Recently, we had an interesting case study. This one started as a techline call which I thought I had put to bed until the technician contacted us saying that the repair led to a different problem.

The vehicle in question is a 2019 Jeep Grand Cherokee with a 3.6L V6. The original complaint was a misfire with codes P0302 and P0300. The technician did the usual checks of the coils, plugs and injectors with no problem found. The technician performed a compression test and found 140 psi on Bank 2 and 210 psi on cylinder bank 1. I knew from experience that these engines have roller rocker and camshaft issues, so I had the technician remove the cylinder bank 2 valve cover and inspect the camshaft and rockers. Several worn cam lobes were found. The technician replaced the bank 2 camshafts and roller rockers which should have fixed the vehicle.

Lean code

The technician called shortly after the repair to report that the misfire was gone along with the misfire codes, but the car was now setting a P0171 code on bank 1. This was not all that unusual— many times when a misfire is present, the unburned air fuel mixture will wind up in the catalytic converter and cause the converter to overheat and melt down. When this happens the plugged converter on bank 2 restricts air flow which forces more air down bank 1 causing the lean code. I had the technician perform a backpressure test on bank 2. The technician reported backpressure of less than 1 psi at 2500 RPM. This is normal backpressure which rules out the plugged catalytic converter theory. At this point I decided that I needed to look at this car instead of trying to solve the problem over the phone. 

My first step was to scan the vehicle and look at some data. After hooking up the scan tool and verifying the codes my next step was to look at the fuel trims. At idle Bank 1 was + 12 and Bank 2 was -2. This is a very large spread as both banks should be within 2 or 3 percent of each other.  I also noticed that the engine had a slightly rough idle but was fine off idle. The idle was not a misfire but it was rougher that my experience of what this engine should be. 

Flow imbalance

The fuel trims indicated an imbalance in the air flow from bank to bank. This happens when one side of the engine is breathing better than the other. The most common cause is a plugged converter, but our prior back pressure test ruled that out. The next step was to see how the engine was breathing mechanically. For this I used a pressure transducer on the intake manifold. The results are shown in Fig 1. The red trace is injector #2 and the blue trace is the vacuum trace. It is clear from the trace that every other intake pull is week. The pull just ahead of the injector trace is from bank 1. Notice the vacuum is not pulled down as far and trends higher indicating that bank 1 cannot maintain vacuum. Clearly bank 1 has a breathing problem. At this point the bank 1 valve cover was removed to inspect the camshafts. The results are shown in Fig 2. New camshafts and rockers on bank 1 finally fixed the issue with this Jeep.