This is happening to friend's MY1997 XK8. The idle oscillates between 700 and 900 rpm, like the tide surging and ebbing cycling every 4 seconds. This happens after the catalytic converters were replaced.

The OBD codes read as follows, but I have no clue how they relate to the fault:

P1224 Electronic Throttle Control Position Error
P1475 Fan Relay (low) Circuit

Any suggestion/solution would be most welcome. Thanks in advance.

 

looks like a possible throttle position control sensor???? Outside chance is that the cars main computer is starting to die. 1997 models are well known for Computer (ECM) units taking a dump on these cars. While this car is showing just ONE of them it is one of the ECM starting to go fritzo.
The ECM board starts to fail (others can explain it better) and you will start getting odd issues happening with the car. You have two if they both started right at the same time.
First thing that should be checked is to make sure the battery is good. Get it load tested, a "weak" battery can also make these cars have issues. It can make the cars various electric components have a hard time reading there correct out puts, inputs (others can better describe that) and make the various thing act weird.

 

One thing to try before going to far along is a hard reset. After the cats were change the tuning characteristics were changed. Take the negative cable loose and hold it against the positive cable for 30 seconds. This will erase and put all of the controls to default. The ECM system can then relearn the driving modes.
Another thing on the 1997's is slack in the throttle cable. To adjust you clip the tie-wrap and rotate the mount to remove the slack and then tie-wrap back in place.
You can do a forum search and find pictures of the sequence.

 

+1 on the hard reset to clean preset values. You might consider checking the fuel trims as well via torque app and ensure they are tracking, several threads on this as well. btw this phenomenon is called 'Hunting' as it's hunting to find it's idle speed.

 

Thank you very much members for pitching in on this thread.

Our mechanic strongly suspected the replacement catalytic converters which were obtained from the local junkyard were the source of the fault and proceeded to take them off and reinstalled the original ones. And the problem simply went away.

Another fine example of "If it ain't broke, don't fix it". Cheers.

 

I am not a fan of cars just FIXING them selves. ha ha ha