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1997 Jaguar XJ6 running rough

6 cylinder 4 liter motor runs rough while it is warming up after I start it. When it warms up it seems to smooth out. Fuel filter changed; fuel conditioner/cleaner used; new plugs; no trouble codes come up. What is the problem?

Some kind of a vacuum leak, which is worst when cold, then partially seals as the motor is warming up . . .

That is one possibility, but I have to say I’m not familiar with this engine