2004 V6 MPG Nosedive
 

Hi,
I'm new to the forum and I hope you guys can help me.
I have a 2004 Dakota Quad Cab V6
For the past year and a half my MPG has been getting progressively worse. Lately I have been averaging about 10 mpg in the city whereas I was getting 14 when it was new. However, I fueled up the other day and I noticed that my mpg has gone in the toilet. I've only driven about 80 miles and I'm in between half and 1/4 left in the tank. This is ridiculous. That's like 4 or 5 mpg. The truck is running fine otherwise.
It was in to Dodge for service last month for the check engine light and they changed the plugs, decorbonized the FI and changed the o2 sensors. (Or so they told me they did). BTW The plugs were changed in Oct 2008 by them too. (again - so they tell me)
I don't want to bring it back to Dodge because I don't trust them. Anyone having this problem? Or have any suggestions?

I would pull a plug or two and see if they are new or old and make sure the gap is correct. They probably used Champion replacement plugs, Autolite 3923's work a lot better in the 3.9. I would get under the truck and check to see if the O2's had really been changed, maybe check for marks from the socket or wrench they would have had to use to R&R the O2's. Which ones did they replace anyway? And how long has it been since the truck was at the dealer? Usually it should not take very long, less than 50 miles for the PCM to learn the new O2's and adjust the air/fuel ratio. Be sure they used a Dodge O2 sensor and not Bosch.

Do you have any codes from the PCM stored? Turn the ignition on and off 3 times and on the last time leave it on and any codes you have will show in the odometer. Did the dealer reset the PCM when they finished the job? You may have to take it back and complain that the MPG is in the toilet since they did the work. Something there is just not right, and 5 mpg is crazy.

Jimmy

Well there are a lot of simple things to check first. Air pressure, air filter, plugs, wires...

Even though dodge told you they changed somethings, I would do your own tune up and see if that helps. Does it feel like it has less power then it already did? I mean, losing gas milage is usually just simple maintence things that need to be adressed.

And yeah 4-5 mpg is unreal... Thats like stock cars running at 8 grand.

EDIT: Dam you CC, you always just beat me with a 10x better post lol.

Thanks for your help, guys. I'm definitely checking into all of that.

I just put in a K&N air filter and put air in the tires. (They were low - only 32psi).

I'm going to pull the plugs that are in there now to look at them. The dealer supposedly changed them but maybe I'll change the plugs and wires anyway. Pep Boys didn't have the Autolite 3923's but they did have the platinums. Should I try to get the 3923s or do you think the platinums will make a difference?

The only difference platinums will make (other than they cost a lot more) is they last longer than copper core plugs. The 3.9 and other Magnum engines run best with a plain copper core plug, like the 3923. There is no need for platinum, iridium, plutonium or kryptonite whatever plug in the 3.9. I think it's a waste of money when the 3923 works so well. Dodge engineers even did a study and found out the Magnum engines performed better with a copper core spark plug.

If you are having problems finding 3923's just get them at Walmart. You will find them in the oil filter section, around here they cost about $2.80 for 2 plugs in a pack. If you go to Napa or Advance you have to specifically ask for 3923's because their books won't list a 3923 for any Dakota.

I have no info on K&N air filters. I never used one but others here have. What I did do though on my truck is I removed the right angled snorkel tube off the air box (it is very restrictive) then I took a dremel tool to the front of the box and made the opening a lot bigger. It gets a lot more air flow that way and is just as good as a high performance air filter. I just use a plain paper air filter in it. It howls a bit if I am going up hills or passing someone. If my exhaust ever breaks I will get a better flowing muffler then.

32 psi is probablytoo low for the tires. It's interesting you mentioned it because I just tried an experiment of my own on my tires. I run Firestone Destination LE 235/75R15's and always kept them at 35 psi and usually got about 16.5 to 17 mpg. I write down my mileage at every fuel fill to track my MPG. A couple of weeks ago I bumped my tires up to 40 psi. I filled up yesterday and calculated my MPG when I got home, it was 18.36. I was pretty happy with that, and the increased psi gives a little firmer ride quality, which to me is better.

One other thing I do is add 2 ounces of Marvel Mystery Oil to each 10 gallons of gas when I fill up. I have tested how the MMO affects my MPG too, and it is better on MMO, by about 1 to 1.25 MPG. Plus I get a little better performance and a smoother idle withit, because of the upper cylinder lube properties of MMO. I think you have other things to get fixed on your truck though before MMO would help.

Jimmy

Air pressure can make a big difference if they're low. It is obviously not the reason you are getting 5 mpg, but like CC said it is the difference between 16 and 18 mpg...

remember guys, he has an 04 , so he has the 3.7.

I know for the 4.7 NGK 6987's work best, so i would ASSUME that is the case for the 3.7 but I wouldn't know for sure.

Thanks for the input.
Filling the tires to 45psi did make some difference...and the truck has a stiffer ride now. I also added a bottle of Lucas Upper Cylinder Lube and Injector Cleaner when I fueled up. I'm definitely noticing a change in the MPGs. I've gone about 30-35 miles since I fueled and the needle barely moved. Maybe I also had a bad batch of gas but it seemed to burn through my last tank like I had a leak. I'm definitely going to pick up the 3923 plugs as soon as I can find them. We'll see what happens.

Good. Better be careful with higher psi in the tires, it can cause steering and control problems. 40 psi is working great for me though. I have heard and read on other forums Lucas helped on MPG. I tried it once and my truck did not run very well with it.

Jimmy