Is this a 5.9 gas or diesel

Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice so we may have to fine tune the answer. Feel free to add any additional info you feel is needed as we go on. Always keep in mind that I am here to help you with any questions.

I was thinking it was a diesel. I would look for an air leak on the suction line, between the pump, and the pickup tube in the fuel tank. If this happens the pump sucks air in, and has trouble moving enough fuel, because under suction the air is drawn into the fuel pump. They call this aspirating. Thie can be tough to find. What I do to check for this is to put a piece of hose on the suction side of the pump, and place it in a big can of diesel, then recheck the fuel pressure at the filter housing. If the pressure is significantly improved, then it is time to go over the suction line, and part of that is dropping the fuel tank, and replacing all the rubber lines, and checking all the hard lines. If the hose in the bucket of diesel doesn't help the pressure I would be worried that the cam might be worn where the pump contacts the special cam lobe that moves the lever on the pump. You can use an electric pump if this is the case, and it will work just fine. Perform the diesel in the can test, you might have to buy a few pieces of hardware to do it, but it will tell you a lot. I have seen a lot of these older cummins Dodge trucks with deteroriated rubber suction lines going into the tank that cause this problem, I have seen a few with a bad lobes on the cams too. If you have any more questions, as always, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.

I hope this helps, 100% satisfaction is my goal. Myself personally, and Just Answer are here to serve you. If the answer is not clear, let me know what additional help you need, and I will assist you further. A bonus and positive feedback are always appreciated, good luck and Thanks.