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Gas MPG

3K views 15 replies 7 participants last post by  cbr600rx7 
#1 ·
I have the V8 5.9 L slt and I am getting 8.8 mpg around town and I am not driving like a mad man. Could this be the result of a bad fuel pump? I am out of clues and have no idea why I am getting such horrible gas mileage
 
#3 ·
You need to do a search on bad gas mileage for 5.9 V8's on here and it will tell you where some of the common areas for these engines are the cause bad gas mileage.
Also, the 5.9 is going to a gas hog. That's how it was designed, because gas was less than a buck a gallon and trucks are on long manufacturing life cycle, so no one at Chrysler could see that gas would go up anytime soon.
 
#4 ·
A 5.9 should get around the same mileage as a 318 under normal driving assuming all other variables are the same - it's larger displacement but more torque so it's not working as hard to get you up to and maintain speed. 8.8 is very low, it should be getting at least 10-11 around town, maybe 18 on the highway.

O2 sensors would by my first suspect. They don't always throw a code when they go bad, the engine just starts running rich. Replace them with Denso or NTK (those are the OEM brands depending on the year). If you have two in the front, replace those. Don't worry about the one(s) behind the cat(s) because they only monitor the cat(s) efficiency, they don't affect the mixture. DO NOT use Bosch. Also do a full tune up - new plugs/wires/cap/rotor, good washable/rechargeable cotton air filter like K&N or Spectre (basically the same, Spectre is just cheaper), make sure tires are aired up properly (adding 5PSI over the sticker pressure definitely helps mileage but might cause them to wear faster). If you have a regular station you go to, try switching it up. They might be getting contaminated gas or have a problem with their tanks. If you can get ethanol-free gas and it's within 20 cents of E10, it's worth it to pay extra for it. You could also have an issue with brakes not fully releasing or bearings that are not rolling freely.

If this is a Dakota, make sure your tailgate is on. It's a proven fact running with your tailgate down/off or using a net gate costs you mileage.
 
#5 ·
I view my mpg tonight on my 4.7 through the rear view mirror outputs. I got an average usage 19.2 L/100 km's, which is fucking horendous! 363 km's on a full tank of gas, thank god gas is cheap as dirt now compared to last year this time! whats the model or part number on the O2 sensors that you mentioned? i got a code for mine telling me that mine is bad, P0138. so i want to replace them ASAP as i'm planning a 950 km drive in about a weeks time.
 
#7 · (Edited)
That be the one. You only have one before the cat, right?

And 19.2L/100km = 15mpg.. Now is that on the highway, or the average? Also keep in mind the Durango is about 500 pounds heavier than the Dakota, and the 4.7 just about the same torque/hp as the 5.2 (but its power band is about 1,000rpm higher).. If 15mpg is your average it's spot on. It should still get about 17-18 on the highway. I got 15 highway with mine while pulling a 6,000lb trailer.
 
#8 ·
i was thinking there would be 2 before the cat? one for each manifold? i'm probably wrong but imma buy 2 just incase, worst case senario i'll have a spare. yeah i'm not sure if that was the average or not, i was just idling in a parking lot when i discoverd how to use the buttons on the mirrow to be honest. so if that is spot on then its not so bad as i thought then lol. i'm new to truck/suv driving. i own a honda civ as well and i mean i get 700 km's on a 45L tank lol so its certainly an adjustment period. that and all the extra hp and tourque i have now! either which way i can smell gas in the exhaust at times which is telling me its too rich of a mixture, correct me if i'm wrong on that. and its throwing a code for the sensors so i figure it cant hurt to change em out. the rig is 14 years old and i got no previous driving information on it seeing as i'm the 3rd owner. wish i had though because the seller had the rig inspected for me before i bought it, but the garage obviously just signed it off and passed it because i'm finding things that should of failed the inspection. that and a lot of rust/eaten out metal in the rear end, particuallry around the fill pipe. i dont know if there was supposed to be some sore of plate or not protecing the pipe, if so, thats pretty much completly gone now lol.
 
#10 · (Edited)
Yeah if you're smelling gas it's running rich, which points to the O2 sensors. There are 2, 3, and 4 sensor systems, and there isn't really much rhyme or reason as to which trucks get how many sensors (except for all California vehicles get 4). You just have to look and see how many there are. 2 sensor systems have one after the Y and one after the cat. 3 sensor systems have one in each pipe before the Y and one after the main cat (you may or may not also have mini-cats on each pipe). 4 sensor systems have one before and one after each mini-cat. There is also a main cat but there is no sensor after it in this configuration.

The ones before the cat(s) are upstream and the ones after the cat(s) are downstream. In 2002 they're the same sensor with a different plug. The upstream ones are what control the mixture. The downstream ones are just to monitor the cat's performance, they do not affect the mixture, so unless you are getting a downstream-specific code, you don't need to change them.
 
#13 ·
It is getting harder to find older vechicles, especially ones that are in good shape, seeing as we are surrounded by salt water and high winds that blow the salt everywhere! that an the fact the roads are salted 4+ months of the year ahha. this is my second vechicle thats 10+ years in age, first one was an 2000 civic, and by comparrison, my durango is in much better shape! had to rebuild the engine, replace ball joints, struts, power steering, the subframe, hood, and a few other things before the motor finally blew, it was a nightmare haha
 
#12 ·
To put it into perspective, I usually get an average of 15 mpg in my V6 5 speed, once I had the exhaust welded, they used the y pipe with the O2 sensor in it and welded without removing it. My truck computer on the overhead console was saying I was getting an average 5 mpg.... Those sensors essentially go haywire, think you're exhaust is too lean and try to overcompensate by telling the PCM to dump in more fuel
 
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