Fuel Pressure Issues

[DMac94]

Hey guys, first time poster. I have gotten a lot of great information off of the site over the years as I've had my truck for about 17 years now, so I figured I would come here with questions.

A little backstory: I have a 1994 C1500 with about 130k miles with a 350 and auto trans. The truck is pretty much bone stock, the wiring hasn't been molested, and the truck has no rust as it has never seen salt (I mention this since it may be relevant to diagnosis since I have read that some folks have fuel pump issues related to corrosion).

On to the issue: OCCASIONALLY when I hit the throttle from a stop, the truck will cough or hesitate for a second and then recover. It doesn't die, but it sputters long enough for me to be weary of pulling out onto a busy road where I need to accelerate quickly. It only does this if the truck is under load and is even more likely to do so if I am starting off on a steep uphill. I performed a fuel pressure test and see a very steady 11-13 psi (TBI truck) at idle and when revving the truck up in my driveway.
I then went for a ride holding the pressure gauge out the window (I teed in by the fuel filter and my gauge hose was just long enough to reach to the driver window). The fuel pressure is still very steady at 11-13 psi under normal driving and hard acceleration.
HOWEVER, when I can get the truck to cough/sputter when taking off from stop, the fuel pressure drops erratically, sometimes even going to 0 psi. I am baffled by the situation since the truck runs great otherwise.

Of note, I did replace the fuel pump about 8 months ago with a Delphi when the truck lost fuel pressure (by some miracle, this happened after a two hour trip right as I pulled into my yard). When I removed the fuel pump, it turned out that the old rubber hose between the pump and sending unit in the tank had split, but I replaced the pump and sock screen anyways since they were 30 years old. *I also replaced the fuel filter* The tank was very clean inside. The truck ran fine for maybe 6 months, and this intermittent hesitation/sputter issue started about 2 months ago.

Any ideas?

The fuel pump doesn't know how much load the truck is seeing and should be running consistently, so I wonder if somehow the truck is cutting power to the pump under only this very specific circumstance for some reason. I know the fuel pump primes for only a couple of seconds after turning the key on without starting the truck, but does the fuel pump keep running if the truck stalls out or thinks it is dying?

Something else I speculated was that maybe, somehow, the fuel pressure regulation in the TBI assembly is allowing a ton of fuel to bypass to the tank return, but only under this specific circumstance, but this seems less likely since the truck runs great at all other times.

I feel like my next course of action is to go to the local junkyard, and grab the two connectors where the sending unit plugs into the truck harness, and rig up a little harness I can plug up inline to check pump feed voltage while I'm driving. Maybe even set up a battery in the bed running to the pump so that I know the pump is 100% getting a steady power supply and see if the problem persists.

Anyways, any insight or suggestions are welcome, thanks.

 

[PlayingWithTBI]

The truck ran fine for maybe 6 months, and this intermittent hesitation/sputter issue started about 2 months ago.

My 1st guess would be, even though it's new, your pump or connections inside the tank, are failing. A bad "new" pump is not unheard of.

I wonder if somehow the truck is cutting power to the pump under only this very specific circumstance for some reason

That's unlikely but, everything's possible. The oil pressure switch supplies power to the pump as well as the PCM activates the Fuel Pump. IF you are losing power to the pump, it's somewhere between them and the pump itself, i.e. your sending unit.

I know the fuel pump primes for only a couple of seconds after turning the key on without starting the truc

That's normal, it should prime for 2 seconds with KOEO. This also tells us that your PCM to Fuel Pump Relay are working as they should.

does the fuel pump keep running if the truck stalls out or thinks it is dying?

only if you lose oil pressure and PCM to pump circuit. If you lose Distributor Reference Pulses, your injectors will stop firing as well as your ignition.

maybe, somehow, the fuel pressure regulation in the TBI assembly is allowing a ton of fuel to bypass to the tank return, but only under this specific circumstance, but this seems less likely since the truck runs great at all other times.

Yes, unlikely.

I feel like my next course of action is to go to the local junkyard, and grab the two connectors where the sending unit plugs into the truck harness, and rig up a little harness I can plug up inline to check pump feed voltage while I'm driving

You can but, IMO it's unlikely the issue.

Maybe even set up a battery in the bed running to the pump so that I know the pump is 100% getting a steady power supply and see if the problem persists.

Maybe temporarily. Otherwise that's not a good idea since, in case of an accident, your fuel pump will keep on pumping maybe adding fuel to the fire.

 

[Drunkcanuk]

I had the "hose in the tank" as a problem as well as the solution to the same issue.
Personally I would change the filter again. Highly possible that some gunk got in there again. And it's quick, cheap and easy to do. And then move on from there. And ya, new parts being junk nowadays is not unheard of. I went with a Bosch when I did mine, no issues with it. But that was a long time ago and who knows how they are now.

 

[DMac94]

My 1st guess would be, even though it's new, your pump or connections inside the tank, are failing. A bad "new" pump is not unheard of.


That's unlikely but, everything's possible. The oil pressure switch supplies power to the pump as well as the PCM activates the Fuel Pump. IF you are losing power to the pump, it's somewhere between them and the pump itself, i.e. your sending unit.


That's normal, it should prime for 2 seconds with KOEO. This also tells us that your PCM to Fuel Pump Relay are working as they should.


only if you lose oil pressure and PCM to pump circuit. If you lose Distributor Reference Pulses, your injectors will stop firing as well as your ignition.


Yes, unlikely.


You can but, IMO it's unlikely the issue.


Maybe temporarily. Otherwise that's not a good idea since, in case of an accident, your fuel pump will keep on pumping maybe adding fuel to the fire.

PlayingWithTBI,

Just to clarify when you say "If you lose Distributor Reference Pulses, your injectors will stop firing as well as your ignition," are you saying this will kill power to the pump, or will power to the pump only be shut off as a result of the oil pressure switch sensing low pressure or the PCM turning the pump relay off?
This makes me wonder if my issue is something ignition related, and as a result of the truck misfiring/hesitating, the RPMs are getting low enough that oil pressure is falling enough to momentarily cut power to the fuel pump (or the PCM is briefly sensing the truck shutting down and kills pump power briefly while it is sputtering). Maybe these suggestions are not possible since a brief cut in power to the fuel pump would exacerbate the issue and definitely kill the engine. I'm just spit-balling here trying to determine if I can rule out ignition issues.

I agree that a new pump could be failing already or that the connections in the tank could have bad contacts. It just seems odd that the issue would only arise when hitting the gas from a stop. I would hate to drop the tank again, but if more troubleshooting points to the pump itself, so be it.

Yes, I agree having the fuel pump running no matter what would be a bad idea. Placing a battery in the bed feeding the fuel pump would be strictly for diagnostics. The truck is a secondary vehicle, so I'm not trying to cobble something together just to have a vehicle to drive.

Thank you for the detailed insight. This gives me a few other factors to consider.

 

[DMac94]

I had the "hose in the tank" as a problem as well as the solution to the same issue.
Personally I would change the filter again. Highly possible that some gunk got in there again. And it's quick, cheap and easy to do. And then move on from there. And ya, new parts being junk nowadays is not unheard of. I went with a Bosch when I did mine, no issues with it. But that was a long time ago and who knows how they are now.

Drunkcanuk,

I will probably try a new filter out since this would be the easiest thing. I figure I could check fuel pressure between the filter and pump as well. I've been looking at pressure just downstream of the filter.

Thanks

 

[PlayingWithTBI]

are you saying this will kill power to the pump, or will power to the pump only be shut off as a result of the oil pressure switch sensing low pressure or the PCM turning the pump relay off?

The pump will stay on even with no DRPs. One time, when I was priming my oil pump with a drill, the fuel pump came on, even with no distributor and the ignition key off.

 

[PlayingWithTBI]

Here's a schematic for my 88 but, yours s/b about the same. Note; the power feeding that circuit is "Hot at all times". The power through the Oil Pressure Switch is too when closed.

You must be registered for see images attach

 

[Schurkey]

Low-to-no fuel pressure on sudden acceleration makes me think that pump volume is a problem. That is, the pump can supply low-to-moderate volume, but high volume causes pressure loss.

This can be a pump problem, or an electrical issue where the pump isn't getting enough voltage/amperage. The wiring on the supply side could be compromised--high resistance on one or more wires/connectors--or it could just as easily be on the ground side (or both.)

I think it's an excellent idea to get a "temporary" jumper harness to plug into the vehicle harness, so that you can do electrical diagnosis without damage to the vehicle wires. I did. I built my own "temporary testing" harness for my '88 K1500 using Weatherpack connectors, but Treasure Yard connectors would work as well.

 

[Road Trip]

PlayingWithTBI,

Just to clarify when you say "If you lose Distributor Reference Pulses, your injectors will stop firing as well as your ignition," are you saying this will kill power to the pump, or will power to the pump only be shut off as a result of the oil pressure switch sensing low pressure or the PCM turning the pump relay off?
This makes me wonder if my issue is something ignition related, and as a result of the truck misfiring/hesitating, the RPMs are getting low enough that oil pressure is falling enough to momentarily cut power to the fuel pump (or the PCM is briefly sensing the truck shutting down and kills pump power briefly while it is sputtering). Maybe these suggestions are not possible since a brief cut in power to the fuel pump would exacerbate the issue and definitely kill the engine. I'm just spit-balling here trying to determine if I can rule out ignition issues.

I agree that a new pump could be failing already or that the connections in the tank could have bad contacts. It just seems odd that the issue would only arise when hitting the gas from a stop. I would hate to drop the tank again, but if more troubleshooting points to the pump itself, so be it.

Yes, I agree having the fuel pump running no matter what would be a bad idea. Placing a battery in the bed feeding the fuel pump would be strictly for diagnostics. The truck is a secondary vehicle, so I'm not trying to cobble something together just to have a vehicle to drive.

Thank you for the detailed insight. This gives me a few other factors to consider.

Greetings DMac94,

I'm in agreement with both PlayingWithTBI and Schurkey. The only thing I'd like to add is
the fuel pump wiring diagram taken from the '94 FSM covering the Driveability, Emissions,
and Electrical Diagnosis. Note: The circuit topology looks to be the same, but it looks
like there's been a color coding change and the technical writers redrew the circuit as well.
(No doubt based upon feedback from the dealer technicians.)

You must be registered for see images attach

To answer your question, as PlayingWithTBI said the loss of reference pulses will shut off the injectors *and*
the Fuel Pump Relay. However, as long as the crankshaft is turning the oil pump will be putting out pressure,
so this leg will still be powering the fuel pump. But since the fuel injectors have stopped metering fuel
(assuming an automatic) the crankshaft will stop turning within a second or so. Therefore, oil pressure drops,
and now the other power feed to the fuel pump is removed also.

EDIT:
Note: The only reason I bring this up is because elsewhere on the interwebs this circuit is incorrectly described,
and they talk about how a loss of oil pressure will always turn off the fuel pump. This would be true IF the fuel pump
relay was wired in series with the engine oil pressure switch, but it's NOT. If you study the wiring diagram
you see that it's wired like an OR gate, one side -or- the other can/do feed the fuel pump. Obviously the designers
were more worried about a flaky fuel pump relay putting drivers on the side of the road due to relay failure.

But by the same token, IF the engine isn't running they did NOT want the fuel system to continue to
meter fuel and have it possibly accumulate outside a hot motor. So no reference pulses = no injector
driving AND also no fuel pump relay power, so crank stops, AND oil pressure drops to zero, and this
pressure switch opens also.

In English, there is actually redundancy built into shutting off the fuel pump -- we just have to
take all the rotating bits into account when pondering this circuit's dynamic behavior.

****

As for troubleshooting this circuit, we have to decide is the fuel pump a good one and is a victim
of flaky power? Or is the power solid and the fuel pump is the flaky perpetrator of the malfunction?

IF we think the power is the problem, assuming both oil pressure switch and fuel pump relay
are good, then the only parts of the electrical circuit that can affect BOTH switches simultaneously
can cause this problem. Looking at the circuit, this would include:

* The power input to the 2 switches in parallel. In other words, do we have a flaky fusible link?
Or is it the feed from the battery -to- the fusible link?

* Or on the far side of the fuel pump, there is only 1 ground wire from pump motor to frame.
No doubt that you have already checked/cleaned this frame ground back to new condition?

NOTE: IF this proves to be a tough intermittent, then you could use a couple of cheap LEDs
in the cab and wire one to the output of the fuel pump relay as well and the other to the output
of the oil pressure switch. I would expect with Key On the fuel pump relay LED will come on for
2 seconds and go out.

You then start cranking the engine, and the fuel pump relay LED comes right back on,
(thanks to Reference pulses coming into the ECM) engine fires, and shortly afterwards the
oil pressure switch LED comes on also. And they should both stay on steady for the duration
of the trip.

Of course you could add a third LED, connected right at the fuel pump connector, to monitor
the wiring between these switches and the power feed at the pump itself. IF all 3 stay on
solid and you lose fuel pressure (assuming a perfect ground on the far side of the pump) then
we now know that the pump itself is flaky (perpetrator) since power never flickered in this scenario.

For what it's worth, I've only pushed LEDs into a piece of cardboard (with hand-lettered
notes & arrows to each one) and taped this to the dash only a handful of times in my life,
but I have to admit that this removed all the guesswork about if I had a flaky component
or a flaky power feed to said component. Normally we'd only resort to this on a 'unfixable'
vehicle that everyone else had given up on. (Not recommending this in your case, but I
just wanted you to know that there's always the next level of troubleshooting available,
if needed. :0)

But a careful visual inspection/cleaning of all your connections should be more than enough
to help you figure this out. By studying this circuit we now know where the 'single point
of failures' are, and where they aren't.

Apologies for the length, but just wanted to give you the '94 FSM diagram for your viewing
pleasure. (And if you haven't already done so, you really want to download the '94 FSMs
for your vehicle: '88+ FSM links)

Now to figure out if the power or the newish fuel pump is the perpetrator of your
intermittent fuel pressure loss.

Best of luck with the hunt for the root cause of this head scratcher --

 

[DMac94]

I had some time last night to tinker with the truck after getting a better understanding of the fuel pump circuitry thanks to the wiring diagrams supplied by PlayingWithTBI and RoadTrip.

I started by replacing my battery because it was on its way out. It has been weak for a while, and I knew it wouldn't make it through the winter anyways.

I disconnected my oil pressure switch with the truck running, and it stayed running, so I know the FPR is getting signal from the PCM.

I disconnected the FPR connector with the OPS connected while the truck was running, and it died.
This tells me the OPS is bad even though I still get responsive oil pressure readings on the dash.
At this point, I figured since the fuel pump circuit does have the OPS circuit as a backup, maybe the PCM is sending a bad signal to the FPR under certain circumstances and the OPS circuit isn't carrying the fuel pump through periods when I suspected the relay may have been cutting power.
I proceeded to disconnect the FPR connecter and jumper across from orange to gray opposed to orange to pink/black (since my truck is a '94).
At this point, the fuel pump has direct voltage feed from the battery.
The truck fired right up but still had the hesitation/sputter from a standstill. I took a ride with the fuel pressure gauge and still saw brief low readings during episodes where I can get the truck to cough/misfire.

I also moved the location of my fuel pressure gauge from between the engine and fuel filter to between the fuel pump and fuel filter.
No difference in pressure readings, so the fuel filter is probably fine.

It's just so bizarre to me that the fuel pressure is so steady at all times unless I am taking off from a stop or worse, taking off from a stop going uphill. It almost makes me think it more of a mechanical thing than electrical. Like Schurkey said, maybe a volume issue opposed to pressure problem. I will add though that I can stomp on the gas while cruising, and the fuel pressure stays absolutely steady. I would think this would require more fuel than starting from a standstill, but maybe not?
The truck currently has 1/4 tank of fuel, I wouldn't think it would be sucking in air at any point. I haven't wanted to fill the truck up out of fear that I will have to drop the tank again, but I may do so anyways. I can always drain it into gas cans if I decide to drop the tank.

I think my next step will be to rig up a setup where I can quantify amperage/voltage right at the sending unit. It might be time to drop the tank after that.

While I was wrapping up for the evening, I noticed my power steering pressure hose was leaking pretty bad, so I ordered another hose and a fuel filter (just for the heck of it). I'll visit the truck later next week after my parts arrive. I don't want to spray ATF all over my engine bay in the meantime.

I can try to post a super short video of the fuel pressure behavior if that is allowed.

Anyways, thanks for all the input guys.