Decreasing Gas Mileage in Dakota???

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mopowers

mopowers

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I've got a 2006 3.7 Dakota 2WD club cab with 85,000 miles on it. Up until about a year ago, I'd get ~20mpg highway and ~17mpg city. Over the past year, I've noticed the mileage slowly decreasing. Over Christmas, I drove it ~400 miles round trip to my folks house, and got 17.5mpg with no headwind. Lately it's been around 14.5 city and 17.5 highway.

Any ideas why??? Plugs were replace a year ago at 70k miles. I'm also really anal about keeping the tires properly inflated. It seems to idle nice and clean, with no stumble. At this point, I've just sorta chocked it up to the truck getting older. Is there something I may be missing?

Thanks for your help.
 
Sounds like 02 sensors. I would check that first anyway. Downstream one first. Only need a VOM to test them.
 
mine has seemed to do the same 4.7 use to average about 18.5 now its about 17mpg. i to have heard o2
 
Thanks for the input. I always figured a faulty O2 sensor would trip the check engine light. I haven't had the light trip yet. I'll test the sensors though just in case. It does seem a little down on power as well.
 
Might want to check the coolant temperature sender as well. It may not be signaling the ecm that it is warmed up....from what I remember OBDII vehicles will not trigger the light until an emissions reducing problem has occurred. Just because the light is not on does not mean a code has not been set. The light is there to let you know that you have a problem that is increasing the emissions. Before you go tossing $$ would suggest getting someone with a good code reader to see if there are any trouble codes set...
 
All are good suggestions. Start as ink said some will throw a discreet fault with no light. If you need a thermostat, I personally go back to oem on my vehicles due to several of the after markets failing shortly after installing. If as stated the temperature isn't where it should be it will mess with the system. Even if the guage is about where it normally runs. The O2 sensors usually from my experience won't give a light unless the heat circuit is failing. As far as O2 sensors go buy NTK (ngk) direct fit. The one I bought for my Durango from Mopar was made by NTK who is also NGK. It was stamped right on the sensor.
 
still sounds like plugs to me. who changed them/are they the exact same type/were they gapped properly/any crossthreaded/what torque
 
Dubob said:
All are good suggestions. Start as ink said some will throw a discreet fault with no light. If you need a thermostat, I personally go back to oem on my vehicles due to several of the after markets failing shortly after installing. If as stated the temperature isn't where it should be it will mess with the system. Even if the guage is about where it normally runs. The O2 sensors usually from my experience won't give a light unless the heat circuit is failing. As far as O2 sensors go buy NTK (ngk) direct fit. The one I bought for my Durango from Mopar was made by NTK who is also NGK. It was stamped right on the sensor.
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Did the O2 make a difference in mileage ?
 
Mine were faulting due to the heat circuit. It was a 99 5.9l was never really good on gas. After reading thru all this again, moparlover may be on to some good things. Reading your initial post it appears like the issue started after new plugs. If someone threw iridiums or something other than the oem style spark plug it could I believe affect performance. I try to stick with oem style plugs. Lots of parts stores try to push the exotic plugs, but the ignition system isn't designed around that style plug.
 
Any chance that you've been putting in an ethanol mixed fuel? That will noticeably drop your fuel mileage.
 
Thanks for the input guys! I really appreciate it.

a year or so ago, I swapped the old factory plugs with the same exact ones and gapped them to whatever the spec in the FSM called for (0.040" i believe). Some of the old plugs had as much as 0.065" of a gap. I'm pretty sure none of them are crossthreaded. I didn't use a torque wrench. I just snugged them down like I always do. Since it's easy, I think I'll pull a couple and see what they look like.

I also thought about the whole ethanol thing. There is ethanol in our gas here in California, but I'm not sure if the percentage has changed recently. I don't believe so. I think we've got 10% for regular pump 87, but I'll have a check.

So far, my list of things to check includes a couple plugs, ethanol percentage, and any trouble codes.

Is it possible that a faulty O2 and/or coolant temp sensor may NOT throw a code?

Thanks again guys.
 
They have been upping the percentage of ethanol in our gasoline here. I had heard that the decrease in mileage is similar to the percentage of ethanol used. - So a 10% ethanol mix would theoretically relate to a 10% decrease in mileage over straight unleaded. In Nebraska fuels with added ethanol are cheaper than fuels without. People tend to put in the least expensive gas they can find. - But they may not be saving any $$ with the decreased mileage.
 
yes, temp and O2 sensors can become lazy and not throw a code. I would also change the fuel filter and possibly look at the Cat.
 
1967 'cuda said:
They have been upping the percentage of ethanol in our gasoline here. I had heard that the decrease in mileage is similar to the percentage of ethanol used. - So a 10% ethanol mix would theoretically relate to a 10% decrease in mileage over straight unleaded. In Nebraska fuels with added ethanol are cheaper than fuels without. People tend to put in the least expensive gas they can find. - But they may not be saving any $$ with the decreased mileage.
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Yeah, we've been at 10% for a long time now, so I'm pretty sure that isn't problem.
 
GAMoparMan said:
yes, temp and O2 sensors can become lazy and not throw a code. I would also change the fuel filter and possibly look at the Cat.
Click to expand...

Looks like I need to do some research to figure out how to test these sensors before I assume they're bad and throw money at it. Thanks.
 
I am not familiar with that exact engine. Does it have an EGR system? Carbon buildup in the EGR valve or intake where it attaches can account for that over time. The Ford 4.6s were terrible about it. We used to include cleaning the EGR passages in the intake with a tune up because they were always carboned up. That usually will not set a code or light until the passages stop completely up, but will make mileage suffer. Might be something to check IF it has an EGR valve.
 
I still get the same 20 mpg highway - 16.5 mpg city that I did when our truck was new. 8) 2004 Dakota - 158,000 miles - 3.7 club cab 2wd.
 
..............As said get a scan test by some1 that can read the data and know whats going on, not just the code..........maybe the cat is coming apart and getting into the muffler.........kim.......
 
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