Hi,
1997 Ram w/5.9

At 100K miles, I replaced my Cat because the OEM was rattling bad.

Driving and mileage were good(15.5 mpg); always had a slight miss.

At about 128K miles, I noticed I was burning over 1 quart of oil between changes and my gas mileage was getting poor. This was odd. I found that the plenum was bad, so I bought the Hughes kit and replaced the plate and gasket, along with the water pump and timing chain.

Now I have 132K miles. I don't burn any oil, but my gas mileage is still poor (12.5 mpg), power a bit low. I replaced the tune up stuff (plugs, wires, cap, rotor) and did the wires by the TSB routing. I also replaced the module on the front of TB (cant think of name). I checked vacuum and all is good.

What else could it be? Could my cat have gotten blocked with the 3K miles I drove with the plenum issue? Any help is appreciated. Thanks, Jim

 

you could need to replace your o2 sensors since it was expose to all that oil before you fixed the plenum

 

Replace the precat O2 sensor.
I replaced mine when i did tranny and gain 2 mpg.

Dave

 

all the oil may have got into your cat,, slowing down flow also

 

oil getting into the combustion chamber from a bad plenum gasket will be burned to a degree, but a lot of it is spit right out into the exhaust... which makes short work of a catalytic converter..

these guys are right- o2 sensors need to be swapped as they are likely fouled, but chances are you fouled the catalytic converter too- and it will need to be replaced as well.

 

Thanks for the advice so far guys. I forgot to mention that during the period just prior to me learning that the plenum was bad, I did replace both o2 sensors because a code was thrown. The code stopped but the poor performance stayed. Each time I seem to think I have the answer, it just does not work out. Today I performed a 30 minute carbon cleaning using a friends machine (isolated fuel with cleaner mixed in). A miss at idle still remains. We'll fill up tomorrow and see how the mileage does next week.

As a side note - if I were to gut the cat, what happens with the o2 sensor readings? I'm I looking for trouble or should I just buy a new cat??

Thanks again. Jim

 

I understand your frustrations! I'm in the same situation - everything in the sighature line was done and getting 9-10 mpg. ARGH! I even did the electric fan conversion with these numbers!

I used a brass brush and cleaned the sensor to see if that helps any before buying another one with a new cat. According to the mechanic who did the plenum repair and kegger mod, there wasn't much oil in there.

 

What brand O2 sensors?

You can gut the cat, depending on emissions laws in your area..... and it might throw a code whining about cat efficiency, or, it might not..... certainly get better flow..... there is a method for fooling the post cat O2 if you decide to go that route.

High flow cats are under a hundred bucks as well.

 

Just use the non-fouler trick on the rear o2, and you'll be golden. Gutting the cat does nothing to the front o2 (unless you happen to whack it of course).

You will need to check your state's (or county's) emission regulations--a gutted cat will not pass a smog test (hose up the exhaust), but with the non-fouler trick, will pass the regular OBDII only test. It all depends on which test your state/county requires.

 

Just use the non-fouler trick on the rear o2, and you'll be golden. Gutting the cat does nothing to the front o2 (unless you happen to whack it of course).

Whats the NON-FOULER trick?