Mind Blown, dont know where to start. Idling rough code 51

96 xj stock motor ( PO says it was a replacement from jeep IDK bout that)

RECENT WORK DONE
-recently dropped fuel tank to weld some of the unibody frame
-Installed new exhaust from after down pipe & back ( Thunderbolt cat, new Rear 02 sensor, new muffler and tail pipe)
-recently changed oil
-Fuel filter about a year ago
-Air filter clean
-lucas oil -injector cleaner every three months
-No apparent Vac leaks
-Duty Purge sometimes clicks

Issues/SYMPTOMS

ok so here it goes, Jeep runs like a top, flawless. that is until I took a hour long ride (@ avg 45-65 MPH). A trip I frequently did over the summer. On the way home I stopped to take a leak on the side of the parkway and noticed I was idling funny. almost seemed like she wanted to stall out. Also noticed a very unfamiliar smell coming from the jeep hard to describe. (I assumed running lean ) at stop signs it would really grumble hard. In Park at idle was worse than driving.

CEL came on about 10 min later while driving. Code 12 ( battery Disconnect) and code 51 (Oxygen sensor stuck at lean position)
Soo I searched the magical interwebs and found a few possible fixes.

I went out and bout a 85$ MAP sensor. Cleared the CEL and replaced the old MAP. Started her up CEL was gone and she ran great for about 3 days. drove locally.

Today I went on the same hour long trip to pick up a boat trailer. Just as I got to the marina she started idling funny again. started home and the CEL came on again. Same codes (both times this happened was at the same long trip)

I think my MPG went down too.

Stopped at a buddys house (whose 2000 xj has the same or similar issue) and I reset the computer again. I was good for about 15 min of driving at an AVG of 45 MPH

Before I start replacing the TPS, forward o2 sensor, injectors, IAC, ETC.,
Does anyone have any suggestion.

Fuel pump, Vac leak , ? idk where to start.

Thanks all in advance
-Pete

Did you fix the code 51 for the (Oxygen sensor stuck at lean position) ? Did you check for a crack in the exhaust manifold ?

A faulty O2 sensor can cause the symptoms described. I will bet the gas mpg's have dropped by about 30-40%.

Your 1996 has OBD-II self diagnostics, you should be downloading the trouble codes and starting some logical and methodical testing based on the codes. OBD-II trouble codes are much more specific than generic two digit codes for use in your troubleshooting. Randomly installing new parts in hope of fixing an undiagnosed problem gets expensive really fast.


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