Out of curiosity (and a previous post suggested I try it), today I did a cold start test/experiment on our '98 XJ SE 4.0L with the coolant temp sensor UNPLUGGED. Engine started as usual, ScanGauge indicated "open loop" as usual and coolant temp was an indicated -40F (sensor unplugged). 42 seconds after starting engine, the ScanGauge indicates the system switches to "closed loop". Coolant temp is still an indicated -40F. With the engine still idling, I plugged in the coolant temp sensor and at that point the ScanGauge indicated 112F and still "closed loop". I might also mention that when the coolant temp sensor is unplugged, the efan starts immediately and continues to run (engine cooling safety default would be my guess) and you get a CEL. Been using a 180F t-stat since December (when we purchased our XJ) and consistently get mid teens stop-n-go city mpg and low 20s highway mpg.

Don't want to start any flame wars or raging debates about "coolant temp must be at 195F+ before the system enters closed loop", etc., etc., etc. I just wanted to post the results of a very unscientific test on our '98 XJ. It's my opinion that "closed loop" is entered when the heated O2(s) reach operating temp and when coolant temp reaches normal, the heater works best. LOL