1998 Cherokee Laredo 4.0, 201,000 miles.
I can't get the darn Check engine lite to stay off.
Runs well, no stalling, no surging, etc. a few hiccups when idling, but RPMs are pretty steady, starts right up.


Codes are: B21 Engine is cold too long
U3F System Rich, R-idle ADAP at lean limit.


New "live data" info. upstream o2 bounces between "lean, rich, lean, rich..... it will just keep doing that. Engine temp sensor at idle says 207' after some driving 210' Console temp gauge stays just below dead center, but just barely so that's 218-220 on the dummy gauge.


What I have done. Flush radiator, change water pump (bearing was going bad), changed thermostat twice, once with a 195 gates, and recently with an OEM mopar 195. Just put a new exhaust manifold on it and gasket (had a crack), new catalytic converter, new temperature sensor (the one by the thermostat), cleaned up the intake and all sensors around throttle body when had off to fix cracked exhaust manifold. New battery and terminals. Changed upstream o2 sensor. (I do have the down stream o2 sensor but have yet to put it on).


OK, hit me, what have I missed??
Thanks

 

Try a 210* thermostat. That's what is recommended in my WJ.