Hello my 2004 x-type 3.0L 6 cylinder jaguar just got the coils, spark plugs, and gaskets renewed and it was sitting in my garage for 6 months. I have gotten it to start and I can even drive but the brake gets stiff when I use it at a stop sign and my rpm's start to die. When I sit in idle the car just slowly dies on its own, I don't know what the problem is. I have already given the car some STP to help with the old fuel and driven it around for a couple days but I don't know what is causing the car to just die on its own. Any help is much appreciated.

 

With the brakes being stiff and the car dies at idle it is most likely the brake booster check valve is cracked and causing a major vacuum loss. The line from the top of the intake manifold going towards the drivers fender has round object about half way along the line before it gets to the brake booster.

I have attached a picture from the JAGBITS.COM one of our sponsors. The white object is the check valve. Inspect it for cracks and damage. If you suspect its bad you can wrap it in tape and see if things improve. If so order a new one and replace it.

 

I think you're right on the money Vern..Good call.