I just purchased a used 2005 Dakota 3.7L, so I am a bit of a Dodge newbie and am in the process of troubleshooting why this beast only gets 9 mpg, I am wondering if someone can answer an easy question. I replaced the plugs and both upstream O2 sensors with some minor improvement, but it didn't fix the problem. While testing, I noticed that if I disconnect the battery for a few minutes before starting, the overhead reports 19 mpg at 45 mph until the engine goes closed loop, then the mileage suddenly drops to 10 mpg. Of course, disconnecting the battery before every trip would be a bit inconvenient, so I need to get this fixed. I suspect the problem is likely a sensor that is ignored during open loop operation. My question is whether the MAP sensor and IAT sensor are ignored when the engine is running open loop. If so, I think I will replace them next.

Thanks in advance. George

 

well depending on the mileage, it could need a cat conveter. when the truck is running, hold your hand at the tail pipe. if it is not hot, cat is plugged. the only other sensor is the iat.

 

No codes, truck runs good both open loop and closed loop. Plugs looked like it was burning a little rich, but they were clean and a bit worn. The truck has 26k miles and accelerates strongly so I don't think the cats are plugged. I can't find any vacuum leaks using a stethoscope or by spraying starting fluid. The fact that the truck runs and idles equally smoothly open loop or closed makes me suspect a sensor rather than a vacuum leak. The only sensors I can think of that would affect mileage besides O2 sensors is the MAP and IAT, unless the wiring or PCM is hosed. If the MAP and IAT are not used during open loop then I suspect it is likely one or the other is broke. I suppose it doesn't matter, both parts are cheap, easy to replace, and I am driving near by the Dodge place tomorrow so I'll just replace them both to test the theory..

 

My overhead was indicating 9 mpg at a steady 45 mph, replaced front O2 sensors and plugs, mileage increased to 12 mpg at 45 mph. Replaced MAP, IAT sensors and PCV, overhead indicates 26mpg at a steady 45mph. Much better. Next step, clean throttle body, IAC and EGR.

For what it's worth, I just drove from Houston to FLorida at a steady 70 mph, got 21 mpg using Exxon 89 octane purchased in Louisiana, and 23 mpg using Shell 87 octane from Mississippi. Seems like that should be the other way around. I calculated this by dividing miles by gallons, although the overhead was actually pretty accurate. I think I'll double check on the way back using 87 octane from Louisiana and 89 octane from Mississippi.

 

My new 2008 V6 4x4 with 1100 miles is getting around 14.5 according to the overhead. This is around town. I did a hundred mile round trip yesterday and got 16.5 MPG all highway. I attributed this somewhat sucky mileage to new engine break in. Will I see a increase in MPG at some point? If so when or is something wrong with my new toy ? I'm running 87 octane from BP.

 

You should see marked improvement for your Hwy mileage. I averaged 21.5 with my V8 for my 3000+ trip across the country from our move. You should definitely see the same or better with a V6.