When we first got the v6 dakota years ago it would go 280-340 a tank depending on driving style and highway vs city. Now Im luck if I get 220 out of a tank.

So far ive replaced the spark plugs, wires, cap and rotor. throttle position sensor, Idle air control valve, air temp sensor, new air filter and im about to replace the pre cat o2 sensors.

Im wondering if a clogged charcoal canister or faulty purge solenoid would effect mileage? What about the crank positioning sensor or coolant temp sensor? What about the Manifold absolute pressure sensor?

The idle is alittle rough and it seems like the RPMs are higher then they should be at say 55. Anyone know of a chart with RPMS and corresponding MPH?

 

Bad O2 sensors can cause the IAC to act up and make the truck run rough. I would look at those and depending on the mileage it is a very good idea to replace both of the upstream O2 sensors. A slow or lazy O2 sensor can definitely affect MPG and they may not always cause a trouble code from the PCM. I replaced both upstream O2's in my 3.9 just as a maintenance item at 85,000 miles. Don't use Bosch O2 sensors, they do not work well with the 3.9. Buy the new sensors at the Dodge dealer or you can order from www.ngk.com for a lot less money. I got mine from NGK and it was about $65.00 shipped for both sensors. They arrived in just 2 or 3 days too. I have heard NGK is the OE supplier for Dodge anyway. It can also take the PCM 200 miles or more and so many start cycles to learn new O2's and the new IAC.

The MAP sensor can also be a big cause of reduced MPG and power too. The Haynes book tells how to test it but I think you check the center wire for voltage with the ignition on and engine not running, should have about 4.5 to 5 volts there. When the truck is started, the voltage should slowly decrease to about 1.5. Anything less and the sensor is bad. I had 1.4 volts on mine and replaced it. It helped on MPG some, but restored power too. There is a thread on here from about a year and a half ago called "Power Loss and MPG solved" or something like that. There is a lot of good info on the MAP sensor in that thread if you can find it.

If your evap canister were clogged, it would throw an emissions code. I doubt it could cause bad MPG though. What plugs did you use? 3.9's run great and get good power on Autolite 3923's.

That's all I can really think of right now, but my brain is fried from being out in the 105 degree heat working on my truck today. Others will have more info for you.

Jimmy

 

don't know of a chart, but the fuel makes a difference.

most gas now is a methanol or ethanol blend. lets say a vehicle gets 23 mpg highway on pure gas. it might only get 18 mpg highway on blended gas. and most gas now is a 10 to 20% blend.

KillerKilgore

 

I just ordered a new map sensor from rock auto. The o2 sensors I bought were NTKs. I dont know if my truck is different but it has 4 o2 sensors. The dealer told me my truck is an ultra low emission model. They wanted almost 100 dollars apiece for the sensors. I did some searching and found the corresponding NTK sensors they were still around 60 apiece. I replaced the plugs with NGK already as well. Its also getting new cap and rotor.

My future plans for the truck to increase mileage is SCT Tuner, accel coil, CAI, throttle body spacer, ditch the old bed cap and get a tonneau cover. I may go to a V8 throttle body.

 

you said you already replaced the cap and rotor. Dont do it again.

a throttle body spacer is a complete waste. The coil too is a waste.

as for the original question - ethanol blended fuels; MAP, 02, air temp or coolant temp sensors; clogged cat; bad plugs; dirty throttle body and intake; heavy foot; extra weight.

 

How can I tell if the CAT is plugged?

 

Hit it with a rubber mallet or your hand. If it rattles, it's bad.

Jimmy

 

Or if you have a lot of water pouting out your tailpipe.