Car when first started has a rough/stumbling idle, yet after it gets going runs great.
No codes or engine light. Ran Textron cleaner through it but still acting up.
It reminds of the old carburetors when the idle circuit goes bad or clogged but once past that point it then runs fine.
Tried changing fuel filter but fittings are too corroded to turn.
Could it be that the fuel pressure drops until it builds up while running?
Changed plugs about 20k miles ago.

thanks in advance for all your help.

 

When you say "when it gets going" -- is this at speed or when it warms up?

Make sure you don't have any leaks in the tubes going to the throttle body -- they are known to crack ...also the seals can allow air in that will effect the car at start when the engine is cold.

At 120k you could also have an injector issue.


Sometimes the only way to get to the bottom of some issues is to take it some place where they can scope the engine and check the pressures.

 

Hmmmm....XJR with 120K....as has already been suggested, it could well be air leaks getting into the intake system.
Not uncommon for these if they have not been serviced correctly and on time.
Air seals for the TB, intercoolers, part and full load breathers, MAF to inlet trunking, cracked anti resonance tubes in inlet trunking can all contribute to this 'stumbling'.
What's the history behind this car, has it had a good life or been thrashed as so many XJR's seem to be?

 

Clean the MAF sensor; plugs?

 

THANKS EVERYONE!

Here's what happened since, ran two bottles of injector cleaner TEXTRON through it. Now seems to running ok. No codes or engine light. Checked all the hoses, seals for leaks none found.
I always fill up using COSTCO gas never had a problem BUT let girl friend borrow it and she filled up at a Marathon (see this is why we never lend out our JAGS!!) and must have got some bad gas. Can't get fuel filter off yet, may have to give up and take it to the dealer.
Car is not dragged but I will get on it if I have to.
So once again thanks for all your help.