How many diagnostic tests does it take to find the solution to an automotive problem? Sounds like the question of the decade given all the high-tech vehicles we work on today. Truth is, some cars are resolved with one or two tests while other cars will require much more. Sort of like the human body when you think about it.

A recent case at our shop began when a customer called in reporting their 1999 Lexus ES300 died on the highway. Little did we know what a diagnostic dilemma this car would take us on.
The Lexus would start, backfire and then immediately stall. The customer reported that the check engine light had been on earlier in the week yet the car had been running fine.

The vehicle's fuel gauge seemed low so we added some fuel. The car would still not run but carburetor cleaner sprayed into the engine seemed to make a slight increase in running time before the car would stall again. At first, the Lexus service manual led us on a wild goose chase regarding the fuel pump's wiring diagram. To make a long story short, the factory wiring diagram and supporting documentation were both wrong. The fuse pump relay was not located in the slot in the underhood fuse panel as the manual had stated. Next, we removed the rear seat so we could verify power and ground at the fuel pump. We connected a fuel injector noid light to prove that the fuel injectors were indeed working. Fuel pressure and volume were tested by hooking up a fuel pressure gauge to the fuel line. A scan of the vehicle's computer did not reveal any abnormalities in the datastream. Seven history codes were stored related to Oxygen sensor activity and cylinder misfiring. These codes seemed attributed to the current running state of the engine as opposed to the actual cause.

Suddenly, the car started and idled normally but could not be accelerated without stalling. Now we were really scratching our heads. How could the problem suddenly improve? How could the car be idling ok now? Could this be an intermittent electrical problem? We tested the throttle position sensor and mass airflow sensor and found them performing to normal voltage specifications.

Finally a break came in during a timing test. Timing was found to be off by a large margin. We suspected that the timing belt may have jumped a tooth. After removing the timing belt covers to access the belt and timing marks the engine was found to be timed correctly. Again, the Lexus service manual was in error. So much for the "relentless pursuit of perfection" Then again, one could say that's why perfection is a relentless pursuit.

A pattern in the cars performance was starting to emerge.  The longer the engine ran, the more its condition seemed to improve. Now the engine was running well enough for a test drive. The car seemed much better but was again getting low on fuel. Fuel was again added. The engine was running well but would sometimes backfire if the engine were accelerated quickly. One of our technicians then noticed something strange. When the car backfired through the air cleaner, the puff of resulting smoke had the distinct smell of diesel fuel. Our technician started to wonder if the car had been filled with diesel or some sort of alternate fuel mixture. We then called and questioned the customer further. The customer now added some important information. When the car broke down on the highway the driver thought it might be out of gas since the gauge was so low. The customer's roadside assistance plan sent out a tow truck. The tow truck driver agreed that the fuel level was too low and added some fuel. Since the car still would not run, it was towed in.

We believe that this vehicle's initial problem was that it just simply, ran out of gas. Secondly, the tow truck driver may have accidentally grabbed the wrong fuel can, and, unknowingly added diesel fuel to the car's tank. Now that the car would still not run, it was then presumed something major was wrong and then towed to us.

Undoubtedly, one of the worse problems to discover and cure can be a fuel quality issue. Both the car's computer, and we as technicians, "presume" that the fuel tank has gasoline in it.  The biggest issue with fuel quality, or fuel "types," is that they are one of the last things ever tested or investigated providing they are even checked at all. All the tests technicians perform are based on the fact that "known good fuel" is in the fuel tank and the fuel lines. The service manual presumes this as well as does the vehicle's electronics. A fuel injector has no way of telling you its spraying diesel, water or gasoline.

Overall, nearly seven hours of diagnostic time was spent on this vehicle. At one point, I overheard the customer say that it was: "Too bad the fuel quality issue took so long to find." I replied to him that he was: "Lucky it only took, seven hours."

I can't begin to tell you how often I have heard of other shop owners complaining that fuel quality issues can take 20 or more hours to find. Some technicians will swap out all the sensors and computers one at a time trying to resolve an engine running problem that routine tests can't find. One case I heard of resulted in nearly 40 hours of labor.

In the end, the more fuel we added the better the Toyota ran.  After adding a final 10 gallons of gasoline, the vehicle ran good enough to return to the customer.

Fuel type and quality issues are never the easiest problems to find. Sometimes all it takes is a technician with a hunch and a keen sense of smell to find these illusive problems. In addition, getting as much information out of the customer can also be critical to a speedy resolve.

Good luck in your future diagnostic dilemmas.