Hey everybody, I am looking for some help with a fuel pressure issue. I have been on this forum for several years but never really posted. I have also read multiple threads about fuel system problems so I am sorry for yet another post. I have a 1991 F150 XLT Lariat ext cab 5.0 with about 141000 miles on it. I have had this truck since about 2000. In 2010 I started a mild engine rebuild on it but then joined the army so the truck then sat for roughly 5yrs. Over the last 2 years I have gotten back to being driveable but still sits for months at times and then I just take it for a spin. I have been having some trans issues lately which I am dealing with but the fuel problems are this. I have the dual tank setup. The truck cold starts fine, idles up to around 1200rpm for several minutes then settles down around 800rpm. Driving I noticed decreased response when stopped at red light, really have to mash the gas then if I were to stop, turn it off, and re-start it, it turns over fine but sputters and stalls. To get it back going you have to mash the gas while attempting to start several times. I took it took a mechanic several weeks ago, told him this issue, replaced fuel filter and said pressure was fine after that. Today I checked the fuel pressures. I have been having an issue with the rear tank overflowing for quite some time so I figured that needs a new fuel pump, don't really use that tank because of said issue. So when I started truck on front tank fuel pressure reads 56-58psi. According to Haynes this is high (supposed to be 30-45psi?). With engine running I pulled the vacuum to the FPR and PSI did not budge, still read 56-58psi. Re-attached FPR, still no change with engine still on. With engine still running I switched to the rear tank for S's and G's and pressure jumped to 80psi. Flipped back to front tank, turned engine off and immediately pressure fell steadily down to about 15-20psi. Of note, recently started getting codes 173 - HEGO (HO2S) Sensor fault, rich and code 179 - HEGO (HO2S) sensor fault unable to switch/rich during part throttle. I would assume that these codes are related to my pump and/or regulator issue.

I am assuming I most likely need that rear fuel pump and assembly (because of a bad FDM and check valve most likely? and possibly pump?) and I am assuming I may also need a new pressure regulator? Now what about the front tank pump? Are these the correct pressures? I would really like some further help/guidance on this issue before I start throwing parts and more $$$ at the issue. The truck has been fighting me tooth and nail to get her back to a daily driver. Thanks everyone!!

 

The fuel pressure at the rail should be the same. Doesn't matter which pump is turned on, and should be as the Haynes manual indicated. Sounds like you have it figured out. Yours sounds high. The pressure should rise when you remove the vacuum hose as that simulates WOT. Since yours doesn't change, you're probably right that you'll have to replace your regulator. Once you get consistent pressure within spec, you can address the other issues. But I would start with the fuel regulator. The fuel pumps have to be able to supply at least 30 to 45 PSI at all times, so it's normal that they're capable of more PSI without regulation.