should i replace all the check valves, just to be sure?

anything else i should do, while i'm doing to job?

 

If they are working properly then no. You probaly want to replace the FPD and possibly send your injectors off for a good cleaning. If you are considering block off plates it is also a good time to do those. Replace the turbo coolant lines too.

 

Test the check valves with a Mityvac. The blow through test is not adequate. While your at it also check the PCV (plastic connects from the oil filler neck to the UIM) and purge control valve (metal comes from the catch under the intake elbow), you can eliminate them and cap the nipples on the UIM if you find that they're leaking. Definitely consider getting your FIs cleaned as Rated R1 said if your FD is high mileage, makes a huge difference. Those coolant lines going into and out of the turbos are prone to failure and should be replaced at least every couple of years. And clean your IC. Spray a full can of carb cleaner into it, slosh it around some. Then use some Simply Green or other non-corrosive cleaner and water to get all the carb cleaner out, and let it air dry for a couple of days.

 

What exactly is the purpose of the block off plates?

 

To cover up the areas of emission control items (or various other components) that you want to remove. If you remove the ACV, you can't leave a huge hole in the side of your LIM. So, you put a "Block Off Plate" over the hole were the ACV used to be to cover the opening (likewise with other components).