My 2000 XJ has a resonator (pt no 53030797) above the throttle body. The rubber collar that acts as a union between the two is pretty shredded and might be letting air through.
The collar is part of the resonator and they aren't easily available in the UK.

What does the resonator actually do and what would be the impact of throwing it into a corner of the garage and replacing it with a hose straight from the air cleaner to the TB?

 

My 90 had a "restrictor", between the air cleaner and the TB. I don't know it's name. I ripped it out while doing some other improvements and ended up with better mileage. I'd say don't worry, install your "hose".

 

Thanks.
I will probably go for it. My only concern was that the electronics in the late XJs might be calibrated in such a way that they will report the change in airflow as a fault.
Then again, I guess it's not the most difficult modification in the world to reverse.

Still wondering what the resonator actually does though. The engine is such a primitive brute I wouldn't have thought it needed to worry about the way the air gets in, as long as it does. In fact, I would have thought that anything restricting it's breathing was bad.

Hopefully someone clever will be along shortly to enlighten me.

 

I'm guessing it's there simply to make things quieter.

 

It's for noise. Ditch it and don't worry about the electronics, they won't notice a thing.