Last year I bought a 96 XJ6 running on 3 cylinders. it was discussed a bit here and many contributed good information. during testing i was able to pick up the other 3 cylinders and the engine sounded healthy but would not idle down. The usual suspects were interrogated but the problem seemed to be coming from the ECM which was not responding or communicating. removing the ECM and dismantling found the printed circuit board to be shorted out across several circuits . this was a replacement board and i was reluctant to have it rebuilt. I did order a salvage board from a running vehicle. while i have not tried the new board I believe i may have found a problem that i wanted to share with others. My salvage board came with both multipin connectors still attached , the wires from the donor car on the engine side connectors. when i removed them i noticed something different, both connectors (red and black ) had white plastic isolation , or guide blocks in between the connections. whoever removed my original board and sent it out to be rebuilt did not remove the isolation /guide pieces. they were not returned and the installer apparently did not realize they were missing. I think the resulting gap may have caused corrosion to take place as we are in a salt air environment here in coastal florida. My mechanic friend had a heart attack recently and while recovering now is still not able to do the testing with computer connected to the ECM before start up. I only mention all this in case others replacing the ECM could make the same mistake quite easily. I will post start up results ASAP.
Sounds like you may have(had) blocked drains from the sun roof where water runs down the A post and causes corrosion of the ECM Connectors.
Very common issue and well covered in the Archives.
Success! all systems working now, warm idle at 800 rpm. the salvage ECM did the job, still going to run some checks , i couldn’t wait any longer to try it so just cranked her up today… so happy
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Congratulations! Glad to hear that