I just bought a new to me truck. 99 Ram 1500 5.2L. It has 119K miles.
I just gave it a tune up, which it needed badly. (cap rotor plugs wires). It's not throwing any codes.
Here's what it's doing:
Horrible gas mileage
Using oil at an alarming rate (1/2 quart in 100 miles)
When coming to a stop, the RPMs drop to 500 then it pops back up to about 700.
Otherwise, it runs great.

From what I've been reading, it sounds like the intake plenum gasket is leaking. But how do I check for sure?
I shined a light down into the intake and didn't see a bunch of oil, but it wasn't spotless either.
I know about the Hughes engine fix and that's what I'm planning on doing if that is the issue.

Thanks all.

usually it'll be wet with oil. might not be oil puddled up on the bottom, but just wet. you could also have an O2 sensor going south affecting the engines fuel trims.

but, the use of oil is a big indicator too. now that you changed the plugs, how are they looking?

It sounds like the gasket , even if it was replaced it can go bad again depending on when it was done. I went thru that back in 1999 ...

It is an easy test!!!

Take the oil fill cap off.
Start engine.
Hold a piece of paper over the open hole.

Does the paper go in or out?

In is a vacuum leak that would suggest the pan gasket is leaking.
Out is blow by and a little is normal.
(you may want to pinch closed the PCV hoe while doing this test)
I had two of these manifolds in the past thatdid NOT have the holes drilled deap enough for the long bolts used. It never snugged the gasket down!!! I simply added washers to each bolt.

Thanks for the tips guys. When it stops raining I'll try the paper trick.

Okay. I just tested the truck. I could not see any movement on the paper either sucking in or pushing out. This is over the oil fill hole.
Second thing I tried was pulling the hose connected to the PCV and the one on the other side that goes to the air cleaner. I put my thumbs over both hose ends to see if there was pressure. There was pressure on the non-PCV side, but none on the PCV side. I'm not sure if the valve itself prevents pressure during certain conditions.
Here's the TSB, I think I'll try using a gauge to make sure I was hearing pressure and not vacuum.
http://www.dodgeram.info/tsb/2000/09-05-00.htm

If that thing is really slugging down the oil, like you indicate, and shows no real clouds of smoke out of tailpipe, has no leaks and runs well otherwise, I would think that lower gasket is blown out. It's common.

I was told that oil on the butterflys and the throttle body was a sure sighn. My dakota started idling rough, after I sprayed carb cleaner in all the holes in the throttle body and it ran fine.

I sprayed the throttle body as part of the tune up and that didn't help the idle issue.
I think I've decided to go ahead and replace the seal with the Hughes deal just to be sure it will be good in the future. I might do the O2 sensor at the same time because it seems like they fail around this mileage. Those cause drive-ability and mileage issues too.

been there done that! mine didn't affect the rpm as much as your but the intake was coated black with oil! truck runs way better now ! I do have little issues with it but no oil in the intake! not real happy with the fuel mileage I will keep a eye on that with my next filled up.. fuel up yesterday check mileage it wasn't much of a gain which I figure wasn't going to be good I pinch a oring on one injector so I lost some fuel to that then the cpu had to relearned everything so rpm was up and down... then again the good summer blend gas not in the store yet.. truck alway done better in the summer for mileage! I have picture of my intake when I had aparted look up my my screen name the past three week...