I have a 98 Dakota sport next cab 2wd with a 3.9L manual trains. I'm only getting like 11 MPG but the truck runs fine n starts OK. It a little fumey sometimes. Any help would be appreciated

 

Probably a bad O2 sensor or a bad intake plenum gasket.

I will move this to the 2nd generation Dakota sub-forum.

 

11 city or 11 highway?

If the former, you're only 2 off the standard benchmark. If the latter....time for investigation.

 

When was the plugs,wires,cap and rotor replaced?

 

I changed the plugs and cap n button about a month ago.
I'm getting 11 MPG average.
how can I check the intake gasket?
The O2 Senor should turn on the light right.

 

What thermostat do you have???

 

02 sensor wont always necesarily throw a check engine light.

 

get a new cap and rotor. and plug the vent hole in the cap, with JB-weld. thats how the moisture gets in. the rotor is a week point, it gets a week spot under the spring. if you look close you can see it.

 

What brand was the cap and stuff? Cheap stuff usually fails after a few month. Intake gasket can be checked by taking the air hat off the throttle body then use a flash light and open the butterfly's and look to see if you have oil laying in the bottom. A residue is ok but puddles is not.

 

Ditto here. Bad sensor does not always trigger the CEL. My OBDII always shows an error, but the CEL is never triggered.