Austinstrab

Hey all, I'm new to the site and diesels in general, so I posted this in Powertrain and it was recommended I bring it over here for some more help. Just a quick copy and paste, so I apologize for repeats.

Hello all, I'm new to the forum, as well as Cummins and diesels in general. I've been reading up for a few weeks now, and I'm ready to post a question, as I have not been able to find anything that sounds quite like my problem.

First off, 2001 2500 4 in lift with 35's, intake and a super chip; all done by previous owner, and I am working my way through the issues he left me, as it's starting to become clear he drove it hard and put it away wet.

Anyway, I've read extensively about the fuel issues in this truck and I'm trying to nail down what may be the problem, if anything. I noted fairly quickly that I was not getting enough fuel pressure (around 5 psi and less w/ a hard load) and replaced the fuel filter. Solved the PSI problem (idles at ~13, never drops below 10 under load). It did not solve the buck at around 2000 RPM, however. I pulled the codes with the tuner and got the P0216. I have since cleared it and ran the truck in all configurations the super chip offers, and I get the buck and the p0216 in all settings EXCEPT when returned to stock. I absolutely hammered it in the stock setting and could not get the code to return in stock condition, but in every other one I do, including "hi-performance" which I find odd, as I would think I could get after it a bit more in this setting. (please correct me if I'm wrong in this assumption.) During all test drives the PSI never drops below 10, and I almost always get the buck at about 45mph, with the RPM at about 1900-2000.

So, finally to my question. Is this a symptom of a lift pump, an injection pump, my transmission, or is the chip not allowing me over 2000RPM? I've Troubleshot the Lift pump via bluechipdiesel's run down, and deemed the lift pump worthy, if only for the time being.

Any and all help with this is greatly appreciated. thank you!

Update: after posting this, I went back out to the truck for a closer look at the lift pump and noticed it is a FASS DDRP, and says it's an EM 990 wit a date of 03/09 on it. good? bad? indifferent?