Vibration when warm
I own a 53-plate Jaguar S-TYPE 3.0 automatic with 92,000 miles. Starting from cold, the car drives normally, but as the temperature gauge approaches normal, the rev counter starts to fluctuate by about 150-200rpm. This is accompanied by a rhythmic vibration from under the transmission tunnel. The vibration is in sympathy with the fluctuating rev counter. If I try to accelerate, the vibration gets much worse. The problem will sometimes appear until the car is thoroughly warmed up, after which it will perform normally, even if the car is left for several hours. If the car is left to go stone cold – overnight, for example – the problem recurs.
I have found two ways around the problem. The first is to come off the accelerator, let the car coast, then reapply the throttle. The second is to manually select a lower gear, drive for a short distance, then reselect Drive again. However, as time has gone by, the problem seems to be taking longer to disappear. Stephen Hayes
One common fault with the S-TYPE V6 is failing coil packs. However, when they do fail, they are more likely to give a positive misfire and trigger the EML to illuminate. Another common and difficult-to-diagnose problem is that the small elbow breather pipe under the rear of the inlet manifold has a tendency to perish and allow a vacuum leak from the system. It requires the inlet manifold to be removed to enable the pipe to be replaced, and if this pipe has failed then it would also be worth replacing the spark plugs. The spark plugs on the 3.0 are long-life, but can only be replaced with the inlet manifold removed.
One way of detecting if this pipe has failed is to use a carb cleaning spray around the back of the engine while the car is running. If the spray finds its way into the combustion chamber via any vacuum leaks, the engine note will rise.
Although you do not mention the EML being illuminated, you may find that a fault code has been logged. Using a code reader to retrieve any codes may also help pinpoint the source of the problem.