My 2011 XJ/SC has had the problem since new. The service dept says it's "bad gas" and told me to use Shell or mid-grade QT. I did and nothing has changed! It seems like a blocked injector or bad oxygen sensor, but the dealer doesn't seem to have the slightest idea.

 

I have this issue with my car as previously posted. The car has been to the dealer 4 times, one week each time and also a tech from Jaguar have seen the car. All feedback I have got is that there is nothing wrong. I complained to Jaguar US and got no where (of cause).

The other day I started my car and it was shaking like a Harley on a good day. I opened the hood and looked at the engine shaking. Then it suddenly stopped shaking and it idled with absolutely no vibration. Just out of curiosity I opened the oil fill cap on the engine valve cover. The engine went strait back to shaking. There is a slight vacuum in the oil sump. By opening the cap the vacuum is gone and the engine starts shaking.
My theory is that there is a slight gasket leak in the engine oil compartment. It can be the oil fill cap, dip stick or any other gasket. If you got a small air leak on idle, the vacuum pressure is not big enough to suck the gasket tight (which could be the case for a cap or plug). When the rpm increases the vacuum builds up and suck the cap / plug tight and everything is fine. No shaking.

However, this is just an observation. I still feel like a blind finding my way around. There is absolutely no support from my Jaguar dealer coming to being proactive. I went by yesterday and showed my finding and did replicate the shaking. Not to much enthusiasm shown from them.

But this observation do match the fact that the "misfire" cannot be seen on a diagnostic computer.

Am I totally off on this observation??

 

Your analysis and your experiment with pulling the oil fill cap and the subsequent repeatability of the shake indicates that you are on the right tack in thinking that it is caused by an intermittent vacuum leak. Your misfortune (a very common one) is that it is not occurring right at the dealer, at least, not without pulling the cap. Services hate problems that they can not reproduce and thus work to eliminate.

There are methods to troubleshoot vacuum leaks but, they involve getting your hand dirty. Here is one; spraying starting fluid or brake cleaner spray around suspected vacuum areas/gaskets and if the engine RPM goes up, you located the leak.

How do you locate and fix a vacuum leak

I know that dealer should do this but, unless they can catch the car actually shaking this method would be worthless.

Good luck,

Albert