Let me start off by saying this isnt another "i want 25 mpg" type post. I dont usually post on here but i do read a lot thats posted on here on a daily basis. First off i have a 1999 dakota base with 3.9 and 4x4 and 121K miles. Upgrades: magnaflow cat and flowmaster exhaust to dual pipes. im driving a consistent 30 miles one way to work everyday with about 95% highway driving. I understand its a truck and it will get horrible mileage and i didnt buy it three years ago expecting it either. my question is does 17 mpg on almost solely highway driving sound about normal to the rest of you?
I was getting around 15-16 mpg and then i decided to stop doing 75 or 80 on the highway and drop to 65mph on the highway and thats when i saw the 17mpg average. I havent done a full tune up since i've had it ( bought it with 87K miles 3 years ago and now has 121k) i've priced up a new rotor, cap, coil, and plugs at autozone. totaling a whopping $95. i understand these are good upgrades to do but will it truely make a difference? Any advice or opinions will help.
Justin

 

The cap, rotor, and plugs are a good idea. 30k is about average for the stock plugs. I'd hold off on the coil and NOT replace it unless you have a problem.

Shop around for the parts - really. Prices vary considerably at auto parts stores. What plugs have you been looking at? It came with Champion, probably Copper Core. You should be able to find them for less than a couple of bucks each.

On the cap, make sure it has brass terminals.

Hope it helps.

 

17 mpg with mostly highway driving is OK for 3.9. I used to get about 17.5 to 18.5 back when I was commuting to work and home 56 miles round trip, 5 days a week. About 80% of my mileage was on the Florida Turnpike at 70 mph. My best-ever MPG was 22 on a long interstate trip up to Georgia and South Carolina and then back home. I used my cruise control a lot of that trip and only bought Shell gas. I had some Shell gas cards I had bought at a discount, $50.00 gas card for $40.00 at the grocery store.

17 MPG is decent. I would not use any tune-up parts from Auto Zone, except maybe the plugs. Napa is what I use for tuneups. I use their Belden Premium wire set, they are better than OEM and cost about $34.00. I do a tune-up at every 25,000 miles. I replace my wires at every other tune-up and have never had any problems at all with Belden wires. I use NGK plugs in the stock heat range, I think they are FR5 or FR11, I don't remember which right now. The cap and rotor button from Napa with the brass contacts are the very best you can get and they only cost a few bucks more than the cheap parts from Auto Zone. Cap and rotor part numbers MO 26 and MO 28 at Napa, around here it's about $23.00 for both. I would not replace the coil unless you have backfiring or starting problems, the coil is one of those parts that either works or it does not work.

The Napa cap and rotor button will last you a lot longer and get a little bit better spark to the plugs. The brass contacts do not corrode up like the steel ones in the cheaper parts. Belden wires are great quality and very durable. Slowing down a bit will also help save gas but it is not always possible to drive the speed you might like. Keeping a steady constant speed helps a lot too. One thing I did that seemed to help my MPG a bit was to remove the snorkel off the front of the air intake box and then I took a Dremel tool to it and made the opening much bigger, so it draws in more air. It howls a bit when I punch the throttle or go up hills but it helped a little on power and MPG. It might help even better if I had a larger exhaust.

One other thing you can check is the Manifold Absolute Pressure (MAP) sensor. It is on the front of the throttle body. With the ignition on but engine not running, check the voltage on the center wire at the connector plug with a volt meter. You should have between 4.5 to 5 volts. Then start the engine and watch the voltage reading. It should slowly decrease to about 2.0 to no lower than 1.5 volts. Anything lower than 1.5 and the MAP is reading slow and it is best to replace it. Also if you have your original O2 sensors it may be time to replace those as well. Some say the O2's will get lazy around 75,000 miles. I replaced mine at about 80,000 and it did help improve my MPG. You only need to replace the upstream O2's, the rear ones after the cats don't do much. NTK is the best one to use, do not use Bosch. They do not work well at all in Dakota's. Get the direct replacement NTK with the connector plug already wired in and it will be a lot easier job.

Those are a few things to do. There is a good thread about MPG and the MAP sensor here:

https://dodgeforum.com/forum/2nd-gen...er-solved.html

Jimmy

 

Thanks for the reply. the plugs i pulled up were autolites app5224's they're platinum plugs i believe. i replaced plugs at roughly 90k but that was all that was done ignition wise. they were just the cheap copper plugs also. how important is it to replace the wires? they look fine with no corrosion. I do have a friend that owns a shop in town so he can usually also get parts for a cheaper discount as well

 

Thanks for all the info Jimmy.
i know from experience autozone parts can be junk. especially electrical parts and non-hardware things. the best mileage i've had was 19mpg and that was all highway from akron ohio to westvirginia and back. i usually dont have too much of a heavy foot unless im in a hurry like running late to work lol i can easily and comfortably cruise at 65 on the highway. i had the downstream o2 sensor done around 15K miles ago when i had the new cat put on. but not the upstream sensor. i do have an aftermarket intake installed. grant it, its in the engine bay so i can pull warm air from the engine but with cone filter, theres less air restriction. when i step on the gas, the truck roars but thats a combo of the intake, cat and exhaust which i like. im going to check the map sensor too to see if thats getting the correct voltage. i'll stay away from the coil then because the truck doesnt even hesitate when it comes to starting. and thats with an old battery too. lastly i forgot about the small hole in the exhaust in the weld just before the cat starts. i can feel a real small amount of air coming through it. would that disrupt the back pressure that much to rob power and fuel consumption?

 

There is no need for a platinum plug in a 3.9. They work best with a plain copper core plug. Dodge engineers did a study some years ago and found the Magnum engines work best and produce the most power on a plain copper core plug. That is why the truck came from the factory with copper core Champion plugs. I used to run Autolite 3923's, but I changed to NGK a few years ago when Autolite moved most of their manufacturing to China and Mexico and the quality went way down. I think the 5224 is what Autolite used to call a "truck plug" and it may be a bit hotter than the stock plug. You should do fine if you use a stock heat range plug or go one range colder if you have any pre-ignition ping. I like the NGK plugs. Mine are made in Japan and have been doing great.

I guess wires is kind of a personal preference. Some people have run a set of wires over 100 or 150,000 miles or more and had no problems. I just replace mine every 50,000 miles as preventative maintenance. I carry my old set with me just in case I might have a plug wire problem on the road somewhere. The Belden wire set is great quality, every bit as good and better than OEM for a good price. They can help get a bit more spark to the plugs and each one fits perfect and is the exact length you need. Plus they are a cool blue color, not that that matters much.

Jimmy

 

I think any hole or leak in the exhaust would be detrimental to power and MPG, but I guess the size of the leak would determine how bad of a problem it is. My own opinion is it would be a good idea to get the leak fixed anyway.

Jimmy

 

i have found that any chrysler engine with a distributor car will get moisture in the car through the vent in the top next to the center post. i fix it with JB-weld in the vent hole. took me 5 years to understand why i had moisture in the cap.

 

how can you tell if theres moisture in the cap? and yeah i need to get the hole in the weld fixed. i've got some of that exhaust tape stuff but not sure if that will help with all the pressure in the precat. my friend with the shop said he could possibly just weld up a peice of sheet metal around it patch it up so theres options. i think my headers are leaking too. i always hear the puttering sound bouncing off the walls on the highway when im giving it gas and have the windows down.but not such luck finding the holes in the manifolds anywhere.

 

I Ran autolite 3923's duralast gold stuff all from autozone in my 5.2(plugs,wires,cap and rotor) I think I paid 64.00 out the door. Worked great for me and didnt have any problems. Bumped up my mpg 1 or 2 since my plugs were original with 80k on them. Unless you have a 5 speed 16-18 is all the better you are gonna do on the highway. I used to get about 11-13 city and 15-16 mpg highway. Thats why I switched to diesel. My ram gets about 16-17 city and ive saw as high as 22mpg on the highway.