Isx 870 fuel pressure

[Rawze]

(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