I'll try to explain this issue the best I can. I have a 98 with a stock 4.0 138k. I started off with a high idle/ stalling issue no check engine light. After using a pro scan tool I found out that my idle would jump way up when the upstream O2 heating element would short out. I replaced that and it worked fine for about 1,000 miles. Just recently I had another idle issue where it was running rich to the point it was smoking at start up (no check engine light). I pulled 2 spark plugs and confirmed they were black and smelled like gas. I replaced the PCV valve, grommet, PCV rubber connectors on both ends of the PCV line, and the rubber elbow located about 2 inches below the map sensor. I checked the fuel pressure and it was at a steady 46 while running regardless of rpms and 44 with the engine off (held for over 10 min). I believe the fuel pressure gauge was inaccurate, the first one I rented didn't even work at all. The running rich appears to have gone away. Now I still have no check engine light but the idle wont sit still. It goes from 1 to 1.5, the IAC is clean as well as the the throttle body. The cap and rotor has just been replaced as well. I checked the rest of the rubber caps/ connectors on the intake manifold and they appear to be holding pressure. What am I missing? Could it be a bad TPS?

 

Could be a TPS issue. Just because the IAC is clean doesn't mean it's working and there's no easy way to check it's operation without a Chryco DRB scan tool that I know of.

 

Thanks. I just went down to AZ and got a new TPS. I do believe it's fixed now. Its idling the better than it has since I got it. Apparently it was an issue caused by leaking vaccum lines and a bad TPS. I'll change the oil and plugs and go beat it up tomorrow.