My son bought a 1999 5.9 (without asking my advise first) with 225k on it and we are trying to limp it back to life.

DDT is throwing a P0300 random misfire and cat o2s and evp lights are blinking on the DDT
Cat has been removed and a straight pipe was put in place of it
O2 sensor was left exposed tied to the frame

Vehicle idles rough and has to be driven starting in first gear through the gears or it will sputter and stall.

Question:

My State does not have omissions, will the engine ever run smooth without a cat?

 

You need to install the O2 sensor. If it is before the cat, not having a cat will make no difference. The O2 sensor determines the fuel trim. If it can't measure the exhaust flow, the car will run like crap. The front O2 sensor does not care if the catalytic converter is present or not. But it needs to be in the exhaust flow to set the correct fuel trim.

 

as I suspected, you say front- is there a rear also? if so where is it located as I did not see it.

thanks

 

PRE cat has to be in place or it will run like doo-doo. A/F ratio will be all over the place. POST cat has no bearing on driveability will just generate a CEL. Of course you can drill a hole and weld on a nut with same thread, use a $5 non-fouler to get rid of the CEL.

There was a post cat sensor there originally. Will run fine without a cat but you need to have a working pre cat sensor.

I always did the non-fouler even though I've lived in non-inspection states the last 30 years. Having the CEL on for the rear O2 sensor will mask a possible important CEL.

BTW, if he got it for the right price and there is nothing major wrong with it, I like his choice. Aside from the Plenum issue (which is easily and permanently fixed), that 5.9 is pretty solid and stupid easy to work on. Guzzles gas but tough to kill...