Threaded Mode | Linear Mode

Kiko · 05-21-2025, **(Subject: Isx 870 fuel pressure )**

Hello all again. Am having trouble with another truck being hard to start. Cranks for along time and then fires up and runs rough. No problem found when engine hot and running on highway. Lift pump is working and putting out about 15 to 20 psi. Put on clear hoses and no air found. Also with clear hose on compu fitting the fuel doesn’t appear to be leaking off to return. When it cranks after lift pump runs for 30 seconds it is slow to build pressure. After running and shut truck off. 10 seconds later it’s still hard to start. I read on here about main pumps breaking the shaft and still working but just not to their best. This could be possible but have been trying to eliminate everything else before ripping it off and replacing. A question I have is how much fuel should be returning to tank with just lift pump running. With a clear hose on compu fitting and lift pump running it flows out but not even close to what is flowing out of return hose. Thanks in advance.

Rawze · 05-21-2025, **(Subject: Isx 870 fuel pressure )**

A healthy gear-pump will produce 100+ psi at the fuel pressure sensor (can monitor it in Insite) while cranking, above 200psi while idling, and well above 275psi while running.

note: It takes a minimum of 50-psi at the sensor for the ecm to tell the injectors to fire up the engine.

Kiko · 05-22-2025, **(Subject: Isx 870 fuel pressure )**

Thanks for the info again Rawze. I’m still not 100% that it’s the gear pump. I’m at about 80% right now. I know the air bleed valve will put fuel back to return side hose but also if a pressure control valve is leaking it might do the same. When engine is running I’m getting about 240 psi at idle on insight. Would that then rule out pressure control valves or air bleed valve.

Rawze · 05-22-2025, **(Subject: Isx 870 fuel pressure )**

(05-22-2025 )Kiko Wrote: Thanks for the info again Rawze. I’m still not 100% that it’s the gear pump. I’m at about 80% right now. I know the air bleed valve will put fuel back to return side hose but also if a pressure control valve is leaking it might do the same. When engine is running I’m getting about 240 psi at idle on insight. Would that then rule out pressure control valves or air bleed valve.

WHY are you concerned with the return fuel flow?... ITS A CM870/CM871!!!. They are an older design and have a LOT of return fuel flow, as the pump runs wide-open all the time, and all the excess fuel that the engine is not using is dumped directly back to tank. It having a lot of return fuel flow to tank(s) is their nature!.

... and as far as air in the return line(s) ... THAT IS 100% NORMAL!!!, as they are notorious for return line cavitation straight off the IFSM! . It is well known that they do this!. It has to do with the exit port off the IFSM having a sudden change in piping size as it exits the device, returning to tank.
here are some old posts about it from years ago ...

Rawze Wrote:It is normal to get some bubbles into the return line. It is due to cavitation at the regulator. -- It will typically stop if you apply a bit of back-pressure/restrictions to the return fuel line system while testng.

Pressurize the fuel tanks to 10 psi and see if it clears up, or if you find fuel seeping out of one of the fittings,.. pump, etc. while engine is off and it sits for an hour or so with tanks pressurized.

WARNING: -- NEVER -EVER try to remove the fuel caps woth the fuel tanks pressurized. THEY HAVE TREMENDOUS FORCE BEHIND THEM, and will break your arm.wrist if you try to remove them with the tanks pressurized even a little bit. The pressure volume of the whole fuel tank is behind them.

ref: https://rawze.com/forums/showthread.php?...72#pid2372

Pigpen 2869 · 05-22-2025, **(Subject: Isx 870 fuel pressure )**

You have any leaky injectors. Mine has 2 it takes forever and a day to start in the morning. Then it pops and farts for 45 seconds clears up and is good the rest of the day.

Rawze · 05-23-2025, **(Subject: Isx 870 fuel pressure )**

(05-22-2025 )Pigpen 2869 Wrote: You have any leaky injectors. Mine has 2 it takes forever and a day to start in the morning. Then it pops and farts for 45 seconds clears up and is good the rest of the day.

Thanks for mentioning that Pigpen ...

yes... they also need to do injector leak tests on it as well...
info: https://rawze.com/forums/showthread.php?...8#pid67948