Fuel Excessive Fuel Pressure 98 Mustang Gt

stangfreak98

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Oct 31, 2014
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1998 mustang gt 4.6L SOHC
MODS:
-sr lowering springs
-Summit aluminum upper intake manifold(non PI)
-Airtex fuel pump
-Recently replaced idle air control valve and MAF sensor. And all 02 sensors

Throwing code for: all o2 sensors(recently replaced) my car is now having excessive fuel pressure. Supposed to be like 32-36 psi at idle and is running 100psi. The only conclusion I can come to is the fuel pressure regulator however my local ford dealership says the fuel pressure regulator is fine but has the wrong rail on it??? They checked my vaccum lines, and fuel injectors. Everything seems to be fine. My question is if they are saying the regulator is fine but the fuel rail is bad. Is that the fix??? Apparently my car has the fuel pressure regulator built onto the fuel rails. But I have a return line style. All fuel lines were soaked and blew out to clear any blockage still nothing. Right now I'm debating. Should I buy a new fuel rail kit? I found one for $260 that included the regulator. Should I get it? I'm tired of throwing money into it and I'm trying to sell it. I have a family on the way. I'm trying to get out as cheap as possible but IdK what to do. I'm getting mixed answers because apparently they don't know ford did the split year on the fuel systems. 96-97 rails and 98 rails. The fuel rails have been brazed and replaced like the recalls said. But I guess my question is, is my regulator really bad or good? They told me just because it reads good doesn't mean its the right part????? IdK what my next step should be. Why is my car running so high. It starts fine it just bucks badly almost as if it were a miss fire. Due to excessive fuel pressure I'm guessing. What should I do??
 
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Mar 16, 2009
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Springfield, MO
If you never changed the rail, then how would that be the solution!? If the regulator isn't the problem, then it is either the pump, the wiring going to the pump, a faulty MAF or a problem with the PCM. Have you tried putting the old MAF back on to see if that fixes it? Perhaps you got the rouge terminator fuel pump that became aware of its existence ever since Skynet went online and is hell bent on killing you in a fiery blaze? :)

Good Luck!
 

stangfreak98

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Oct 31, 2014
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Well let's see here what I've done to it is check the vaccum lines tested the fuel injectors and fuel pressure regulator. Replaced the maf sensor all 02 sensors( original code) IAC valve ecu fuel pump. What else is left? Everything is working right. Or should be lol
 

wmburns

SN Certified Technician
Aug 14, 2009
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Just wondering. HOW has the return line been CONFIRMED to be flowing freely? A blocked return line will cause high fuel pressure.

Are you aware that it's possible for an excessively over sized fuel pump to over whelm the ability of the return line to carry return fuel. The symptom would be excessive fuel pressure.

Also, the 98 model year has a two speed fuel pump controlled via a relay and ballast resister. Have your confirmed that the 2 speed fuel pump control is working? What is the voltage at the trunk mounted IFS switch with the motor running?
 

stangfreak98

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Oct 31, 2014
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I replaced the fuel pump with a direct OEM fit fuel pump by Airtex from autozone. I also soaked all of my fuel lines including the return line and blew them out their was no blockage.
 

stangfreak98

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Oct 31, 2014
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It is putting out 100+psi... At idle its putting out the exact psi that the pump is rated for. In other words I have maximum fuel pressure.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
It is putting out 100+psi... At idle its putting out the exact psi that the pump is rated for. In other words I have maximum fuel pressure.
Soooooooooo That says to me that the two speed fuel pump relay isn't working. At idle the relay should be in the low speed mode with reduced voltage at the trunk IFS switch. It's not possible for the fuel pump to produce it's full rated output on half the voltage.

However, my gut still tells me there's a blockage in the return line. Perhaps a kink in a part of the line not tested or a kink that happened when the gas tank was re-installed.

Another thought is there is an un-used port on the gas tank hat. The unused port is blocked off internally. Perhaps the return line was connected to the blocked off port. If this were my car, I would want to know if the return line is clear after everything put back together.

As a final thought what if the lines are connected backwards at the fuel rail?
 

1987stangman

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Jul 12, 2006
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Burns isn't the 98 a kinda bastard year when it comes to the 98'? If it's a two speed pump does the PCM control it for fuel delivery?

Unless some past owner decided to change the fuel rail maybe after an intake manifold replacment it seems like the lines and all should be OK. If you have verified the lines are cleared and you have an exact OEM replacement fuel pump I would take Wmburns advice and check the voltage at the IFS switch and start there first. Shop should have done that.
 

stangfreak98

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Oct 31, 2014
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If your talking about the IFS meaning ignition. The car has no problem starting. It idles rough and floats at about 500rpms but is reading extrememely high fuel pressure