hey has anyone here just have there mileage drop..was getting about 14.5 now it about 11 ever thing the same..same gas same drive..had new y pipe w/cat's and o2 sensor's just a year ago...doesn't seam to be any drag from brakes and it running fine ...maybe time for plug and wires???

2003 dakota reg cab short box 4x4 3.9 122000 miles

 

There are a lot of things that can affect MPG. If the plugs are original, definitely change them out. If the plugs you have now are copper core and have more than 25 or 30,000 miles change them out. If the wires are original, change those out too. Stay away from Champion plugs. They might be OEM but still they don't work very well in a 3.9. Autolite 3923 plugs work great in the 3.9 and can be bought cheap at Walmart. You can get a good deal on the Autolite Professional wires from Advance with an online discount code, plus there is a $10.00 rebate from Autolite. Autolite Professional are excellent quality, just as good as OEM. If the cap and rotor have more than 25 or 30,000 miles change those out too, the Napa Echlin line with brass contacts work best and last a long time without getting corroded.

Also if the O2 sensors are original it may be time for new ones, they do get slow and lazy as they age. 80,000 miles is about the right time to change them out. You should only need to change the upstream O2's, the rears don't really do much and should not need changing unless you get a code for them. Use NTK or Mopar OEM, stay away from Bosch O2's, they do not work well in Magnum engines. NTK is the supplier for OEM Mopar anyway, and you can also find them at a good price at sparkplug.com or ntk.com. The Haynes Book for the 3.9 has directions on checking the operation of the Manifold Air Pressure sensor. There is also a really good thread on here about the MAP sensor, search here in the 2nd Gen forum for "Power Loss and MPG Solved" and it should come up. Also check your Throttle Position Sensor, the Haynes Book tells how to check it too.

Check your air filter, replace if it is dirty, clean the throttle body and Idle Air Control Valve, check the exhaust for leaks, get a good fuel system induction cleaning done at a good independent garage. BG and Everwear are very good induction services and you can get those at most any good garage or tire store.

Check your tire pressure and alignment. Too low of a psi in your tires can rob you of up to 10% of your MPG. If the alignment is out of whack it will definitely have a detrimental effect on your MPG. Depending on where you live you might want to use a thinner oil, 5W30 may save you a little gas but Dodge recommends 10W30. Some say synthetic oil helps MPG too, I never have used any synthetic oil myself.

Use a good quality name brand gas, run a good in the tank cleaner a couple times a year. Red Line Si-1, Chevron Techron Concentrate and Gumout Regane are the better ones to use. They all contain Polyether Amine (PEA) which is the best in-tank cleaner because it is the most effective cleaner and burns off cleanly without leaving any residue behind. The other fuel injector/fuel system cleaners are mostly just kerosene. Red line Si-1 is the best and strongest of the PEA based cleaners but it can be hard to find. They sell it on Amazon, and I have heard O'Reilley's sells it. Pep Boys used to sell it but it is not on their web site now for some reason. Regane High Mileage fuel system cleaner has a high concentration of PEA plus it contains friction modifiers. Some people add a one to two ounce maintenance dose of Techron, Regane or Si-1 at each fill up when they can not get name brand gas. Gumout is a good product, even their fuel injector only cleaners still contain some PEA, just not as much as the Regane branded products.

You can buy a fuel pressure gauge cheap at Harbor Freight or maybe rent one at a parts store. Your fuel pressure at idle should be between 47 and 51 or 52 psi. If it is not in those specs you may need a new fuel pump soon. A weak fuel pump can cause loss of both power and MPG. Do the key dance or get a parts store to read your truck for any trouble codes, if you have any codes stored they could help tell you what may be wrong with your truck.

These are a few things to look for and check to get better MPG. Driving like Grandpa helps save gas too. I use www.fuelly.com to track my gas mileage. It is a good tool to see where I am with my MPG and any drop in MPG will let me know something may be wrong with my truck.

Jimmy

 

WOW!!! that was some good info! but i have to ask..how would an exhaust leak effect my mileage? and it funny you said that because i do have a good one!! i was waiting for a little warmer weather to fix it( just hasn't been that warm yet) and my mileage seamed to get worse with the exhaust. just never put the two together..as far as the rest...sunco gas, amsoil 10-30 , newer tire's good pressure 02 sensor just a year old..so i guess i'll start with the exhaust then plug's and wire's..not sure how long they been in there. same one's that came with the truck 2 yrs ago.....thanks again i'll let you know how i make out...

 

Exhaust leak can cause the pcm to dump too much fuel into the air-fuel ratio. It will run rich and use more fuel. Bad exhaust will cause the engine to run poorly so you will use more gas that way too. Maybe you could go ahead and get the exhaust fixed now, get a good garage or exhaust shop to do it. I think exhaust work is best left to a shop especially if you live in an area where they salt the roads in winter.

Jimmy

 

yep your right...lot's of salt here...trying to get in the shop next week...sure glad i asked..never would have thought the exhaust would anything to do with bad mileage. but you make a valid point...

Jerry

 

Jimmy always comes through with well written and thought out responses!

vtlogger, are you in Vermont? If so, where about?

 

Thanks Brad..I like to post and help out when I can. It's good to see you back on here again, where have you been? Did you get your truck back on the road again?

Jimmy

 

As seen in my new thread, my leaf shackle broke so I cleaned it up and parked it for the winter. Next weekend I'll be making a trip down to my parent's place (I've been stealing a bay of their garage), and getting her out.

I picked up a low mileage '02 Legacy GT for a DD (40k miles when I bought it) but it just doesn't cut it for me. Super boring car (automatic and slow as death). I've been dying to get my Dakota out again. Although it has almost 170k miles on it now, I still feel its in great shape and want to continue enjoying it. It's a fun truck (5 speed, 4.7, 4x4). So I decided to start coming on again and seeing whats going on w/ the forum.

Good to see you're still around as well, I don't recognize a lot of the users on here now...

 

Yep.. in a small town called Bridport...about an hour down the lake from you... my shackle did the same thing on a trip back from CT last spring. Not all that bad to change. i did both side's..if one went the other probably not far behind...and your right ..Jimmy did a heck of a job on the write up.. i even printed it and put in the toolbox!!

Jerry

 

Ah, Addison County is probably the part of VT I'm least familiar with. I went to school at Vermont Tech for 5 years and dated a girl that lived up near Jay for 6 years, also have friends down in the Brattleboro area so I actually am quite familiar with Vermont, just not the Middlebury area very much, I'm usually to the east of the mountains.