Hey everyone I have some problems. Ive been chasing fuel problems around for quite awhile now. symptoms were slightly low pressure at the rail hard start, poor running while warm, and pressure leak back. First I replaced the regulator on the rail and bench tested the injectors. no stuck injectors btw. Next I went ahead and replaced the temp sending unit since it was running poor while hot. the fuel filter had just been replaced. now at this point 2 weeks later I drove it parked and it wont start. took the manual tank selector off and well it all sorts of not right. bypassed it still not right. sooo now I took the lines off before the high pressure pump. loads of fuel from the in tank pump ok I figured since the high pressure pump was acting like a pinched line I replaced it...im not getting fuel on the engine side of the high pressure pump but I can feel and hear the new one working whats going on??? weak in tank pump? what psi should a low pressure pump have by its self??? next im checking for low voltage to the in tank pumps they appear to both work fine

 

You didn't mention the year truck, but normally fuel testing on these trucks can be done by adding a test jumper at the diagnostic link under the hood.


Then test your particular part in a systematic way.


The test for adequate fuel flow from the low-pressure in-tank pumps, as that seems what you have, is the amount of fuel dispensed over a 30 sec interval.


If you have satisfactory fuel flow into the high pressure pump, for sure, and it's running, and it's new, it's possible it's hooked up in reverse polarity.


Put everything back regarding your switching etc. to factory configuration, and research your high pressure pump install.

 

1987 F150 302 dual tanks. and the polarity thing never occurred to me mainly because of the lack of instructions that came with it lol I will switch the wires and measure the fuel output as well. It says in the Haynes manual how much. But is it possible that the amount of fuel produced is correct but the in tank pump is still weak as in not enough pressure when resistance is put on the flow?

 

and for future reference for everyone who comes across the thread later for help the electric low volume fuel pump for early efi models should produce 6 0z minimum in 5 seconds

 

Turns out the parts store gave me a faulty pump.

 

Unfortunately that is getting more common. You are the 3rd person today here in the forum that has received junk parts right out of the box.