so just did the plenum and a full tuneup including plugs wires cap rotor coil pack new oil pump air filter pcv and double timing chain and a new fuel pump, truck has 210k on it motor has about 30k and tranny has 30k has no check engine light, truck has a tinny bit of hesitation when first jumping on throttle at a dead stop but its very small amount, i took the after market muffle off and its still pretty quiet i know the plenum was bad for a long time so im thinking the cat is clogged up so this weekend im gonna try to hollow it out and do the non fouler trick (do i do it to both o2 sensors) so so my question is what is my next step? new 02 sensors? map sensor? its getting like 5mpg but it runs amazing and idles smooth its a 1998 1500 5.9 gas 4x4

 

Front o2 needs to be working in order for the computer to run the right fuel trims

 

Is that 5mpg city or highway?

 

New front O2 sensor for sure, only do the non fouler trick to the rear sensor. You want to keep the front sensor in the exhaust stream so it sends the correct signals to the PCM to control air/fuel ratio.

 

LOL..

replace front O2.
think twice about hollowing out the cat. it'll sound like ****.
but - if your going to remove it - be aware that you can sell your factory cat for $75 at sellyourcatsdirect. enough to pay for welding in a replacement straight pipe.

 

Im having almost the same issues with ny 98 dodge ram 5.9 gas mlg is kill my wallet.

 

Ham Bone is spot on as usual, I've been running a 98 5.9 for years. Also look at
their profiles for tire size and gearing (that'll give you indicators on fuel mileage).
These guys can also coach you on mods like CAI and opening up your exhaust.


I'm a poor example for a profile. Best of luck K.

 

When doing a plenum you should replace the cat regardless, same with front o2. Just non fouler rear o2. I would get a replacement cat over no cat...

This truck lifted at all? What size tires ya runnin on? Whats your gearing?

 

Ok some more info about truck its lifted only 3 inches with some 285 75 16s i went ahead and got new front and rear 02 sensors from rockauto and also ordered the jegs off road y pipe and a dynomax muffler 2.5 inlet 2.5 outley and will run a down pipe off it also got a drop in k&n filter so should have it all mid week, now on the hunt for a new steering box once thats done truck should be brand new ive gone thru motor rebuilt trans new lift new tires and wheels new bed new tailgate new hood new bumpers striped interior down to nothing and rhino lined the cab floor had seats redone new headliner new dash new stereo and speakers with sirius new control arms and track bar with the 3in lift went with the hd 3/4 ton steering 4.10 gears front and rear and am currently working on building a tube bumper around the front bumper to house a 12k winch im sure i forgot something but this has all been in the last 3 weeks

 

I would install a new high flow cat to replace the old one and to offset the cost sell the old one to a recycler for around $80.