Jump to content
  • 0

2008 Silverado 5.3 fuel pressure drop


Josh29

Question

Hello. New poster here. I have a 2008 Silverado extended cab 4x4 with a 5.3 j vin code.

 

I replaced a leaking fuel sending unit a few weeks back. Truck ran ok for a few days then quit. Would turn over but not start. I had no power coming out of the fuel control module above the spare tire. So I replaced it with a GM oem control module and had a dealership program it.

 

After that was programmed it felt real sluggish and slow. Started pinging bad under heavy loads. So I checked the fuel pressure. Should be between 55-62psi. With the key on engine off it jumps right to 62-65psi. Start the truck and it maintains that pressure for about 10-20seconds then drops straight down to 43psi. It's not a slow drop, it's immediate. So I assumed I had a faulty fuel regulator on the new sending unit, so I replaced the whole sending unit with another brand new one (ACDelco). Didn't change anything. So I replaced the inline pressure switch even though it read the same on my scanner that a mechanical gauge read on the fuel rail. No difference.

 

I took it back to the dealer that programmed the new FPCM and asked them to diagnose the pressure drop. Meaning I wanted them to see if the new FPCM was faulty or possibly the program job. They charged me $220 for diagnostics and said I had clogged cats and said it could be causing the computer to force the pressure down. So I went home and replaced the Y pipe and cats which were clogged. Fuel pressure still drops.

 

Anyone have any Idea how this can be happening? I can command the fuel pump on with my scanner and it will jump right back up to 62psi no problem. So why is the computer commanding the pressure down like this?

Link to comment
Share on other sites

Recommended Posts

  • 0

Where was the fuel sending unit leaking from? Top seal on the tank? Did this assembly include the fuel pump itself? (just making sure we're on the same page, because when I hear "sending unit", I think of the tank float assembly for the gauge, only). I think this is where this whole thing spiraled out of control ...

 

Highly unlikely the FPCM failed right after you replaced the unit. I'm thinking that the problem was created during the swap - maybe a wire got pinched, or a corroded wire in the FPCM connector broke when it was moved, or any number of things. More than likely it was a wiring problem there. The fuel drop is probably from a faulty regulator, which should be part of the pump / sender assembly on that. It's a single line out to the engine - the regulator is in the tank, and dumps excess fuel back inside - no more return line. You probably got a defective unit. It's been happening alot in auto repair circles - the joke there is, "New means: Never Ever Worked". The dealership took advantage of this, and pocketed your money.

 

The ECM will NEVER command a fuel pump to run at a lower pressure - the dealership is full of shit, to put it nicely. Since your truck is out of warranty, there is absolutely ZERO reason to have your truck diagnosed there! The pay scale at dealerships encourages SPEED - that's NOT what you want for an electrical diagnosis! Find a small local garage that specializes in drivability and electrical troubleshooting. Had you done that, it would've been fixed after the first trip there, sorry to say, and you'd probably still be $100 or more richer.

 

I don't know how far Niagara County is from Avoca, but look up South Main Auto Repair - Eric O. is one of the best auto techs in your state - would be worth the drive over there ( no matter how far ) to have him straighten everything out. He WILL fix the truck right, and will not take advantage of you. His labor rate is $60 an hour last I heard, and he's HONEST and thorough. He's on Youtube too, and his channel is growing by the day.

 

https://www.youtube.com/channel/UCtAGzm9e_liY7ko1PBhzTHA

 

Hope this gets you headed in the right direction.

Link to comment
Share on other sites

  • 0

Thanks for the response. Both new sending units I put in the tank contained everything. (Pump, regulator, level sensor, filter and tank pressure sensor). The tank was leaking from a rusted metal pressure line coming out of the top of the sending unit.

 

And I agree, I normally would never have a dealership diagnose anything! But I always had good pressure 55psi+ before the new FPCM install and program. And I wanted them to replace their work for free if it was faulty. Which they agreed to. Then when I got the truck back and they told me about the cats I learned that they didn't test the FPCM at all. They are clueless at the dealership.

 

If it was a pinched wire I would assume I wouldn't be able to command the pump on with the scanner and see full pressure.

Link to comment
Share on other sites

  • 0

Yeah, you're right about that, but a pinched or broken off green crusty, corroded wire would most certainly cause a crank, no-start. I suspect your original FPCM was probably fine, otherwise a new unit and program would've fixed everything.

 

I would highly recommend paying Eric O. a visit at SMA in Avoca - you'll thank me later.

Edited by Jsdirt
Link to comment
Share on other sites

  • 0

If I didn't explain it well. This pressure drop is consistent and repeatable on both brand new ACDelco units I installed. And I purchased them online and installed myself. So two different regulators both brand new.

I'll have to look up where he is located from me. Thanks. Never heard of that town.

Link to comment
Share on other sites

  • 0

Ahh, gotcha. What is the scan tool reading on the fuel pressure sensor PID?

 

You might have to go into generic OBD-II on the scanner to get the correct reading - alot of the time, the factory side uses substituted values, which can send you on a wild goose chase ...

 

The PID is labeled " Fuel Rail Pressure Sensor (PSI)".

Edited by Jsdirt
Link to comment
Share on other sites

  • 0

It's reading 43.5 psi on the scanner and the fuel rail. and it maintains that the whole time your driving within a psi or two. No matter what load or throttle position. When I first start it the scanner and the fuel rail mechanical test gauge reads the proper 62psi for 10-20 seconds then it drops to 43.5. The only reason I say it's being commanded down by the computer is because I can command it right back up with the scanner. It seems like they installed a program in the FPCM that is for a different vehicle or something.

 

The old FPCM just rotted out and got water in it. And when I installed the new one it did fix the no start because after that I had power to the pump again. It just supplies a different pressure. I don't know enough about programming to know if it's possible to mess that up when programming the new FPCM.

Link to comment
Share on other sites

  • 0

Just checked and Eric at South Main Auto is about 2-2.5 hours from me. Not bad but I can't imagine I would be able to get there during business hours with my schedule.

 

Can't believe how difficult the original fuel leak fix is turning out to be. CAN OF WORMS!!!

Link to comment
Share on other sites

  • 0

Yeah, these modern vehicles are rolling cans of worms! I don't do much electrical troubleshooting since I'm still in the learning stages on that, but I've seen enough of them to know that much!

 

 

 

Ahh now I see. It's certainly possible (especially knowing GM dealers) to install the incorrect programming. It also could've got corrupted if there was a glitch in their internet connection, or if battery voltage wasn't maintained throughout the procedure.

 

No idea how it could maintain a lower than normal fuel pressure. I would imagine it's getting bad info from other sensors. Are your o2s looking ok? MAF ok? I would imagine your fuel trims are counting rich to compensate for a lean condition, due to the low fuel pressure.

 

In the process of refreshing my memory on this system (I had better, since I own pretty much the same truck). I did discover just as an FYI that there are TWO gray wires coming out of the FPCM (FSCM in GM terms) -- one is to pump +, and the other is labeled "FSCM SHIELD". I would imagine the wire gauge size would be different enough to tell one from the other - the pump should have heavier gauge wire, obviously. But that's neither here nor there at this stage ...

Edited by Jsdirt
Link to comment
Share on other sites

  • 0

Thinking outside the box here ... I would double check the line connection on the new fuel pump - maybe the o-ring slid over and is blocking flow, or something got in there unexpectedly? Long shot, but you never know.

Link to comment
Share on other sites

  • 0

Since I just got the new cats on after removing the plugged ones, I haven't got to check my fuel trims yet. Gonna do that in about an hour. The 02 sensors seem to be ok and I haven't checked anything to do with the MAF sensor. Even though the cats were pretty damn clogged I never got a code for them being clogged, overheated or out of parameter.

 

Before removing the clogged cats, i had -6% LTFT bank 1 and +7% LTFT bank 2. Which I assumed it was trying to lean out bank 1 because that cat was more clogged than bank 2 cat. I'll report back new LTFT numbers later this afternoon. Thanks for replying and brainstorming with me. I needed somebody else's input after I exhausted the knowledge out of everyone I know. I had a really good mechanic at a private shop but he passed away a few years ago now.

Link to comment
Share on other sites

  • 0

Sorry to hear that. Everyone always tells me how hard it is to find decent mechanics out there today. Few and far between these days. Wish I had a concrete answer for you, but that's tough even in person, let alone over the net.

 

I've never heard of a code specifically for clogged cats. There's the P0420 / 0430 cat efficiency codes that might indicate they've melted, but nothing specifically indicating that they're plugged up. A vacuum check will find that issue pretty quickly - lower than normal vacuum at idle, then drops like a rock, and sometimes even goes positive at light throttle, higher RPM. On Fords you can test for that right on the DPFE sensor ... but that doesn't apply here. Trims will certainly go rich though with clogged cats, so that coupled with a vacuum test usually tells the tale. Also, removing the upstream o2 sensor and running the engine will be a dead ringer on really blocked up cats.

 

Just a few things I've been thinking about on this. In addition to the o2s and MAF, also check to be sure your air filter is good and clean, make sure the MAP sensor isn't reading improperly, and if your scan tool is capable, I'd do an injector balance test using your fuel pressure gauge - maybe there's a bad injector that's reducing your fuel pressure. That would also certainly melt down and plug your cat too.

Link to comment
Share on other sites

  • 0

That's a fix! Confirmed it on Eric O's channel - this is part 2 of a FPCM diagnosis - note the scan tool reading. He had replaced and programmed the FPCM here, and is now checking scan data:

 

 

 

The fuel trims are well within the acceptable range. Anything ±10% is good. My truck has been up to -15% in the long term, and no light yet.

 

If you're like me and you will sleep better if everything is perfect, you may have a very small air leak on bank 2 to diagnose. See if that trim number gets better with heat - if it's +8 when cold, then drops as it warms, that's probably an intake gasket leak in the beginning stages. If temp has no bearing, then it could be an EVAP vent valve that's partially stuck open.

Edited by Jsdirt
Link to comment
Share on other sites

  • 0

Thank you for that video link. In those videos the old and new FSCM's were asking for a desired pressure of 43.5

Psi while it was running. It was doing exactly what mine is doing. I guess that's normal. Nobody at the dealership I had it programmed at could tell me that was normal. I feel much better now. The lack of power and hesitation must have just been from the plugged cats.

 

Also I do have an exhaust leak now at the back of bank 2 exhaust manifold where it connects to the Y pipe. So I'm sure that could be causing that side to run a little richer trim. When the Y pipe was off we removed a few broken exhaust manifold bolts that were leaking a little. So technically I just moved the exhaust leak back 18" or so! Haha.

 

I appreciate the help. Now I'm off to drop the tranny cross member and pull the Y pipe again. I despise exhaust leaks.

Link to comment
Share on other sites

  • 0

That right there is the #1 reason I HATE GM dealerships - DISHONESTY!! I hear so many stories of it, and have experienced it myself first hand.

 

Ahh, yep - exhaust leaks will do that too - forgot about that variable. They annoy the crap out of me too, lol.

 

 

Glad this all worked out good, and glad I could help! :thumbs::cheers:

Link to comment
Share on other sites

  • 0

was there a solution to this problem? I have a 2010 gmc sierra 1500 5.3 with the same issues I've replaced damn near everything this guy has and still having problems except my truck doesn't always demand 55psi+ when in wot.  starts up with 60-70psi then drops to 42-43 after a few minutes truck has ran like this for about a year and a half now I got it back from the stealership after they replaced to much in the timing chain department and its never felt the same since the test drive bought it and 2 minutes after I left the show room it had a check engine light for timing so I brought it back if anyone has any insight on what I could be dealing with I appreciate the help 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.



×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.