1998 K3500 5.7 gasoline truck cranks, but will not start. Found low fuel pressure @ about 35 PSI. Replaced fuel pump assembly in the gasoline tank, started truck drove it home, but next day it wouldn't start, I measured fuel pressure, and now have 60 PSI fuel pressure, but engine will start, only if I pour in a little gasoline. This must be an electrical perhaps missing an input signal to the computer, since some common signal enables the fuel injectors to fire. My assumption is the fuel injectors are not turning on. I do not have a schematic diagram of the computer wiring, so do not know what to check next.

Sounds like a fuel starvation issue. You may want to start with something simple like changing the fuel filter, and move on from there. After replacing the fuel filter, check fuel system pressure to ensure it's in the right range (around 47 PSI for most fuel injected gasoline (petrol) engines). If the fuel pressure is low, it may be the pre-filter on the fuel pump, or the fuel pump itself.

Make sure you get a fuel pressure reading to know if you need to replace the fuel pump assembly...it should have 43 psi reading. If it is bad the fuel filters need replaced too. A fuel pump should never be ran lower that a quater of a tank because it kills the life of the pump, it acts as the pumps cooling system. Check for spark and see if there are any engine codes like a bad crank signal. Good luck

Check fuel system pressure if the pump has a bad check ball or the fuel pressure regulator is bad, pressure is hard to pressurize in the lines. Test dead head pressure then operating pressure the a fuel pressure leakdown test. That way you know what actually needs replaced so you don't guess. When you first pressurize the fuel system it should have 30 to 40 psi if it falls fast before you initialize starting the vehicle the check ball in the fuel pump is bad so the whole assembly and fuel filter will need replaced. If you start the vehicle and pressure does not regulate at 30 or 40 and you accelerate and the pressure does not max out it is a fuel pressure regulator. It might be in the fuel rail or in the fuel pump assembly in the tank.

I know you put a new fuel pump on but you can get a bad one, test first = 30-45 PSI (Pounds per Square Inch)Attach fuel pressure gauge test valve as to releasethe pressure in the fuel system, then place the openhose end into a suitable gasoline container. Nowproceed to bleed down the pressurized fuel system.Close the test gauge valve and place the ignitionin the on position, utilize a scan tool to energizethe fuel pump.This will bring the system to a stabilized fuelpressure that can be visually verified on the gauge.

It sounds fuel related, so lets start here.

the first thing I would do is check fuel pressure. The pumps on these vehicles do not like to be run without fuel in the tank since its the fuel that helps keep the pump cool. Running low or out of fuel will cause the pump to overheat.

If fuel pressure is less than 35-40 psi than the pump is bad.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Check your fuel pump pressure, it is probably down in the 50 lb range and it requires a bit above that to start. Once it starts it will run on that pressure. This is a chronic problem with these, my 1998 2500 was a GMC but it's the same vehicle. I put 385,000 miles on it with no engine or transmission trouble but I was on fuel pump number six when I sold it.

may have a bad inj . doese it blow black smoke when irt starts.??

30psi seems pretty low, should be around 45 or so. It sounds like your fuel pump is done again. What did you replace the pump with, OEM or a parts store part?

Don't worry, there is only about 100 things that can keep the EFI system from working properly. Try this 1st: My '91 4runner wont run properly, or at all, when the Air cleaner / intake assembly is not hooked up / installed, it needs the MAS air flow sensor to measure the incomming air or it will not run. Then, Are you getting positive pressure of fuel to to the engine from the tank yet or is the pump just running? Thread a pressure gauge to the fuel rail on the engine & check pressure with key on / engine off (should be between 35-45 psi) Check / replace the fuel filter & try disconnecting the fuel & return lines lines @ both the tank & Throttle body & blowing out with 25-50 psi air to ensure the line is clear. You may need to vacuume test the fuel pressure regulator on the fuel rail. If you are still having problems, it can be the injector(s), the fuel pump relay or any of the sensors in the EFI system including the VSV sensor, which measures the fuel pressure & relays the info to the ECM (computer). To check & test all of the sensors it will be worth the $20-30 Haynes or Chilton Manual. start with the easy stuff though, then work up to the sensors & electronics

Same problem with my 85 F-150 5.0 EFI---turned out that the fuel pump relay under the hood was teriffically corroded on the inside (I opened up the relay to look inside). When I checked the voltage at the fuel pump it would show 3.5-6 volts instead of 12 volts.

I was thinking fuel blockage or perhaps failure of the in-tank pumps. Sometimes the truck would start and run, othertimes it would neither start nor run. Typically, if running it would stay running until turned off (where the relay had to break).

Changing the green fuel pump relay fixed the problem--12 volts now at the pump and no reoccurance.

Good luck--let me know if this fixed your F-150.

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