Bought my xj 4.0 from a buddy of mine that was crashed from the front drivers side fender. ever since i bought it i've had the damn CEL on and i tried everything i could think of and everything on this thread to fix it. finaly decided to replace the ECU. so i ordered it and the next day my water pump went bad. Ordered a new high flow water pump and thermostat housing and a mr. gasket thermostat 195* all from HESCO. anyways. for almost a year i was driving with a bad ECU! Performance Cooling sysytem and a new ECU later and it runs like never before!! I highly recomend the Hesco waterpump. my aux fan never goes on and i stay at a very steady temp. so THATS what the 4.0 is like 
