the car was pinging on the highway at all times. The customer also said that he had been through a full tank of premium fuel but the pinging remained. My first two highway test drives revealed a well running car with no pinging at all. During my third test drive I finally heard the pinging. The nearby highway entrance ramp is at the end of a steep grade. I found that the car now pinged moderately, then severely, during this climb to highway speed. In addition, the car was also pinging on a steady grade when loaded. Strangely though, the pinging would sometimes stop and not occur for several minutes. I found it odd that the pinging would be intermittent under a constant load. No codes were stored and nothing obvious jumped out at me when viewing the datastream. A search of TSB's led me to one that recommend replacing all six coils with a redesigned type. Not a cheap repair at close to 100.00 per coil. Even then, it would be a trial repair at best. Thinking back to my GM days, I decided to give the engine a top-end decarbonizing treatment. After the decarb I noticed the pinging had disappeared. Unfortunately, my cure was short lived. The pinging had returned on a road test several hours later. I started wondering if the coils were the cause as the TSB stated. I decided to swap the coils from a late model Maxima in inventory that I knew was not pinging. First I swapped over the front three coils. The car still pinged intermittently. Then I swapped the rear coils over. Now the pinging had vanished. This time the pinging returned on the third test drive. A search of the IATN (International Automotive Technicians Network) e-mail archives revealed a similar case. Another technician had the identical problem on a 2000 Maxima and stated that the car was fixed by replacing: O2 Sensor Bank 1 Sensor 1, the Mass Air Flow Sensor and a top engine decarbon. At this point the easiest item to swap over was the MAF. That was easy to rule out as the car pinged on the first test drive with the other MAF. Now that all the easily accessible items were tried, it was time to dig a little deeper. Thinking the EGR may be inoperative or stuck I decided to remove the top, electrical portion. Unfortunately, the mechanical side would require some other disassembly to access. Instead of getting my hands too dirty I decided to grab the Snap-On Vantage. Placing the Vantage in "dual meter mode" I decided to monitor both front O2 sensors. During the pinging events, the front bank O2 sensor hug up reporting rich. I was wondering if this rich report was due to a problem sensor or if the rich report the result of a pinging mixture. Hard to tell since this was the first time I had ever monitored an O2 sensor on a car that was pinging. The sensor also had a tendency to be sluggish on the lean-to-rich transition. I thought to myself: "What the heck, I'm buried in this thing anyway, I might as well bite the bullet and replace the sensor." Again, another futile attempt, as the car was still pinging. At this point I started thinking way back to the gas issue again. I ran the car as low on fuel as possible and then filled it with high octane. Finally, I went through five complete test drives without any pinging. The cause was the fuel's octane rating. Most likely the customer did not believe the fuel could be the cause. Perhaps he did not fill the tank with premium initially. Sometimes people just don't realize the complexity of the systems and programs in these cars. Back in the late 80's we went through a similar problem with the GM 2.8 liter engines. On cold mornings customers would have their cars towed in for "cranks but won't run" complaints. By the time we got to those cars in the afternoon they all started and ran fine. For those of you that remember, the 2.8 engines had to have 87 octane or less. The ECM's strategy was so specific that high test fuels would prevent the engine from running in cold weather. We later learned in a GM TSB that the higher octane fuel was less volatile in cold ambient temperatures, thus causing the no starts.  Most of the customers I explained this to thought I was nuts. Perhaps I was, but switching to the low octane fuel fixed all those cars.