i have a 91 f150 xlt with a 351w lots of miles. after sitting for the night i find it hard to start. checked fuel pressure and got

30-32psi while running at idle (steady)
20-24psi at full throttle (steady)
28-30psi with vac line removed from fpr (gauge flutters)


any ideas is the fpr bad or something else

seems like fuel pumps are doing there job

when starting after being cold it tends to take a few trys to get it to kick over on its own then it sputters to a decent idle is this related to fpr or maybe electrical

 

Not an expert but I would have thought the fuel pressure should be around 40psi with vacuum removed. See what pressure you get with vacuum pipe removed and the return line from the Fuel Pressure Regulator pinched tight. If still low I would have thought the pump is weak.
Hopefully some of the experts will give an opinion soon.

 

That fuel pressure is not right.
First change the fuel filter.
Recheck the fuel pressure and if still low at full throttle and the vacuum line removed you need to check your Fuel Delivery Modules (FDM).
Do you have two tanks? If so is the fuel pressure the same on the other tank?
Does one tank fill while running on the other tank?

Fuel pressure should be about 40 PSI at WOT.
And near 45 PSI with the vacuum line off the FPR.
Did you see any fuel when you took the vacuum line off the FPR nipple? If so the FPR is bad also.
30-32 PSI is OK at idle.

I would never do this unless you like to replace fuel lines.

 

i didnt see any fuel from fpr when i took off vacuum line and pressure dropped to 28-30
i do have two tanks one is no good pump is bad ive been using front tank with no other problems ill change the fuel filter tonight and let you know results tommorrow thx

 

Is the tank you are not using now full but was empty to start with?
When you take off the vacuum line the pressure should go up not down.

 

Ill check the back tank tonight. As for the fpr. It doesn't really move when I take the vac line off. Just starts making the gauge flutter

 

Sounds like lack of fuel to it.

 

i know mine dose that because one of my pumps dosent hold enogh presure to keep some fuel in the lines and to start it you have to prime it 2-4 times for it to start on the first pop but i can live with mine until it gose out, i think its the pump mounted on the frame under the cab because it whines really bad sometimes but oh well

 

i checked and the tank i dont use now has 1/4 tank in it

i changed fuel filter reading
35psi at idle
when i remove vac line on fpr no change
wot pressure drops 5-15 psi

 

Either the running pump is bad or the non running pump has and open valve or the FPR is not closing.
This is why if you take it to Ford they will replace both FDM and the FPR. That way they know the problem is solved. $$$$

With engine at idle and the gauge on it shut off the engine and see how much and fast the gauge drops. It should not drop more than 5 PSI in an hour. Mine will drop 5 PSI in two hours.
If the pressure drops fast the the FPR is off its seat or you have a valve open. I would start by changing the FPR.

If the pressure holds you more than likely have a bad pump in the running tank.

 

i replaced the fpr still having a problem starting next step is fdm? which is where? or a fuel pump?

 

The FDM is in each fuel tank. The fuel pumps are inside the FDM.

FDM (Fuel Delivery Module/Assembly):








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