isx derate problem

2005 IH 9400 ISX 400 Had water pump leaking so replaced it myself. Also put a new fuel gauge sensor in the fuel tank. I had to uncouple two 3/8" fuel return fittings on top of the fuel tank to get the sensor installed. Anyway...The truck started right up, ran good, let it idle for 20 minutes, then went to drive it and it stalled, started up again, now can't get more that 1500 rpms. Turned it off several times, starts right up, but still nothing over 1500. Truck ran great before. Did I bump something during water pump replacement or is it something to do with the fuel line being disrupted. Thank you for any ideas.

 

The check engine light has been off and on for 6 months. Never seemed to affect the performance. Just filled the coolant after water pump replacement. Thanks for ideas.

 

I'm not totally sure what those lines are. They both were just those push in O-ring fittings with the C clip to hold it in. They went into the top of the fuel tank. I think they sealed up good again. Thanks.