Engine doesn't idle properly
 

Hopefully someone can help. I have a 4.4 N62 V8 which I just replaced the coolant transfer pipe,valley pan, intake manifold. Now it idles strangely, it't not rough and it's not missing it's just got a very small vibration - you know when you move your car from neutral to gear and it drops a little, mine is always like that...

It idles around 550rpm and sounds/feels like it has a load on it. I'm thinking that the coolant issue is unrelated but thought I would put it in just incase.

Have you checked for vaccum leaks?

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I'd also wonder if there is a vacuum leak (something unplugged or not seated correctly) or a sensor not plugged in. Did you plug your car into a Peak tool, INPA, GT1 or similar to read your codes? The car IS working to tell you something - you just need to find a way to speak it's language :)

It could be a vacuum issue - everything is plugged back in - I'm a bit OCD with that. Maybe dust got into the MAF or I damaged it when I removed the intake components and placed them on the ground? I disconnected the MAF while the engine was running and after it's initial spurt sounded like it idled better (normal) so I'm leaning towards that. I don't have the money to replace it so I'm hoping that it is just dirty but will keep trouble shooting...

If you didn't have the issue before the work you did, it is most likely this issue is connected and likely a vacuum leak as suggested. Before I would do something like a smoke test or replacing anything, I would try spraying carb cleaner or starter fluid where a vacuum leak may be. If there is a vacuum leak you should hear the idle increase somewhat. The MAF is easy to clean, be sure to use MAF cleaner.

To OP : You can also check your 4 sensors cam position... Mine had some leaks (oil) creating unstable idle.

I'm struggling to work out where a vacuum leak could come from, anything in particular I should be looking for? I didn't really disconnect any vacuum sources so any help with trouble shooting would be very much appreciated...

All around intake manifold, usually. Even places not anywhere near where you were working can be affected from pulling on that and tugging on this. Old brittle lines are easy to crack and it's hard to see.

I'm not familiar with your engine so pardon the lack of specifics.

I recently tracked down a Leak to the drivers side rear wheel well which I thought had something to do with cleaning MI idle control valve..... It was just coincidental on the timing and not really related. Good luck with the search...... If you have any hair to pull out you may need to. :)

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It's not the one's you disconnected you have to worry about. It's the one's that may have come disconnected without your knowledge.