M42 Running Lean/Fuel Delivery Issue

[ScottySbk]

Alright here again and I need some help, turning key to ON position and allowing the pump to prime doesn’t allow pressure build up, FPR and injectors were replaced leaving me lastly with my fuel pump. Now idling i get between 28-30 psi and NO MATTER what I do (besides shutting the engine off) I cannot get the pressure to decrease or increase. When shutting it off it’ll drop the 0 psi within a minute. My fuel pump was replaced months ago and ohms and voltage read good. Now the conditions i’m having are weak acceleration at low RPM and rough idle, then after a few minutes of driving i gain power again and it’ll idle great and run great so at that point i believe the ECU reads a lean mixture therefore compensating more fuel or something ? I wanna whether or not a weak pump would cause pressure loss or could it be the built in check valve ? Could I buy a check valve and fit it in after the pump ?

[Panici]

Alright here again and I need some help, turning key to ON position and allowing the pump to prime doesn’t allow pressure build up, FPR and injectors were replaced leaving me lastly with my fuel pump. Now idling i get between 28-30 psi and NO MATTER what I do (besides shutting the engine off) I cannot get the pressure to decrease or increase. When shutting it off it’ll drop the 0 psi within a minute. My fuel pump was replaced months ago and ohms and voltage read good. Now the conditions i’m having are weak acceleration at low RPM and rough idle, then after a few minutes of driving i gain power again and it’ll idle great and run great so at that point i believe the ECU reads a lean mixture therefore compensating more fuel or something ? I wanna whether or not a weak pump would cause pressure loss or could it be the built in check valve ? Could I buy a check valve and fit it in after the pump ?

Bad pump, or maybe the in-tank hose is split?

I used Submersible-rated Gates 27097 (3/8" ID) on mine.
Note that it's an early model/M20 pump assembly with an aftermarket Aeromotive pump in my case.

[ScottySbk]

Bad pump, or maybe the in-tank hose is split?

I used Submersible-rated Gates 27097 (3/8" ID) on mine.
Note that it's an early model/M20 pump assembly with an aftermarket Aeromotive pump in my case.

yeah so where you have that hose there’s something else in place, like this plastic thing which im assuming is the check valve, i suppose i’ll replace the pump because even clamping the hose after the pressure change i cannot get my fuel pressure to get up past 30 psi. Just find it dumb considering i just replaced it, just curious whether or not fuel check valves also prevent fuel from back flowing while running, then maybe if i put a new one in at the pump it’ll allow more pressure when running.

[twright]

Hook up a fuel pressure gage so you can see it when you are driving it. If it stays at 28-30 psi, it’s probably not the fuel pump. If it drops off on accel, start looking for a restriction or a bad fuel pump

[ScottySbk]

Hook up a fuel pressure gage so you can see it when you are driving it. If it stays at 28-30 psi, it’s probably not the fuel pump. If it drops off on accel, start looking for a restriction or a bad fuel pump

ok that’s what i wanted to do but i’ve tried to not drive it because when at those low rpm’s trying to accelerate it’ll pop real bad until it catches up speed. Do you know the usual pressure it’s suppose to be ? i’ve read 43 psi but idk how much it’ll fluctuate with idle and accelerating and such, so isn’t it suppose to increase when accelerating not stay around 30 psi ? Would a restriction in the return line cause this ? i ran new high pressure fuel injection hose from pump to filter to fuel rail and still no change in psi.

[twright]

The fuel pressure is supposed to be somewhere around 40 psi, but they seem to run fine until it drops to about 10 psi. I don’t think a return line restriction would cause this. It’s more likely a fuel filter or pump.

[ScottySbk]

The fuel pressure is supposed to be somewhere around 40 psi, but they seem to run fine until it drops to about 10 psi. I don’t think a return line restriction would cause this. It’s more likely a fuel filter or pump.

yeah i mean at this point it’s gotta be the pump, even though i’ve replaced it months ago. My filter seemed fine ? I replace it at the same time as the pump, i pulled it a few days ago and it seemed fine. I also did a pressure check before and after the filter and it was the same. Im gonna jump the pump and do a volume test and also disconnect the hose from the rail and block it and shut the pump off and see whether or not the pressure loss is at the rail or pump.

[twright]

I haven’t had much luck with the volume test.YMMV

[KI4UJO]

Pump supplier? 28-30 is definitely low... worked on an e28 a few years that idled at 30psi, it would drop to 15 before trying to completely stall while accelerating up a hill.

I would definitely inspect the line/fitting connecting the pump to the metal carrier. They explode/leak more often then you would think. Also, it is worth measuring voltage at the pump while the car is running. You could theoretically have an electrical issue supplying low voltage when under a load.

[Tom Galloway]

-I get grief from " real mechanics" for suggesting this but it works for me. Take the plastic cover off the air/fuel meter and rotate the geared wheel 5 clicks or so to re calibrate the fuel supply to a richer mix. Be sure to silicone caulk when replaced to eliminate vacume leaks.

[KI4UJO]

-I get grief from " real mechanics" for suggesting this but it works for me. Take the plastic cover off the air/fuel meter and rotate the geared wheel 5 clicks or so to re calibrate the fuel supply to a richer mix. Be sure to silicone caulk when replaced to eliminate vacume leaks.

While an AFM can be calibrated with the proper equipment, I wouldn't recommend doing this. Also, it won't resolve a low fuel pressure issue.