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As my signature states: 87 4.0 Laredo. I have Nick's REM to give me readings from the ECU. Lately, the engine is running stuck in open loop (OL). I have the REM set to display engine temp., Open/Close status, O2 volts and (something else). I start the truck up in the morning and let it run for a few minutes - 2 to 3. The REM shows it switched to CL. I start to drive away and it switches to OL. It will remain in OL. It does not switch back to CL. The ECU came out of a 90 XJ with a 5 spd. The O2 sensor was replaced last fall with the NKG, or whatever # Cruiser states. The O2 volts at start up are 4.+ volts. I can watch the volts drop to ~ +/- 2.5v when it switches to CL. I will drive only a short distance when it switches back to OL. The O2 will stay at or below 2.5. The O2 usually reads below 2.5 most of the time. The engine temp per the REM is at 185* and below while moving. At idle, it will rise up in to the 190s. Once I start to move, the temp drops back down. Before I pull out a parts cannon, does anyone have a suggestion on where to start my testing? The Renix fuel injection manual only describes how the system works in a perfect world. It does not give a detailed trouble shooting guide. I am going to replace the t-stat to address the temp issues. What about a CTS? (Part # for a CTS?)