03 Mustang Mach 1 fuel issues

[another Todd]

Helping a buddy with his brothers Mach 1. Car barely runs. Fuel pump replaced 3 months ago. Scanner reveals:
STFT -20 both banks similar
LTFT -7 both banks similar
o2 sensor .975-1. does not alternate between high and low
MAF 70
Fuel Pressure 7

Now I know 7 psi fuel pressure is way to low, but why is the computer trying to lean it out with neg. fuel trim?
Why does the o2 sensors stay above .975 and not fluctuate between that and say .100 as it should (both sensors the same)

 

[mechtech2]

If the O2 sensor is fully hot, and the engine is fully warm, it should fluctuate. But if you have a massive rich or lean situation, it may peg at one end or the other. So at this point I don't know if it is the sensor being bad, or being OK and reacting properly.

That fuel pressure seems way too low. A plugged filter may be all it is, or maybe the pump.

we always have to be aware of multiple problems.

 

[stroked93]

the low fuel pressure is likely just a symptom as ecu will pull voltage from pump to lean out. Or it could be that the fuel pressure sensor is defective and the ecu cannot regulate the pressure. what modifications have been done to the car?

 

[another Todd]

No mods, but I really have no history, just stepped into this today. I was not aware the ecu could lower voltage to the pump.
I agree the filter will be the starting point.

 

[stroked93]

99 up have return less fuel system,

 

[another Todd]

Turned out the new pump installed a couple months ago was bad.

 

[GMBoy]

Originally Posted By: another Todd
I was not aware the ecu could lower voltage to the pump.



That's because it DOESN'T. That is bad information. The only way an ECM can control fuel is thru the injector pulses. It can completely cut off an injector on up to keeping it open and everywhere in between. A fuel pump ALWAYS runs FULL SPEED - no variable speed. Either the excess fuel is returned to the tank (in a return system) or the pump cuts off as needed based on the pressure sensor (in a returnLESS system).

 

[mechtech2]

Originally Posted By: GMBoy
Originally Posted By: another Todd
I was not aware the ecu could lower voltage to the pump.



That's because it DOESN'T. That is bad information. The only way an ECM can control fuel is thru the injector pulses. It can completely cut off an injector on up to keeping it open and everywhere in between. A fuel pump ALWAYS runs FULL SPEED - no variable speed. Either the excess fuel is returned to the tank (in a return system) or the pump cuts off as needed based on the pressure sensor (in a returnLESS system).


Actually, Ford and others DO use a variable voltage pump system on some cars.

 

[bdcardinal]

Originally Posted By: mechtech2
Originally Posted By: GMBoy
Originally Posted By: another Todd
I was not aware the ecu could lower voltage to the pump.



That's because it DOESN'T. That is bad information. The only way an ECM can control fuel is thru the injector pulses. It can completely cut off an injector on up to keeping it open and everywhere in between. A fuel pump ALWAYS runs FULL SPEED - no variable speed. Either the excess fuel is returned to the tank (in a return system) or the pump cuts off as needed based on the pressure sensor (in a returnLESS system).


Actually, Ford and others DO use a variable voltage pump system on some cars.


Bingo, the returnless fuel systems use a fuel pump driver module(s) to vary the voltage to the pump(s).