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

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?

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

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?

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

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

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

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

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

What year is the truck?
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Re: fuel pump only pushing 15-18 PSI???? Please help...
 

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.

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

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!

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

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.

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

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.

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

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.

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

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.

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

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.

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

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?

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!

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

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.

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

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.

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

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?

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

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

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

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

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

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.

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

Is the fuel filter plugged?

When is the last time you replaced the fuel filter?

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

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.

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

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.

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

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.

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

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.

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

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.

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

Thanks ChevyTech, just trying to help out and understand more and more about these trucks. I actually can "usually" diagnose and figure out my own at some point, but I normally only work on the ones I drive, ocassionally a few others. It's great to have guys like you willing to share the knowledge and help us out, maybe I should leave the diagnosis to y'all also.

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

Please help out anywhere you can. Sorry if I sounded like I was scolding you. I did not mean to sound that way. I just meant to say you were talking TBI and he has a central port injected Vortec.

The TBI is the easiest to help with and the Vortec mess he owns is one of the worst systems to diagnose the GM ever built. I dislike this system so much that I have avoided ever owning any central port injected vehicle.

I only have a little free time that I can spend at the site right now, and any help anyone else can give is great with me.

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

HaHa at myself, I didn't even realize there was that much difference between the two. Thanks for the info, and apologies to OP. :D

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

TennesseeZ, no apologies necessary. I appreciate all input. Thanks!

^^^ You got that right, what a pain in the ass. But the good news is.......wait for it......

IT RUNS!!

I really want to thank everyone who helped out, and will never turn down any advice or someone just suggesting things. Especially with a situation like this, because I was ready to give up and burn this thing down. So a big thanks to you all.

So now on to the resolution. When I got home from work, I made up a cable and grounded one wire with the fuel pump ground, stuck the hot into the fuel pump 12v connector on the sending unit plug, and powered it with 12 volts from a jumper box. Presto, the pump fired right up, nice and loud, and 58 psi on the fuel rail. I knew then for sure it was wiring, but I really thought it was going to be the gray hot wire to the pump. Started narrowing it down after multiple continuity tests at the distribution block/relay. That's when it was found. Shame on me, believe it or not, it ended up being a bad ground. But not in the usual spot one would think, so before I get scolded, let me explain. I had a bad ground strap under the engine. As probably everyone knows, this grounds the battery, chassis, frame together. It was so corroded it broke into little pieces just from touching it. I do take responsibility for not checking the ground the way I should have. Every time I checked the ground, which believe me was a lot, I was not checking to the battery. I was checking the wiring to the frame, which the fuel pump was grounded to, duh! I knew that ground was good there, but should have taken the extra time to run a strap from the battery to my fuel pump ground and do a continuity test, would have saved me a lot of trouble. The frame ended being isolated from the ground from the battery. I know bad grounds cause some crazy things to happen, but this was insane.

I can't take all the credit though. My neighbor came over and as we were testing he thought of the chassis ground, not sure how long it would have taken me to figure that out. Really helps having another mind thinking of different things, which is what makes these forums so great. I was ready to just splice in under the distribution block and ground to the chassis, because that's one way I got it working. But in the end, it is fixed correctly, all new grounds from the battery to the chassis/frame done the right way.

Thanks to everyone who chimed in and threw out ideas, much appreciated. I hope documenting this situation helps people out in the future trouble shoot fuel pump problems, or any electrical problems for that matter. Just remember, always check your grounds, all of them!

Thanks again, Steve

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

Oh yeah, still got a check engine light, but I will address that after some time off from working on this thing. haha!

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

Glad to hear that you got it fixed, and thanks for sharing what the problem was.

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

Yeah I was getting pretty frustrated that I couldn't get it to work. But all is good now.

I try to document on the boards a resolution anytime I can. I know how much I rely on these boards, so anything I can do to help someone else out, I will. I am glad I had as much response on this that there was and hopefully, it will help someone else out in the future with similar types of issues. I know what the first thing is I will check next time. I am going to reground the pump off the frame and run it to a spot which is more accessible, or just double up the ground just in case.

I am actually going to throw the old pump back in tonight just to see if it will work. I just can't understand why when I beat on the bottom of the tank it started up originally.

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

Great you found it, and once again it turns out to be something basic that how many? poeople overlooked? I know a ground never even crossed my mind, I was off on a totally different goosechase, but it's great you found it, and also on the documentation here, no telling how many peeps it will help in the future. That's something I notice, and try to pass along, is going back to the basics, and K.I.S.S., seems it's so easy to focus on that one big part you're sure it's got to be, ya change out three of them, then remember to check the fuse or find something else equally simple. I'm glad you finally got it sfran, now get out there and get busy on that CEL! :D

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

I totally agree with overlooking the basics and the K.I.S.S statement.:lol: But at least I found out from the hundred tests I performed that my frame and fuel pump ground wire are grounded together very, very well.....lol

That's usually how it is with me though. Dig in way to deep, because there's no way it could be something that easy, not with my luck. Oh well, at least I got the frame and undercarriage undercoated real good while the bed is off.

Still going to check the old pump before I put the bed back on.

And just for future reference, the bed does not have to be on the frame or the wiring plugged in for the fuel pump to work like someone had mentioned to me.

Thanks again, Steve