CM570 Low Fuel pressure woes

[Rawze]

It sounds to me like your over-thinking it. Those older IFSM systems are pretty straight forward.

250psi is about normal for idle pressures. - What is NOT normal is It falling off when engine is loaded up. This leads towards a fuel starvation issue, or a bad gear pump.


Also, unlike common rail systems... it is normal to see a lot of fuel going back to the return lines, so your barking up the wrong tree on that one. - This is how they work... Once the pressure is met,.. ALL the fuel gets circulated back to tanks to keep the tanks warm (preventing gelling in winter while engine is running) and bleed off heat from the fuel system. It is also NORMAL to see bubbles in the fuel return lines btw. It is due to cavitation at the exit port of the IFSM and nothing else.

#1... Start by REMOVING all the guts from the fuel shut-off solenoid. They are notorious for going bad internally and starving the fuel system. The engine should shut down just like it did with the solenoid guts in place. If it has hesitation in shutting down with guts removed from shut-off valve, then you have leaky injector(s).

#2. DO A INJECTOR LEAKAGE TESTS... for all anyone knows, you may have a bad check-valve in an injector, blowing combustion gasses into the system and starving it of fuel. This is also possible... and not too uncommon. Use OEM ONLY replacement injectors, as they have updated check valves, etc.. and are not as prone to these types of failures.
info on how to do it: https://rawze.com/forums/showthread.php?...8#pid67948

#3. If the truck has a glass-bowl type of pre-filter... the type where the glass boll hangs down.. those are NOTORIOUS for letting air in at the seals after a filter change... and you will never see bubbles in the bowl. - Ensure this is not the issue.. bypass the thing if you have to for troubleshooting.

#4. Pressurize the fuel tanks to 5 psi, put lines that feed the engine into bucket, and see if there is air in the fuel. For all anyone knows, there could be a cracked pick-up tube in a fuel tank.

#3. Replace the gear pump (OEM ONLY) if it is more than 8+years old. A snapped shaft will often let the engine crank and it run, but not keep up when under load. Also an OEM-only part.