I've got a 2008 5.4L I just bought. The previous owner just ran around the city and never pulled anything with it. After the truck has been pulling and the engine is hot, when I slow down to an idle the engine starts to diesel. This lasts for a few seconds then goes back to normal. Is this just because the engine isn't used to working and the computer hasn't adjusted yet? is this something that could be serious and I should have checked out by a garage?

 

dieseling, is when you turn OFF a gas engine but it continues to run.

more detailed information is needed.

first guess.. check the cooling system completely.
clutch fan not locking
fan belt loose
radiator front is clogged with grass/leaves..
low on coolant.
bad radiator cap.
or a bad/sticking idle air valve.

 

You probably mean that it sounds like a diesel at idle. That is a common issue that needs to be addressed before doing any major harm, probably cam phasers.

 

Yes, cam phaser issue. Lots of good instructional videos on cam phaser and VVT solenoid relacement on YouTube.

 

Yep cam phasers.