I have a 1994 Dodge Ram 4x4 5-speed manual with a 318.
I am getting 7mpg at best in this truck regardless of how I drive it, and I hear about people getting 19-20 with the same truck. The truck also seems to have a rough idle after it gets warmed up.
After reving it, the tach drops to almost below 200 rpm and revs up to about 1200rpm and then stabilizes the idle.(still rough though)
The vacuum will read around 20 inches after starting, but then will slowly drop off.
I haven't noticed any usage of oil

What I have done:
- It was throwing a 53 error code so i went to the junk yard and found the same truck, including having the manual trans and the 318, and got the computer. This showed no change in my gas mileage.
- Changed the O2 sensor
- Checked the voltage readings on both the MAP and throttle position sensors
- Changed the EGR
- Changed the whole throttlebody
- Removed the Catalytic Converter
- Checked for vacuum in the crankcase
- Changed the IAC motor
- Changed the thermostat and coolant temp sensor and coolant
- Changed the oil
- Compression test (all were around 133 +/- 5 psi)

When I get on it, you can see black smoke coming from the exhaust, and there is a lot of carbon in the pipes. It seems to be running extremely rich, it smells like its running rich.

I was wondering what sensors cause the PCM to richen the air/fuel mixture.

Any help would be great, I'm running out of ideas of what to try next. It doesn't seem like there could be a plenum problem, because there is no oil in the intake (checked when I had the throttlebody off) and there is no vacuum in the crankcase.

Thanks,

 

Air temp sensor.

Check voltage to injectors. One may be stuck on. You may also just need new injectors. What is your fuel pressure?

 

I had the truck in the shop, and they said that the fuel pressure is where it should be, I don't have the tools to check the fuel pressure myself, unless there is a simple way.

What should the voltage be for the injectors.
I have tried unplugging one at a time to see if this made a difference in the idle, and i couldnt notice one.

I will try changing the air temperature sensor tonight or tomorrow, I honestly didn't even know the truck had an air temperature sensor.

Thanks,

 

What brand O2 sensor did you get?

 

+1.... and I see no mention in the original post, about whether or not the plenum was checked(regardless of oil usage)or replacement of ECT(engine coolant temp sensor).

 

i'd guess one or more injectors is dumping gas.
does any spark plug look bad?

 

what size tires do you have? are they the stock size?

 

I bought a BOSH O2 sensor,
I did replace the coolant temp sensor,
All 8 spark plugs are dark brown-ish black, none were wet.
I have not pulled the intake off, so I haven't checked the plenum.
And the truck is completely stock, including the tires and gear ratios.

I am about to go out and put the Air temp sensor I bought in right now. Ill post back the results.

 

ouch....

they suck dude... No personal experience, but tons of bad reports here and other Dodge sites..

NTK/NGK... not so much suck..

 

by the way, I have two brand new NTK/NGK sensors- you can get 'em cheap if you need them.. I ordered 49 state o2's, when I needed cali..

PM me if you're interested... I have two, I think you only need one, right?