Is carbon buildup really a problem in FJR engines?
I do essentially the same that Jeff suggested but give it about half hour to an hour while doing other stuff around the garage. Usually trying to find something in the mess takes that long.
I've experimented with Seafoam in a couple of engines that I either was about to take apart or had pieces from. SeaFoam definitely works better in a pan of parts on a hot day than in the winter, which supports the idea of using it in a hot engine. Like naphtha, it evaporates pretty quick too, so don't expect stuff to soak overnight in an open pan.
The big thing that I found was that instead of touching the actual cooked-hard carbon, it reduced the tacky brown hardening crud before it became the hard black stuff. Which is a good thing, because from the injector bodies to the intake valves is where that crud collects and it will eventually bake on. Everybody talks about the valves and the piston crowns, but forgets that rough walls on an intake manifold also massively affect air flow.
I had a Ford 4-cyl that I SeaFoamed, removed the throttle body, looked in the manifold, then re-assembled, and tried again. The reason is that after the first treatment I got lots of black but it didn't totally clear the engine, so I took a look at what the first treatment did. It really did take several treatments over an evening, but the last time I removed the throttle body the manifold had lost much of the tacky touch and looked a bit better. Not fantastic but better and the car ran much better.
As for hydro-lock, you'd need to have an appreciable amount of actual liquid in the cylinder for this to be a concern.
