Ok, I've got a 2001 4x4 1500 with the 5.9 Magnum that is driving me crazy.

First off I've done the hughes plenum fix, and its holding fine.

I'm averaging mileage in the single digits, usually 8 or 9mpg. Running 3.55 gears with stock sized 245 tires. Bought the SCT tuner too, it didn't really seem to make a noticeable difference. Sometimes I get in and it runs like a bat out of hell. Othertimes I feel like I'm driving an amc gremlin.

I've replaced the TPS sensor(MOPAR); pcv valve; front O2 sensor(NTK), oil changes and transmission fluid and filter. I recently went to change the plugs to autolite 3923's and I started it up to pull it in the garage, and shut it off (30 seconds) and when I pulled the plugs it smelled like they were all wet with gas.

I have gotten P0138 error which is the downstream O2 sensor, but it comes and goes. would it cause problems?

 

I had the same PO138 as you for three weeks and it ate major gas, put in a new post O2 and well much better, you could also check/test your temp sender for the pcm.

What shape is your cat in, could be plugged.

 

You might want to look into replacing all your O2 sensors, and your cat might have been clogged from it burning oil if your plenum was really bad for a long time.

Im not sure if you can do this with your cat but i do know you can wash old cat convertors with a special cleaner instead of buying expensive new ones.

 

+1... I've been having second thought's on how much of an effect the post cat o2 really has. And yes, have the cat pressure tested.

 

When I was thinking about gutting my rattling cat, most shops told me the post cat 02 would affect mileage (after gutting the cat). Most told me the post cat 02 did fine tune the air/fuel ratio. Only one shop out of 8 said the post cat 02 "probably didn't do anything to the fuel mix". These guys weren't trying to sell me anything either, so I believe them. So I went with a magnaflow cat, but have never replaced the post cat 02...yet. That being said, I agree on checking the cat and possibly replacing both 02 sensors.

BTW, did you get the CEL before or after the autolites? If it was after the plug change, I think I'd go back to stock plugs before I did anything else.

 

Check engine light was before the plugs, I only did the plugs last week, and haven't seen it since, but it wasn't frequent.

 

so a clogged cat would give you bad gas milage? i got the same truck 01 4x4 1500 with the 5.9. and im only gettin 8 mpg and i did the plenum n all that crap to. but im on 35s but still id like better MPG... not trying to hijack the thread but ive wanted to know how to get better milage because i cant afford it anymore

 

Re-gear. Going to 4.56 gears would be one of the best things you could do for your truck.

 

Ok, well I replaced the rear O2 sensor, and... The truck seems like a brand new truck. I've never seen the truck respond so well. I'm not sure what the mileage is going to do, but the truck wants to run now. And as a plus, I've been driving it for almost a week, and the gas gauge isn't dropping nearly as bad as it was before (so it seems, maybe its all in my head). I'll find out the next time I fill up I guess.

 

hmmm, me thinks i might have to give the rear o2 an update after reading this