Hey guys, I bought a 2000 Dakota Quad cab, 3.9L Auto., 79,000 miles. This thing is in GREAT condition. When I bought it the only thing wrong with it was the tires needed to be replaced, and a full tune up. (I only paid $4200 for this truck) The tires that were on it were the Good year (I think) KO's, I wanted something a little less noisy, these seemed very noisy..... so I thought. I went and bought some new tires, supposed to be great truck tires. Nothing off-road or anything like that, your basic truck tire.... well I replaced them and it sounds like I have some oversize Off Road tires..... (it sounded like this before but I thought it was the tires) I took it back to the place who put the tires on, because I thought maybe they were not balanced properly.... well they "re balanced" them and it still does it..... when i turn right or left, the sound goes away, but as soon as my tires a straight the sound comes back!! Does anyone have any ideas what is causing the sound???
Also it seems like there is some type of restriction with the (limited) power..... I know, I know, it is a V6.... but it seems like there is... .I can push the pedal all the way to the floor and it will take a few seconds before it really starts to move!! it just feels like there is some type of restriction......
And what is the gas mileage supposed to be??? I am only getting like 13mpg on average... I drive 26 miles roundtrip to and from work, and 23 of those miles are on the freeway.... I already did a full tune up, new plugs, wires, cap and rotor.... any ideas on how to improve MPG??!!!
Let me know if you can help me with any of these problems, THANKS!!!!


Chris

 

Sounds like the front wheel bearings... I either look at getting them replaced or replace them yourself.

 

Well sorry to burst your bubble but the QC w/ the 3.9 is only going to get 13-14 mpg. With a CC they get around 16 so it only goes down with added weight.

As far as power "restriction". It's called the 3.9 is a dog. They are slow. End of story.

A tune up is the easiest way to get a slight bump in mpg. But if you want to do more, you need to spend money. A CAI will help, but it costs money. How long it will take back to make the money, I don't know, but it will be awhile. Same with an exhaust. Helps your mpg but will likely never pay back for a longgg time.

As far as the sound goes, I think its probably a wheel bearing like bolt said.

 

Most definitely a wheel bearing, especially if the sound goes away when you laterally load the wheel (turning).

 

I have an 01 with the 3.9. My MPG improved when I put in a 180 degree thermostat and a V8 throttle body. It improved the power some too. If you replace the tstat be sure to use a quality part like the Napa Super Stat or a Robert Shaw. The V8 TB made a noticeable gain in the power at highway speeds, it's easier to pass slower drivers and I move quicker through the gears to get up to highway speed faster. My truck is a 5 speed manual. I get about 19 MPG combined highway and town driving. I used to get close to 21 MPG but I had Michelin LTX tires then. When they wore out I couldn't really afford another set so I went with Firestone Destination LE's, 235/75R15's. They are a great tire, nice smooth solid ride and great traction wet or dry and really quiet for a truck tire. But they have a much more agressive tread pattern than the Michelins, so my MPG dropped a little.

I don't know what kind of plugs or ignition parts you used but the 3.9 works best with plain old copper core Autolite 3923 plugs. Also the distributor cap and rotor button from Napa with brass contacts, part #'s MO 26 and MO 28 work real well in the 3.9, and I personally really like the Belden Premium wire set from Napa.

If it's never been done, maybe try getting an induction cleaning service like BG or Ever Wear at a local garage or tire store. Then every 5,000 miles or so run a bottle of Gumout Regane through the gas tank. There are other fuel system cleaners out there but they are mostly kerosene and really don't provide much benefit. Redline is a good one too. Regane has the highest concentration of PEA and it does work better than the others.

If you have a bad wheel bearing or hub that can affect the MPG too. I'm surprised the tire store didn't notice it. Also if you have an automatic, well the MPG will be less. I had a 2000 CC with an automatic before my 01 and it was a real pig on gas. It probably got 13 to 15 MPG on a good day.

You also want to pull the throttle body and clean it if that hasn't been done. Check the MAP sensor. With the ignition in the run position and engine off there should be about 4.5to 5 volts on the center wire. Start the truck and let it idle. The voltage should slowly decrease to 1.5 to a maximum of 2 volts. If not, the MAP sensor is lazy and you may get better power and MPG by replacing it. Check the Throttle Position Sensor, with the key on and engine off you should have between .35 and .90 volts on the center wire. Some say .75 is the best and there is a mod for that. With the engine off, slowly open the throttle and the voltage should increase to about 4.5 to 5 volts with the throttle wide open. While you are cleaning the TB, remove the Idle Air Control valve and very gently clean the pintle end with a shop rag soaked in carb cleaner. Don't drop it or handle it roughly or you will need a new one. You just need to clean the black crusty stuff off the pintle. Check the o-ring when you reinstall it. Use a new gasket when you reinstall the TB too.

It could be lazy O2 sensors but at only 70K it's not likely. When it is time to repalce the O2's you should only need to replace the 2 upstream ones and be sure to use either the dealer parts or order from NGK online. Don't use Bosch, they don't work very well in the 3.9. Check the air filter too, that can cause MPG to suffer.

Be sure to use a good quality oil and filter too. I use Napa brand 10W30 conventional and a ProSelect filter and change it every 3000 miles. Napa oil is just the same as Valvoline, only without the Valvoline price. I used to use Maxlife and Napa High Mileage, but my truck was a bit sluggish and the MPG was a little less on it, so I switched back to conventional. High Mileage oil is at the thicker end of its given viscosity, so it can be detrimental to MPG and power.

Those are a few things to check to help your truck run better. Hope it helps you.

Jimmy