Austin91

Hey guys, I have a 91.5 D250 that has a misfire. Pretty sure its fuel related but not 100%. When it first happened I checked the lift pump and it was failing so I replaced it with a HVLP piston pump. Did not fix the misfire. If I crack the injector lines at the injectors it gets worse on all but #5. I swapped #5 injector with another to see if it was the injector but still had a miss on #5. #5 cylinder is also ~30° hotter than the average of the others after only running for a few minutes when checked with an infrared gun. I also noticed that when I crack the injector lines I get an atomized mist from all but the #5 line, it is more of a squirt... I am thinking it is an issue in the IP but would like to know if there is anything else it could be before I pull it. I did pull the valve cover on #5 and it seems to be okay but I did not pull the rocker to check pushrods. I'd like to do a compression test but my mechanic uncles do not have the fitting for a 12v... I have also swapped the delivery valve on #5 from a "thought to be good" pump.

Any help would be greatly appreciated!

Notes: I have pressure tested the lines from the tank to the lift pump and changed the fuel filter. Also tried an electric lift pump off a 24v and no change.
Mods include pump tuning, 3200 Gov spring, fuel pin, KSB shim plate, 5x14 sac, HE351, THD HVLP pump..

thrub

I am definitely not a pump expert but I did just have one apart for a reseal and there isn’t much in there that can go wrong and just affect one cylinder. The delivery valve which you checked and the small fuel passage to the delivery valve. At least as far as I can tell. I kind of think it more likely that your issue is maybe compression or lack there of. Not sure if lack of compression explains the higher temp on that cylinder though.