I have a 98 Ranger XLT 4x4 with the 4.0 OHV, around 157K miles. It has a rough idle once I reach speed and hold it, and it sounds a little rough in park. So far I have replaced the plugs and wires and the thermostat, plus I ran some seafoam through it. Today I finally checked out the exhaust and the catalytic converter has been removed and straight piped to the muffler. I'm guessing this is why my CEL is on. Will the cat being gone cause the idle problems? I have cleaned the MAF sensor and the IAC is fine. The plugs are gapped correctly and I had an oil change about 4 days ago. Could the idle problem be caused by the Throttle Position Sensor? Will a de-fouler help with the lack of a cat? It has a K&N cold air intake, and since the exhaust is missing the converter, do I need to do the relearn procedure? If so, how do I do it? Sorry for the ton of information, but the idle is killing me.

 

No, the missing Cat won't cause the running issues, but it is a $2,500 Federal Fine for not having it, so don't take it to a shop for any work, they are required to report it or install one.
Yes without the cleaning of the exhaust by the Cat the rear O2 sensor, if it is still there, will turn on the CEL, if it has been removed then the CEL will come on because of that.

With engine warmed up and idling, unplug the IAC Valve, idle should drop to about 500rpms, or engine should stall, either is fine, it means no vacuum leak.
If idle doesn't drop below 600 then you have an air/vacuum leak.
PVC valve hose is a common leak on Rangers.
Air Plenum tube from MAF to intake also needs to be air tight, that is a "vacuum" too.

 

I actually did unplug the IAC and it started to stall, so I guess the vacuum lines are fine. That idle is really annoying. I would like to be able to turn on the cruise without it sputtering.

 

If you can rent/borrow a live scanner you may get some info on the problem from the live data while you are driving.

You can buy the Bluetooth OBD II adapters for $40 they interface wirelessly with Smartphones to display the live data.

Check STFT(short term fuel trim) while its stalling out
Also TPS voltage and MAF voltage

 

i wonder if the vacuum line to the EGR valve is still connected

computer is not reading the fuel emissions from the catalytic converter and is activating the EGR valve