fuel pump only pushing 15-18 PSI???? Please help...

[sfran]

Please help me sort this out. I need my truck this weekend!

So my truck has been acting funny lately and finally wouldn't start the other night. I crawled under and banged on the tank, and presto, the pump came on and it started up while still running not that great. So obviously I assume fuel pump. This is my 4th fuel pump I've done in the 10 years I've owned the truck, now after replacing it this time, the damn thing won't start. I have never had an issue after replacing one before but this one is driving me nuts.

I have checked everything I can think of. I have spark, 11.2 volts (with a weakish battery from cranking, now charging) at the connector on the pump in the tank, I have a good ground, I have tried 2 new pumps, and a new sending unit to compare voltage and pump operation, but can't drop the sending unit in the tank, because then I will have to buy it. I know the pump is kicking on, but it is very quiet. I put a fuel pressure sensor on the fuel rail and I can only get it up to 15-18 psi.

What am I missing, or where should I turn next? My Napa guy said it might be the ignition module. I think this is the one with the heat sink?

I don't want to just throw parts at it so any help would be appreciated.

Thanks

****Update*** just thought to put starter fluid in throttle body, well actually carb cleaner cause its all I have. And voila, it started. So I am assuming I am the unlucky bastard that gets two bad pumps on the same day? Has to be fuel right?

[sfran]

Also, I don't recall having to have the bed on the frame in order for the pump to work in any of the previous times I've done it. I am having some people tell me that the brake/parking lights and ground need to be hooked up due to it being part of the grounding system. I have the bed on a rolling cart next to the truck and have hooked up the taillights, and extended a ground to the ground wires. Still same issue
Any of this make sense?

[dragginmetal]

Originally Posted by sfran

I am having some people tell me that the brake/parking lights and ground need to be hooked up due to it being part of the grounding system.

On my '88, i dont have a bed, tail lights, or ground hooked up. Mine runs and drives fine. It might be different on a vortec tho.

[BigBlocksRule]

Make sure you have the right pump - sounds like yours is a TBI piece.

[sfran]

I will have to double check the part number. Thanks, didn't even think of that.

I was wondering if it could possibly be the pressure regulator? I am assuming that the pressure gauge sits on the pressure side line and not the return side correct? And if so, I wouldn't think the regulator would play a factor in the reading? I would think it reads the pressure directly from the pump before the regulator if it's on the pressure line.

Thanks

[93redsled]

What year is the truck?
Posted via Mobile Device

[sfran]

Originally Posted by 93redsled

What year is the truck?
Posted via Mobile Device

Sorry, the truck is a '96, Vortec 350.

I am really thinking it has to be the fuel pump. I pinched off the return line going back into the tank to rule out the pressure regulator. I wanted to see if my pressure jumped with ignition on. No change, still only 15 PSI. This should have ruled out a stuck open regulator. I also pulled the pressure gauge off and put it back on my car just to make sure it is working properly and it was.

Think I might go buy a Delco pump. Just hate grasping at straws with this situation and spending unnecessary money. Damn fuel pumps!

[bighaas79]

Delco or delphi pump is the only way to go. My experience with cheapies is exactly what you have now, replacing way to often. Sounds like pump, but you have to remember if the regulator or injectors are leaking, pinching off the return line, won't find those issues.

[sfran]

Originally Posted by bighaas79

Delco or delphi pump is the only way to go. My experience with cheapies is exactly what you have now, replacing way to often. Sounds like pump, but you have to remember if the regulator or injectors are leaking, pinching off the return line, won't find those issues.

I agree. If the injectors or regulator are leaking shouldn't the pressure drop off on the gauge? Smell fuel in the oil? The pressure seems to be holding over night at 15 psi.

I am going to leave the line clamped over night on the return line to verify it is not leaking past the regulator, will check again in the morning. I tried to start the truck while holding the pedal to the floor. I was thinking by doing this it would bypass a flooding situation and identify if leaky injectors were flooding the engine. I would think if the injectors were leaking bad enough to only produce the PSI I am getting, the gauge wouldn't hold for any length of time.

I am still scratching, but definitely still thinking pump, and it is just stupid to think that two pumps are bad out of the box. I also checked the part number from the box online and it does say it is for my truck. It is a NAPA part# P74167, Carter pump. Maybe they had a bad day and packaged these things in the wrong boxes, lol, I've seen crazier things before. I did read that the old TBI only took like 15 PSI of pressure, which got me thinking of that scenario.Thanks for the replies everyone. Keep the suggestions coming, even if it is just a stab in the dark. I am up to trying anything at this point and will get to the bottom of it.

[TennesseeZ]

Originally Posted by sfran

Please help me sort this out. I need my truck this weekend!

So my truck has been acting funny lately and finally wouldn't start the other night. I crawled under and banged on the tank, and presto, the pump came on and it started up while still running not that great. So obviously I assume fuel pump. This is my 4th fuel pump I've done in the 10 years I've owned the truck, now after replacing it this time, the damn thing won't start. I have never had an issue after replacing one before but this one is driving me nuts.

I have checked everything I can think of. I have spark, 11.2 volts (with a weakish battery from cranking, now charging) at the connector on the pump in the tank, I have a good ground, I have tried 2 new pumps, and a new sending unit to compare voltage and pump operation, but can't drop the sending unit in the tank, because then I will have to buy it. I know the pump is kicking on, but it is very quiet. I put a fuel pressure sensor on the fuel rail and I can only get it up to 15-18 psi.

What am I missing, or where should I turn next? My Napa guy said it might be the ignition module. I think this is the one with the heat sink?

I don't want to just throw parts at it so any help would be appreciated.

Thanks

****Update*** just thought to put starter fluid in throttle body, well actually carb cleaner cause its all I have. And voila, it started. So I am assuming I am the unlucky bastard that gets two bad pumps on the same day? Has to be fuel right?

Spray a little starting fluid in the intake and see if it will fire up and run a couple seconds.

EDIT: Just saw your post where you did that. Now search for ChevyTech's instructions on how to test the TPS. Check it. If it's good, pull the ignition module and get it tested at Auto Zonedout or Sadvance.

[sfran]

Originally Posted by TennesseeZ

Spray a little starting fluid in the intake and see if it will fire up and run a couple seconds.

EDIT: Just saw your post where you did that. Now search for ChevyTech's instructions on how to test the TPS. Check it. If it's good, pull the ignition module and get it tested at Auto Zonedout or Sadvance.

Not sure the TPS would have anything to do with the pressure would it? That's a good idea to get the ignition module checked. I wasn't aware they did that. Keep in mind that the truck ran with the old pump after beating on the tank, and has never run on the new pumps.

Just checked the pressure this morning. It has only dropped 5 psi overnight on the gauge.

[ChevyTech]

The low pressure is what you need to find the cause of.

Did you replace the hose in the tank that connects the pump to the sending unit?

It is common for the hose in the tank to leak.

I suggest only using OEM (Original equipment manufacturer) fuel pumps.
I have seen so many aftermarket fuel pumps fail during their warranty, or just after the warranty expires, that I strongly suggest you stay away from aftermarket pumps.

If you are shopping around, only get an AC Delco or Delphi pump.

[sfran]

Originally Posted by ChevyTech

The low pressure is what you need to find the cause of.

I agree and am trying to narrow down what might cause the low pressure by process of elimination. Right now I am convinced it is the pump. Is there anything that I have forgotten to check or am not thinking of?

Originally Posted by ChevyTech

Did you replace the hose in the tank that connects the pump to the sending unit?

It is common for the hose in the tank to leak.

I suggest only using OEM (Original equipment manufacturer) fuel pumps.
I have seen so many aftermarket fuel pumps fail during their warranty, or just after the warranty expires, that I strongly suggest you stay away from aftermarket pumps.

If you are shopping around, only get an AC Delco or Delphi pump.

I did replace the hose that connects to the sending unit with the new one that comes with the pump. The old one was all dry rotted/cracked. The only thing I didn't do is change the first new hose I got with the hose from the second pump. Maybe I will try that, maybe that hose is defective from the first pump.

I also didn't change the wiring in the tank with the wiring that comes with the pump. That all looks good, and see why that would cause low pressure unless voltage has something to do with it.

I am going to try and get to the store tomorrow and pick up an oem pump, didn't have time today.

Thanks!

[sfran]

I was thinking that if the line from the pump to the housing was bad, wouldn't it not hold pressure? I just changed out the hose anyways going from the pump to the housing with another brand new hose with No Change


I also changed out the wiring from the 4 prong connector on the sending unit to the 2 prong connector at the pump just to rule out the wiring to the pump, No Change.

15 PSI and holding strong though, lol!

One quick question. What voltage should I be seeing at the connector that plugs into the pump and is it possible that if the proper voltage isn't distributed to the pump that it won't provide full pressure? I think I mentioned I was getting 11.2 volts when I checked it the other day. The battery may have been a little weak.

[ChevyTech]

If it can only produce 15 PSI even with the return line blocked, my guess is they are selling you a pump for a TBI truck, and a crappy pump if it can only produce 15 PSI.

You should get full battery voltage to the fuel pump when testing.

11.2 volts is way low. Charge the battery up, or replace it if it won't charge up to around 12.6 volts with a good charger.

[sfran]

Originally Posted by ChevyTech

If it can only produce 15 PSI even with the return line blocked, my guess is they are selling you a pump for a TBI truck, and a crappy pump if it can only produce 15 PSI.

You should get full battery voltage to the fuel pump when testing.

11.2 volts is way low. Charge the battery up, or replace it if it won't charge up to around 12.6 volts with a good charger.

That's what I thought about the TBI pumps. I know those put out around those numbers. It seems the numbers are correct from what I could find, but maybe carter pumps had a really bad day and marked a bunch wrong, lol.

The battery is at 12.3 volts. I checked voltage at the truck end of the connector at the tank where the sending unit plugs into. 12.2 volts while priming on the green wire (left pin), 8.2 volts at the red wire (center pin) w/ key on and 7.3 while cranking, right pin (black wire) seems to be a ground, checks out OK. I didn't pull the pump back out to test at the connector there.

Should I get full voltage constant at this connector, or only while priming?

[ChevyTech]

The fuel pump should only get power for about two seconds when the key is turned to the on position, without the engine cranking.

[sfran]

Originally Posted by ChevyTech

The fuel pump should only get power for about two seconds when the key is turned to the on position, without the engine cranking.

This is what I see on the green wire, only voltage for a couple seconds while priming. I guess the true test is going to be from an OEM pump, hopefully tomorrow.

Thanks

[sfran]

So I got a Delphi pump today, and felt good all day about how well it was going to work. That is until I installed it and had absolutely no change. I am so pissed right now, that I am ready to just part this truck out.

I am thinking now that it has to be electrical some how. Some sort of voltage drop when the pump is under load? I have good voltage with the pump disconnected. I don't even know where to begin except for running new power lines to the pump.

[ChevyTech]

Is the fuel filter plugged?

When is the last time you replaced the fuel filter?

[sfran]

Originally Posted by ChevyTech

Is the fuel filter plugged?

When is the last time you replaced the fuel filter?

Just put a new one on this weekend. I even checked the flow direction thinking that maybe I installed it backwards. I also just installed a new fuel pump relay.

I just did a voltage test with the pump plugged in and under load from priming. It seemed as though the voltage dropped to around 4-5 volts while priming. I moved the distribution block under the hood around, and now the pump is not even priming, but I still have voltage with the pump unplugged.

[speedygonzales]

Originally Posted by sfran

Sorry, the truck is a '96, Vortec 350.

I am really thinking it has to be the fuel pump. I pinched off the return line going back into the tank to rule out the pressure regulator. I wanted to see if my pressure jumped with ignition on. No change, still only 15 PSI. This should have ruled out a stuck open regulator. I also pulled the pressure gauge off and put it back on my car just to make sure it is working properly and it was.

Smell your oil on the dipstick to see if it has gas on it. I think your FPR has a hole in it. If you want, prime the pump while you hold the throttle plate open and listen for fuel spray out the FPR. Your assumption that you ruled OUT the FPR is only correct if the diaphragm does NOT have a hole in it, just that it is sprung and won't regulate properly.

That would obviously account for why a number of fuel pumps did NOT fix the problem.

[sfran]

Originally Posted by speedygonzales

Smell your oil on the dipstick to see if it smells of gas. I think your FPR has a hole in it. If you want, prime the pump while you hold the throttle plate open and listen for fuel spray out the FPR.

I tried that, no smell of gas in the oil. And I can't hear any fuel spraying out. The pressure holds strong overnight, just only at 15psi. If I had a leak the pressure should not hold over night.

I believe it has to be in the electrical between the distribution block and the pump. I had a voltage drop under load after the truck/pump connector at the tank while priming the pump. Now I have moved the distribution block around looking for bad wires, and the pump won't even prime??????

I think my next step is I am going to run 12 volts from the front to the pump and splice it in to see what happens. I should mention that I did put a hose from the pressure line of the pump to a clear container and primed the pump to see what the flow looked like, and it was extremely low flow. Not quite a dribble, but not 60 - 80 psi for sure.

[TennesseeZ]

Originally Posted by sfran

Please help me sort this out. I need my truck this weekend!

So my truck has been acting funny lately and finally wouldn't start the other night. I crawled under and banged on the tank, and presto, the pump came on and it started up while still running not that great. So obviously I assume fuel pump. This is my 4th fuel pump I've done in the 10 years I've owned the truck, now after replacing it this time, the damn thing won't start. I have never had an issue after replacing one before but this one is driving me nuts.

I have checked everything I can think of. I have spark, 11.2 volts (with a weakish battery from cranking, now charging) at the connector on the pump in the tank, I have a good ground, I have tried 2 new pumps, and a new sending unit to compare voltage and pump operation, but can't drop the sending unit in the tank, because then I will have to buy it. I know the pump is kicking on, but it is very quiet. I put a fuel pressure sensor on the fuel rail and I can only get it up to 15-18 psi.

What am I missing, or where should I turn next? My Napa guy said it might be the ignition module. I think this is the one with the heat sink?

I don't want to just throw parts at it so any help would be appreciated.

Thanks

****Update*** just thought to put starter fluid in throttle body, well actually carb cleaner cause its all I have. And voila, it started. So I am assuming I am the unlucky bastard that gets two bad pumps on the same day? Has to be fuel right?

If it runs after being cranked with an accellerant, how would that lead you to conclude low fuel pressure was the problem? Would it not still have low pressure and therefore why would it run?

These TBI engines, I learned from ChevyTech and others, need to get above like 400 RPM before the ignition module starts sending the "run" mode to the ECM. I had the same experience several years ago. In my case starting fluid would crank it, and it would idle and run fine till you shut it down, then same problem again, till I would give it another dose of starting fluid. But altho the engine would operate fine, the transmission would not shift correctly, another of our ace techs suggested trying a TPS, which I did, it solved both the "no crank" and the shifting issue, and the truck has been doing fine for 3 or 4 years now. That is why I suggested the TPS, I know from experience that a malfunctioning TPS can cause issues you'd never expect. Not sure about your year model, but don't these trucks only require something like 9-13 PSI in the first place? Like ChevyTech said recently, hard to diagnose something you can't see, hear, feel, smell, just trying to help you out.

[ChevyTech]

TennesseeZ – he has a 1996 vortec, he stated in his 7th post.

If he only has 4 to 5 volts at the pump when it is running, that is a problem.

He has a wiring problem.
He should look closely at his connectors for melted plastic and black connectors.

The distribution block should be tested with a volt meter. Sometimes the connection feel tight, but have very poor electrical connection. If the problem got worse while working with the distribution block, then take it apart and clean the connections with a wire brush and spray contact cleaner.