My 1999 dodge ram 2500 hd quad cab 5.9 v8 seems sluggish and gets low mileage. 380 km for 100 liters. I have done a complete tune up, plugs,wires,changed all fluids. No codes, cat is gone, stock muffler, 4.10 gears, 250000kms. What can I try? New muffler, o2 sensor????? I had a 94 v10 that got better mileage.

 

Do a search on here for the plenum gasket. Fixing that will help.

You should have two O2 sensors. The first one, the one that went before or into the catalytic converter, should still be there hopefully. (Depends on what you mean by "cat is gone".) If its not, that may be part of the issue. If it is still there, replace it with a good brand (NTK/NGK or OEM). Do you have a CEL or any trouble codes?

 

Inside intake is clean, both o2 sensor are there, no codes, might try an new o2 sensor. the cat has been taken out and a pipe put in but both o2 sensors are there, sometimes there seems to be a whistling at part throttle.

 

Try replacing the at one time... pre-cat O2. Make sure you get a good brand, OEM, NTK, or Denso.