I have a 99 Dakota club cab 4x4 sport automatic with the 3.9. My normal mileage has been just under 16 city/highway combined. My last two fill ups the mileage dropped to 13.2 combined and 13.6. The one can be explained from the weight that I threw in the back for winter, but there's no obvious explanation for the other since it was before I put any weight in. The truck has not thrown any codes or anything. The only other problem I've had is once in awhile the oil pressure gauge drops to 0 and the check guages light comes on. The first two times this happened, it was fixed by smacking the dash board. The rest of the time the truck just needed to be turned off, and then started again for the pressure to come back. It also sounded like a rocker or something was rattling after the pressure dropped the first two times, but as soon as pressure came back the noise stopped. A few of the times this happened, it seems to be related to getting the r.p.m.'s up between 3,000-4,000. I'm not sure what the deal is with the oil pressure, but would that problem have anything to do with my mileage dropping? Since there's no codes the only thing I can figure is the cat, but I don't really notice any rotten egg smell from the exhaust.

Any help for either problem would be appreciated.

Thanks

 

might want to have the dealer check that out....i kno this isnt quite related, but before i bought my dakota, i had a 88 Chevy 1500 with a 350 and that happened to me and it was a combination of the oil pressure regulator and about every gasket that retains oil being rotted out. but in any case, my truck would do the same thing where it would drop to 0 oil pressure and then fluctuate from 0 to normal. now, im not exactly sure what the oil pressure regualter actually does, but i can say that the chevy is out at my friends proporty and we drive it around his land all the time and it still runs fine a year later (however we have to dump a ton of oil in it to make up for the leaky gaskets). hop that helps, but i would serriously get it to the dealer or whatever trusted mecahnic you have as soon as possible and dont drive it unless you really have to to. oil is something you really don't want to mess around with, and when you are having problems with oil pressure and/or oil leaking, it would be deadly to your engin, so my advice would be stop driving it and get someone to look at it as soon as possible. hope that provides some insight to the problem.

 

Thanks for the reply Mike. Trust me, I know oil isn't something to take chances on. In this case though I'm not quite as worried about that part simply because it doesn't leak oil. In between oil changes I might lose 1/2 quart, but there are no visible leaks. Plus the couple times that the pressure dropped, and it came back after I hit the dash, that seems more like it's in the wiring and/or gauges. Right now I'm more curious as to why my mileaged dropped as bad and as quick as it did. Right now on my latest fill up I've got about 90-100 miles out of almost a half tank. I've usually gone around 140-150 miles at a half tank. I included the stuff about my oil pressure thinking maybe somehow they were related and if someone else had ran into the same situation maybe they would know what it was. They're probably just two seperate issues.

 

I have an 01 Dak with the 4.7 and when I bought the truck I was getting 12mpg then last year it dropped to 9mpg!

I took the truck in about 5 mounths ago for it's 30,000 check up and now it's back around 12 again.
Plus there is a boost in power now, I hadn't noticed the truck performing any worse but it is alot better now!!

If your's is a 99 and you haven't had it in for a tune up you should bring it in.

 

12mpg seems Absurdly low ... Especially for the 4.7 which was supposed to be about 5mpg better than the 5.2. ... is it just me?

 

I get 12 in my 5.9 quad.

I'd suspect the O2, but really where have you been getting gas? did they switch formulas to winter oxygenated stuff? That alone can cause a dip in economy.

 

I've been getting gas at the usual places, Speedway, and Flying J truck stop. I've never had this problem in winter before. After I did a tune-up this summer I finally got 17mpg once, other than that it was almost always 16, that's why I think there's a problem when I just lose 3 mpg. I thought about the o2 sensor, but I thought that would throw a code if it went bad? I was told the cat was going bad awhile ago because of a rattle in the exhaust, but I found that where the muffler connected to the exhaust pipe was loose, so I tightened that and fixed the rattle. There is nothing that sounds loose inside the cat, but I have wondered about it for some time since the truck never has seemed to have much power. I've searched the net for a direct fit cat, but haven't had much luck.

 

Hello Muy_Pyro
I have two strikes aganist me for gas milage.
I work only 5 miles from my house so the trips are absurdly short and the AWD option on my Dakota doesn't help either.
I was just stating the the MPG went down to 9MPG and a tune up at 30,000 jumped it right back to 12MPG <--which is good for me.

 

Well if the poor economy isn't limited to a tank or two I'd rule out the gas. I'd seriously consider the O2 sensors, as they do go bad over time and when they do you're generally going to start dumping more and more fuel, and if you dump enough unburned fuel into your cat it will clog up. You would think that bad O2's would throw a code, but they don't frequently... how's the ECU to know if they're going bad or if you put a cam in it and you're drawing a lot more air through, requiring more fuel?

 

It happens when it gets cold. Your IAT senses the cold air, more air needs more fuel, you run richer, AND your timing gets advanced.