My 1995 XJ6 Vanden Plas just failed smog in CA. The #'s were off the chart, HC-269, CO 2.31, NO 1166.
In addition I have been having idle problems that can be the justification for running "rich". It just started running at 1500rpm

The 10amp fuse for the O2 heaters was blown and replaced with no problems

Working with my mechanic we replaced
the Idle Air Control, no luck still 1500rpm
Coolant Temp Sencor, no luck still 1500 rpm
Throttle Body, no luck no luck still 1500 rpm
mass air flow, no luck still 1500 rpm

Put in a used ECM and it ran at 1900 rpm, when the Idle air control was manually set it ran perfect. Drove fine, Idled fine, could turn it off and would start fine.

HOWEVER, wait 5 minutes and it is 1900rpm again

Because they had other cars we tested other computers for the heck of it
Original in mine made another car idle at 1500rpm (just like me)
Used would not even start the other car (In mine 1900 rpm)
When I put their ECM(1996 XJS and this is only experimenting) mine idled at 800 but was rough (to be expected)

I have 2 guys with 30 years each stumped

Any suggestions or help would be GREATLY appreciated

 

By the way we tested the EGR and it was fine as well

 

I wish I could suggest something else for you. All I can say as that I really hope you fix the problem and soon you have dropped a lot of $ already I would imagine. Good luck bro
Others will chime in hopefully