11/11/2020
FROM THE FILES OF "THERE'S A FIRST TIME FOR EVERYTHING"
Yesterday we had another shop brought us a diagnosis that they didn't have the time to deal with. The vehicle is an 06 Jeep Grand Cherokee with a 3.7L V6 engine. The vehicle had two problems. First problem was that the vehicle didn't want to start when you turned the key..until you released the key a little. This was easy. On these vehicles, the electrical part of the ignition switch is housed in plastic. This allows wear which then allows movement, which messes with the contacts within. Very easy to diagnose, kind of a pain to repair.
Second problem was the vehicle running rough, no power, and a single random misfire code. Now, as i tell my regular customers, the computer codes don't tell us what is wrong with the car, but rather they act as a sort of roadmap(I actually confused a very young customer with that reference, as she'd never seen or heard of a roadmap....think GPS). It gives us clues as to which road to go down to find the problem. Sadly in this case, the map was wrong. The random misfire code is P0300. Any misfire code in the "P03's" is generally a problem in the ignition system.
With this in mind, the very first thing we did was to verify the misfiring cylinder. We found cylinders 1, 3 and 5 all misfiring. We also noted that at the back of the vehicle, we could smell raw fuel from the tailpipe. On this vehicle, these three cylinders are all on the same bank(remember the engine is a V-6). Very odd. We pulled the easiest to reach coil pack and spark plug. The spark plug looked relatively new. We hooked up a tester to the coil, and re-started the engine. Very good spark. OK, the ignition system's not the issue. We listened to the fuel injectors through a stethoscope, and found them all sounding normal.
Before going to a compression test, now we started the engine, and looked at the live data on the scanner. This showed us misfires on 1, 3 and 5, which we already knew. scrolling through the data, we can watch the fuel trims, and the oxygen sensors. This is where it caught us off guard. One single oxygen sensor wasn't reading quite right. Short term fuel trim was going nuts , while long term fuel trim was going nuts taking fuel away. As in -30%. This is essentially leaning out the air/fuel ratio on just that bank. These fuel trims are how the computer constantly adjusts the air/fuel ratio, giving us cleaner running engines. Both long and short term fuel trims were normal on the other bank.
Now seeing this, we went back to the oxygen sensors, both up and down stream. Both sensors on the other bank were moving as they should, and rapidly. The upstream oxygen sensor on this bank was moving, but not enough upon closer examination, and nowhere near fast enough. It was essentially stuck showing full rich. The computer seeing this kept trying to correct for the problem by subtracting fuel from this bank.
Now I have to be honest here. In 35 years working on and diagnosing these things, I personally have NEVER seen an oxygen sensor create this type of problem. I was hesitant to call our customer(the other shop) and tell them that the problem was an oxygen sensor, but the call had to be made. After explaining all of the readings to them, they agreed, and told us to go ahead and get it installed. I personally still had doubts due to having never seen this before, so I think you can imagine my surprise when we installed the new sensor, cleared the codes, and re-set the computers memories, then started the engine...and she ran smooth as glass.
After 35 years, we are STILL learning. This is the kind of job that makes this profession fun.
