I have an ED Falcon which I've only owned for a month or two. I am able to do mechanical tasks, but I am always learning along the way (mostly from mistakes). Last weekend I replaced the Intake Manifold gasket because it had a bad vacuum leak as a result of a busted gasket and this was making it idle way too fast. Being the first time I have done one it took me around 5 hours with basic tools. All the time the battery was disconnected. Once I had completed the job and put everything back together I started it and it ran like a dream, perfect idle speed, no jumping when changing gears. Now it been about a week the high idle has returned. I was wondering if it could be the ECU because that would have reset with the disconnected battery. Either that or i didnt do a very good job of the gasket replacement. any help you could provide would be great.

If it took you 5hrs to replace an inlet manifold gasket on a ford 4.0 doesn't sound like you have the skills to be working on your own engine. No one here will be able to tell you what your problem is. It could be a bolt or two that have worked there way loose could be a loose vacuum hose could even be the idle speed out.

Take your car to a professional they will find the problem, will end up saving you money in the long run

Thanks for the response.. I think the main reason it took so long was i just didnt have the right tools to do it. Most of the time was spent undoing the bolts, some only 1/8th of a turn each time.
Oddly enough after disconnecting the battery for half an hour, running through the steps in this thread viewtopic.php?t=33496 it was still idling high for around half an hour of driving before suddenly coming good. Its now running well again today.

But yes, if it returns i think the mechanics is the way to go.