I've owned my XF Fairmont Ghia EFI since 1994, and I have never seen the thing stall or even falter in the slightest after removal of the oil cap. I can watch my rockers at work through the oil filler hole and the engine doesn't miss a beat.
This would be because I have a vacuum leak somewhere and the computer is compensating for it, thereby reducing my all round performance. Add to this the fact that I can hear a very obvious hissing noise coming from around the EGR valve region, where there are a lot of vacuum hoses and valves and stuff. I've sprayed WD40 all over the area to try and find the leak, but to no avail. I've jiggled and prodded, you name it. If I take off some of the hoses the engine stops, but if I do it slowly, so it has time to adjust before the hose is all the way off, it can keep running.
Anyone have any advice on this? The engine's been rebuilt in 1999, so it's not a major thing like manifold gasket, also the EGR valve has been replaced before. I'd love to sort this one out.
If it ain't broke don't fix it.