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1994-1995 5.0 Mustang SN95 Tech Forum

5.0L Talk | 5.0L Tech | 94-95 Tech | 1979-1993   1994   1995   Upload Pictures | Timeslips | Power Adders | Mustang Tech 


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Unread 06-06-2011   #1 (permalink)
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Default 1994 Mustang GT low fuel pressure

Ok, so here's what i've done so far: replaced fuel strainer and filter (fuel pump new), new adjustable FPR, checked all lines for leaks. I am only getting about 24-26psi on fuel pressure, even with the adjustable regulator. I tried adjusting it but the gauge doesn't budge. Any ideas to what the problem could be/is?
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Unread 06-06-2011   #2 (permalink)
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Could be a power issue.
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Unread 06-06-2011   #3 (permalink)
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Default 1994 Mustang GT low fuel pressure

What aspect of the power system would affect fuel pressure? I'm running a 195lph pump w/ 24lb fuel injectors and a 80mm mass air tuned for 24lb injectors.
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Unread 06-06-2011   #4 (permalink)
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The fuel pump is electric so you may have a bad wire/fuse/ground somewhere. I believe the fuel pressure is also electronically controlled. Could also be a fuse/sensor.

How many miles on the car? There's also a possibility that you have a blockage or kink in the lines somewhere. Overtime with low quality gasoline, you can build up a block inside of the lines. When was the last time you ran some fuel injector cleaner or fuel system cleaner through the system?
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Unread 06-07-2011   #5 (permalink)
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I'd suspect a bad AFPR or bad gauge. Our "primitive" SN95's don't do much by way of electronic control - the fuel pump is all-or-nothing. If you did have a fuel pump / blockage issue, you could still adjust downward (under where you're presently reading). Since it's not budging at all, it sounds like your fuel pressure regulator could have been bad out of the box. Did you have the pressure problem before the new AFPR? If so, I'd lean towards your gauge.
 
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Unread 06-07-2011   #6 (permalink)
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Default 94 mustang gt low fuel pressure

I had this issue with my fuel pressure before purchasing the new adjustable regulator. I have tried running injector cleaner and seafoam to ensure that the system had no blockage and still i got the same results. This engine has been newly rebuilt and i only put about 75 miles on it since. Could an exposed wire inside the tank (perhaps the ground) cause this low pressure reading or would it be all or nothing like you said? The only relay that I am aware of that would have anything to do with the fuel system is the CCRM and that is new as well. If there is any other, let me know where they are. Back to work on this beast that I can't drive!
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Unread 06-07-2011   #7 (permalink)
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Possibly the MAF or the TPS is bad. Doesn't the voltage of these 2 items also control how much fuel is sent into the system?
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Unread 06-07-2011   #8 (permalink)
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Default 94 Mustang GT low fuel pressure

Well, the TPS is new because I replaced it. The MAF sensor i bought off of someone. The MAF sensor tells the computer how much air you are recieving and the TPS tells the computer what position the throttle is in.
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Mustang 5.0 GT- 302, 3:73 gears
BBK Long tube headers w/off road X-Pipe, SLP I exhaust dumps, BBK Fenderwell CAI, Mallory wires, 24lbs injectors, Steeda X2 ball joints, steeda bumpsteer kit,Eibach Pro Kit suspension w/Stagg Shock & Struts, 180 degree thermostat, high performance oil pump, Walbro 155lph Fuel Pump,Saleen Rims & Body Kit EGR Eliminator, 75mm throttle body, E303 cam, GT40P Heads, Trickflow Street Burner upper/lower intake, King Cobra Clutch, SCT 4-Bank Eliminator Chip
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Unread 06-07-2011   #9 (permalink)
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Quote:
Originally Posted by johnGT View Post
Possibly the MAF or the TPS is bad. Doesn't the voltage of these 2 items also control how much fuel is sent into the system?
The control you speak of here is the computer changing the injector pulse width to increase or decrease fuel needs. There is absolutely no link between the computer and fuel pressure.

A working pump will produce about 90 psi to the regulator. The FPR adjusts the pressure to maintain an even flow rate through the injectors. At idle this should be about 35 psi. As the engine load increases the pressure will rise to a max of about 42 psi at WOT. KOEO the pressure should be about 40 psi. If it doesn't get to that pressure either the pump isn't seeing 12v or isn't working and/or the FPR isn't working.
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Unread 06-07-2011   #10 (permalink)
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Default 94 Mustang Gt low fuel pressure

Well, as I increase the throttle, there is no change in fuel pressure +/-.
When i turn the vehicle to KOEO (key on engine off) i can hear the pump prime. The fuel regulator i purchased is new and adjustable. Adjusting the screw on top of the FPR shows no change in pressure whatsoever. Would a vaccum leak cause the issues i'm having? Even though my fuel pump has less than probably 5 hours of operating time, is it possible that could be the culprit or an exposed wire in that area?
Thanks guys for all your help! "Its funny how i can fix everyone else car and catching h@!! with mine!"
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BBK Long tube headers w/off road X-Pipe, SLP I exhaust dumps, BBK Fenderwell CAI, Mallory wires, 24lbs injectors, Steeda X2 ball joints, steeda bumpsteer kit,Eibach Pro Kit suspension w/Stagg Shock & Struts, 180 degree thermostat, high performance oil pump, Walbro 155lph Fuel Pump,Saleen Rims & Body Kit EGR Eliminator, 75mm throttle body, E303 cam, GT40P Heads, Trickflow Street Burner upper/lower intake, King Cobra Clutch, SCT 4-Bank Eliminator Chip
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Unread 06-07-2011   #11 (permalink)
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Quote:
Originally Posted by toyman View Post
The control you speak of here is the computer changing the injector pulse width to increase or decrease fuel needs. There is absolutely no link between the computer and fuel pressure.
Gotcha. I always thought it worked differently.

Sounds like you got one hell of a gremlin. I'm out of ideas.
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Unread 06-09-2011   #12 (permalink)
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The electronically controlled fuel pump (returnless system) came on the later Mustangs -- the 94-95's had the old fashioned fuel-pressure-regulator and return-line configuration (thank goodness, much easier to upgrade!).

Anything is possible if you had a bad pump or a wiring issue. But... you should still be able to adjust the pressure downward using your AFPR, and your pressure should still drop when you attach the vacuum line to the AFPR (if it's pulling any vacuum at all, even if you have a leak -- just check the hose with your finger).

Have you eliminated the bad fuel pressure gauge from the equation? Your symptoms are pretty unusual. You can probably borrow one from an Autozone, or at least try your gauge on another car. My car probably wouldn't even run with 26psi at least not without throwing a lean code.
 
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Unread 06-10-2011   #13 (permalink)
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Default 94 Mustang Gt low fuel pressure

just though of something here guys and i was wondering, with my setup of E303 cam, trickflow street burner intake (upper and lower), 24lbs injectors, 80mm mass air, and GT40 heads if i could still run the stock throttle body? Im asking to figure out if thats where my problem is coming from. I think the stock throttle bodies for those cars were like 55-60mm stock.
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BBK Long tube headers w/off road X-Pipe, SLP I exhaust dumps, BBK Fenderwell CAI, Mallory wires, 24lbs injectors, Steeda X2 ball joints, steeda bumpsteer kit,Eibach Pro Kit suspension w/Stagg Shock & Struts, 180 degree thermostat, high performance oil pump, Walbro 155lph Fuel Pump,Saleen Rims & Body Kit EGR Eliminator, 75mm throttle body, E303 cam, GT40P Heads, Trickflow Street Burner upper/lower intake, King Cobra Clutch, SCT 4-Bank Eliminator Chip
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Unread 06-10-2011   #14 (permalink)
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Your signature indicates you have a 75mm TB. However, I see no reason for the stock TB to be an issue with your setup. Besides I thought we were chasing down a fuel pressure issue.
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Unread 06-11-2011   #15 (permalink)
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^+1 seriously friend, you ask for advice and people take the time to respond to your request with intelligent troubleshooting advice. if you want to fumble in the dark and replace parts, by all means have fun. otherwise, if you're asking for assistance, a response to the troubleshooting steps provided to you is good etiquette.
 
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