Are XJ6 and XJS ECUs Interchangable?

I have a 96 XJS 4.0 with high idle issues that I have narrowed down to a faulty signal to the idle control valve. I have have no vacuum leaks, properly cleaned the throttle body and replaced the temp sensor. I have tried three different idle control valves and continue to get the same high idle issues after going through the start up sequence for 'calibrating" the ICV.

There is a 95 XJ6 sitting in my local Pick and Pull and I was wondering if the ECU from it would be interchangeable with the 4.0 in my 96 XJS.

95 XJ6 part number - LNA1410AH
96XJS part number - LHE1410AL

Thanks - Kevin

You will have a constant Check Engine Light. P1199, but it should work just fine.

As a test to back check an existing XJS ECU, it should do a good job at that!

I have done just as you described with a 95’ XJ6 ecu into a 95’ XJS and the idle was much higher. Did not get below 1200rpm.

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How did you end up resolving your high idle issue?

I am currently running with the Idle Control Valve disconnected in the fully closed position. I am not seeing much of a difference other than a slight fluctuation at idle. ~550 - 650rpm and no check engine lights illuminated.


My original ecu appeared fine. It idled properly although would fluctuate between 650-725 and I tried to get it perfect.
I have changed usual suspects like the EGR, cleaned mass air flow, and also changed all associated sensors-IAC, and throttle position sensor(which reminds me, make sure yours is not soaked w oil).
I tried the ecu from the sedan just to see if it made any difference which was making it have high idle.
Unlike cars 94’ and younger, we cannot really change the idle as the idle screw is not present. And when you do adjust the linkage, the computer will compensate and maintain its “idle”.
If your having a high idle still; make sure that the air induction “accordian” that slips into the mass air flow is securely connected on the bottom. These cars seem to be really sensitive to any extra air and the idle will respond to it.

Thanks James,
I have triple checked for vacuum leaks - all good
Removed throttle body and cleaned with damp rag/throttle body cleaner - no change
Replaced IAC with two new ones and a used original IAC off the 95 xj6 - no change
Replaced Temp Sensor- no change

I did not attempt to clean the mass air floe unit. Should I spray MAF cleaner in the unit?

Everything I have done has been in steps. I am convinced the ECU is sending a faulty signal to the IAC but not sure if that is a result of the ECU receiving a faulty signal from something else. Bad data in = bad data out syndrome.

Every time I replaced the IAC, I would go through the calibration sequence of starting the engine - disconnecting the IAC connector… The idle would always settle on 650-700rpm. I would let the car sit at idle for five minutes and then take it for a short drive. It seemed that it would lock into high idle mode when I got above second gear. It did this after every attempt to reset the IAC.

I’ll clean the MAF and inspect the TPS for oil as a next step.

Thanks for the feedback

Yes, I would remove the butterfly housing and then remove and inspect. They are very expensive but try cleaning yours first. You can see in photo mine was a bit oily since it sits on bottom of housing.

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Will do and thanks for taking the time to provide the photos! Never a substitute for photos.

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I took the photos couple months ago, i intended to write about it in near future on a blog and I hope it solves your problem.

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