I have the van out for a pre-vacation shakedown trip and gas mileage is horrible. I am getting 10 MPG when I should be getting 15 MPG. The van runs pretty good but has a tiny miss on idle and it is sluggish on heavy grades.

From what I can tell Code 51 talks about the van appearing to be running lean when it is actually very running rich. http://dodgeram.info/Engine-Gas/OBDI/pcm_fault_code.htm

I pulled the plugs and they are black so nobody is steeling my gas...it's definitely going through the engine. The rotor also looks cooked.

So far I have an NTK O2 sensor, a thermostat, cap/rotor, and air filter on order. Do I need anything else? Due to the ridiculous fuel costs I need to throw parts at it and get it working properly immediately.

 

Spark plugs too.

O2 sensor will likely fix the major issue though.

 

I replaced the cap, rotor, and air filter before a 200 mile jaunt but that did not help. The new air filter seemed louder so I actually stopped and checked to make sure it was installed properly but it turned out that it was OK.

The old O2 sensor was a Denso, hopefully the NTK lasts longer. The original O2 sensor lasted for over 2 decades before I changed it.

The new fuel filter is in now in but no luck tackling the thermostat yet. I'm in the midst of changing the engine mounts too so I haven't run the thing yet. The engine mount replacement job is a PITA, you have to take out the engine adapters too because the transmission hits the floor before you can get the mounts out. grrrr

 

The code 51 has went away with the O2 sensor and fuel filter. The mileage has also bounced back to normal.