98 5.7 fuel pump intermittent no prime / dies while driving

[Schurkey]

So if it only has the one wire, it means that the truck should cut off with the relay removed because the oil pressure switch will be doing nothing to counteract that, correct?

NO.

SOME vehicles have two oil pressure sensing devices--one sending unit to drive the oil pressure dash gauge (one wire) and one switch to bypass the fuel pump relay (two wires.)

SOME vehicles--tending to be newer like your '98--have a single, three-wire combination switch/sending unit that does both things.

If you're looking at a one-wire oil pressure sending unit, you're looking at the WRONG ITEM. The 3-wire combination sending unit/fuel pump relay bypass; or the 2-wire fuel pump relay bypass switch tends to be near the distributor, while the one-wire sending unit is down by the oil filter.

 

[Schurkey]

crank position sensor

can you elaborate some?

If the computer doesn't receive an RPM signal from the crank sensor, it'll disable the relay.

Potentially a faulty sensor, or faulty wiring between sensor and ECM, with a wild, outside chance of a faulty ECM.

 

[Pollo Dustino]

I suggest you do a fuel pressure check. If you get full fuel pressure at initial prime but it quickly drops off then you have a "leak" of sorts in the fuel system, and it is likely leaking back to the tank.

My 1998 K2500 is having a slightly similar problem with losing fuel pressure prime. If I cycle the key a couple times it will start, but sometimes it has a crank no start. Also threw a P0171 "Bank 1 Too Lean" code. When it starts it runs okay the majority of the time, but sometimes does sputter. Long term fuel trim is at +20, and short term is at +9, definitely indicating a too lean condition.

My very mechanically inclined friend suggested the fuel pressure regulator, which is located on the fuel rail. If it's failing it may be dumping fuel back to the tank at too high a rate, which can cause loss of prime and rough running concerns. I am inclined to agree. The fuel pump is new with only 400 miles on it, and I do not have any additional codes such as crank sensor or anything like that. I also have poor fuel economy, which would make sense if the computer was trying to add as much fuel as it could. A failing regulator would be a likely cause.

 

[Keeper]

can you elaborate some?

sorry, I was running out the door earlier. I actually didn't remember I hit the Post Reply button. Your descriptions and the lengthy timeline reminds of a failing CPS. It seems to be one of those sensor's whose failing deathspiral comes and goes like this over a longer period of time. Fuel issues would manifest on a much more frequent ongoing basis imo. The non-priming fuel pump was a memorable symptom when I dealt with a failing CPS. But it should throw a code, so.... it should be an easy check.

 

[TheTruckOfTheseus]

Have you connected to a scan tool or any codes?

yes, on my innova 5110. no codes whatsoever and emissions test reads green

 

[blackburb]

Potentially a faulty sensor, or faulty wiring between sensor and ECM, with a wild, outside chance of a faulty ECM.

I have had the same issues you described twice. Both had separate causes. This first time it was the wiring harness from the Crank position sensor shorting to the block. The scan tool did not have any codes. My mechanic noticed intermittent signals coming from the CPS during a test drive. Mechanic repaired the wiring harness and it has been good for over 300k miles. The second time was a faulty fuel sending unit. I had the fuel pump replaced and still had intermittent issues with fuel pressure. Replacing the sending unit fixed the issue.

 

[SableSlayer]

I suggest you do a fuel pressure check. If you get full fuel pressure at initial prime but it quickly drops off then you have a "leak" of sorts in the fuel system, and it is likely leaking back to the tank.

My 1998 K2500 is having a slightly similar problem with losing fuel pressure prime. If I cycle the key a couple times it will start, but sometimes it has a crank no start. Also threw a P0171 "Bank 1 Too Lean" code. When it starts it runs okay the majority of the time, but sometimes does sputter. Long term fuel trim is at +20, and short term is at +9, definitely indicating a too lean condition.

My very mechanically inclined friend suggested the fuel pressure regulator, which is located on the fuel rail. If it's failing it may be dumping fuel back to the tank at too high a rate, which can cause loss of prime and rough running concerns. I am inclined to agree. The fuel pump is new with only 400 miles on it, and I do not have any additional codes such as crank sensor or anything like that. I also have poor fuel economy, which would make sense if the computer was trying to add as much fuel as it could. A failing regulator would be a likely cause

I suggest you do a fuel pressure check. If you get full fuel pressure at initial prime but it quickly drops off then you have a "leak" of sorts in the fuel system, and it is likely leaking back to the tank.

My 1998 K2500 is having a slightly similar problem with losing fuel pressure prime. If I cycle the key a couple times it will start, but sometimes it has a crank no start. Also threw a P0171 "Bank 1 Too Lean" code. When it starts it runs okay the majority of the time, but sometimes does sputter. Long term fuel trim is at +20, and short term is at +9, definitely indicating a too lean condition.

My very mechanically inclined friend suggested the fuel pressure regulator, which is located on the fuel rail. If it's failing it may be dumping fuel back to the tank at too high a rate, which can cause loss of prime and rough running concerns. I am inclined to agree. The fuel pump is new with only 400 miles on it, and I do not have any additional codes such as crank sensor or anything like that. I also have poor fuel economy, which would make sense if the computer was trying to add as much fuel as it could. A failing regulator would be a likely cause.

I went through a lot of this on my 98 vortec. Have you put a fuel pressure gauge on yours? Have you done a smoke test to see if the 20 percent ltft is from a vacuum leak?

 

[SableSlayer]

yes, on my innova 5110. no codes whatsoever and emissions test reads green

The crankshaft sensor is super easy to remove. It's just behind the harmonic balancer between it and the oil pan. Pull it and see if there's any scrapes on the metal of the sensor if so it needs replaced and you'll need to make sure you replace it with one that comes with the metal shims. You can also inspect the harness for it follow the wire up and see if it's been laying on the exhaust manifold if so it could be burnt and needing a new harness.

 

[Pollo Dustino]

I have a hunch the fuel filter has never been replaced. I bought the truck surplus from my job and I see no records at all for a fuel filter. I'm going to replace it first and see what happens.

 

[SableSlayer]

I have a hunch the fuel filter has never been replaced. I bought the truck surplus from my job and I see no records at all for a fuel filter. I'm going to replace it first and see what happens.

Probably a good idea to change that regardless then. With the vacuum leak only showing on bank one and not both it's probably something just on that side. Pull your pcv valve while it's idling to see if it's stuck open. I had that happen on mine with a brand new pcv valve. The valve is supposed to close at idle. I also had in internal leak on the intake manifold gaskets which are prone to fail.