Multi displacement Woes
I have a fun case study for you this week. Last week I had a shop call me to come diagnose a misfire on a 2006 Jeep Command with Multi-Displacement . For people that don’t know what that is, simplest terms would be the PCM turns off certain cylinders for better fuel economy. The back story on this vehicle goes as such, vehicle came in to the shop with a blown engine. When they replaced the engine they also replaced the solenoids that activate the cylinder deactivation during the replacement. The vehicle drove for about 2 months after the replacement, then returned to the shop with a cylinder #4 misfire. During their diagnoses they replaced the lifters on that cylinder and spark plugs (this vehicle has 2 per cylinder), after this repair the misfire on cylinder #4 was still present. The shop then replaced the engine on this vehicle again in hopes of fixing the misfire. After the engine was replaced the misfire was still present, and this was the point where they called me in. When I got there I started my standard misfire diagnoses, I performed a clear flood and the cadence of the engine sounded uniform. Then I went to spark and fuel checks by performed a current ramp check on the ignition coils and fuel injectors at the fuses (injectors and coils had their own fuses). This checked good as well, which lead me to check my secondary ignition pattern for this cylinder and I noticed the burn pattern looked a little strange. So with seeing that I decided to do a little swap tronics and swapped coils and plugs with a know good cylinder, during this swap I noticed that the spark plugs were saturated in fuel. After swapping parts I checked my secondary pattern again and it still looked off. And here is where things got interesting with the diagnoses. Cylinder #4 is one of the cylinders that will deactivate when the multi-displacement happens. So I decided to scope the command on the cylinder #4 & #6 deactivation solenoids, which showed the PCM was not commanding this cylinder to close. So I decided to take my pressure transducer and run an in-cylinder check to see what my valve activity looked like, and this is when it got real interesting. You could see on the capture that you had valve activity on cylinder #4 until the engine built up oil pressure, and as soon as the oil pressure was present the valves would stop moving. This showed me that the deactivation solenoid was stuck open keeping cylinder #4 shut down at all times. Since the PCM wasn’t commanding the cylinder shut down, it was still supplying fuel and spark to that engine. This is why my secondary pattern had looked strange, because the cylinder was fuel saturated with no fresh air being pulled in to the cylinder. Also since the cylinder had valve movement during the first few revolutions, it had passed my first check of listening to the cadence of the engine. You could have diagnosed the valve movement (or lack thereof) with the use of one of our VA sensors in either the intake or exhaust while the vehicle was running/extended cranking check.
