Fuel Pressure issue?

Discussion in 'Engine & Drivetrain' started by TahoeInFC, Apr 30, 2017.

  1. east302

    east302 Full Access Member

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    Here you go, it's for a 1998 350...

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  2. o_fatmike_o

    o_fatmike_o Member

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    gotta remember "theoretically" the pump doesn't create pressure just flow and the regulator creates the pressure . but yeah with that other info , im thinking you may be right with the pump on its way out . but let us know what you find
     
  3. TahoeInFC

    TahoeInFC Member

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    Thanks! That's a good way to look at pump vs regulator. Either way, I'm in for some fun. It's either pulling the plenum or dropping the tank.
    It still could be the regulator starting to fail, because I can faintly smell gas in the intake, but my luck it's both the regulator AND the pump. Ugh.
    However, I'm also leaning toward the pump since pressure doesn't leak down after key-off.

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  4. TahoeInFC

    TahoeInFC Member

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    Hey All,
    I think I'm going to have a little time to chisel on this problem this weekend.

    I probably won't have enough time to drop the tank, but may have enough to pop the upper intake and have a look. Still, that's a major pain, so I'm thinking of some other tests I can run before I get extreme. I'm planning to look at the plugs, the cap and rotor, and check for vacuum leaks. I might even grab the pressure gauge and do another leak-down just to verify what I saw the first time.

    I've read that a vacuum leak can cause the fuel pressure regulator to misbehave, and cause the engine to run rich. If I have a vacuum leak, it's not severe enough to make it run super bad (yet). Not throwing codes or mis-firing. Seems to run OK. Still gets around 15 mpg, so mileage hasn't suffered much. Occasionally idles a little rough if I've been driving on the highway and stop at a light, but then seems to straighten out, sometimes by the next stop light.

    I'm still running rich, though. Sooty coating on the inside of my tailpipe. I can clean it out, and it's back in a few days. I can smell the richness in the exhaust, and I'm thinking I'm going to end up trashing my catalytic converter(s) if I let this continue.

    I can also smell gas in the intake plenum when I remove the air intake and sniff down in the throttle. I did a quick leak-down test, but pressure stays solid when engine is killed, so is it possible that the gas smell could be a by-product of a vacuum leak somewhere? Also, is it possible for a fuel pressure regulator to 'sort of' fail? Meaning it would not leak down quickly, but could still be leaking enough fuel in the intake to cause the engine to run rich, but not rich enough to throw codes?

    I'm thinking I need to attack this 'rich' issue before worrying too much about the fuel pump.
    I'm in that weird no-man's-land where the truck isn't really running all that bad, but I know something is wrong.

    Any more ideas?

    Thanks folks!
     
  5. TahoeInFC

    TahoeInFC Member

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    Hi All,
    I messed with it a little more today. Grabbed the fuel pressure gauge and took some more readings. I really didn't do a long-term leak-down the last time, but I did today.

    At key-on, the pressure gauge quickly jumps to 60psi for about 2 seconds, then drops to 50-52psi, and slowly creeps up to 56psi. It sits there and hangs out as long as the key is on.
    When I key-off, it again jumps to 60psi for about 2 second, then drops down to 52 and again creeps to 56psi and holds there for at least 10 minutes, then slowly drops to about 54psi over the course of 5 more minutes. No quick bleed off that would indicate a completely failed regulator or leaking injector lines.
    When I start the car, it again jumps to 60psi at key-on, then drops to 52psi while the Tahoe runs. Goosing it jumps up to 58-60psi, but drops and holds at 52. Shut it off, and it holds at 52, then slowly creeps up to 54-56 psi again. Key completely off. It'll hold this pressure for a long time. I let it sit for 15-20 minutes while I did other things, and I had to bleed off the gauge with the relief valve because it was still holding about 54psi.

    I've attached a video here if you want to watch it. It illustrates what I'm seeing. Please keep in mind while you watch that I'm not letting it get all the way back to 56psi in the interest of time for the video. It will go back to 56 if you wait about 30 seconds. Just wanted to add that, because I know someone will say "What do you mean 56psi, it's only hitting like 52?"
    Video is in YouTube, here -->

    I still think the fuel pressures are low.

    Additionally, I popped off the air cleaner and using a mirror and a small flexible LED flashlight, peeked around inside the plenum through the throttle opening. Key-on, I didn't see any fuel dripping anywhere, but can sure smell it. It's hard to tell if it's wet in there, because the light glares off everything. I could see the pressure regulator from the bottom, and no drips that I could see. Doesn't mean it's not on the verge of failing, but again, no pressure gauge leak-down to speak of at all.

    Your thoughts after seeing the video?
     
  6. east302

    east302 Full Access Member

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    I'd agree that you're low.

    Following the factory test procedure, it looks like you're going from step 2 to 8 to 13 to 15 to 19 (correct?) ending at a fuel line restriction part. #21 would seem to be the next test.


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  7. TahoeInFC

    TahoeInFC Member

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    Yeah man! east302, Thanks again for that procedure. I was trying to work through it today, and I think you described the same path I was following. Step 21 describes my next steps for sure, but not sure if I'll have more time this weekend to dig in. I changed the fuel filter at 117k, but they're not expensive, so I might just pick up another and toss it in.
    I also started to track the short and long-term fuel trims today, and will do a test if I get time tomorrow at idle and at 2500RPM. I've read that elevated long-term trims can indicate a fuel delivery problem, and in some of the quick scanning I was doing today that seemed to be the case on my truck, but I ran out of time. Low pressures and elevated trims...hmmm...sounds like I'm in for a fuel pump if I can't find another fuel restriction of some sort. That pump is starting to get noisy, and from everything I can tell, it's also 18 years old.
     
  8. TahoeInFC

    TahoeInFC Member

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    Hi All, does anyone know if these 99 Tahoes have a 'Fuel Pump Test Terminal', like is described in step 21 of the fuel pressure test sequence above? I know some GM cars have them...the S10 trucks have a red wire that hangs out somewhere close to the fuse/relay box. Tie it to battery through a fuse, and voila! Fuel pump comes to life without starting the truck. I think some of the wiring diagrams call them a 'pump priming terminal'.
    Is there something like that in these OBS full-size trucks? I've looked around in that vicinity under the hood, but haven't seen one yet. Maybe just a spare terminal in the fusebox someplace?

    Thanks
     
  9. east302

    east302 Full Access Member

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    This looks to be something at the fuel pump relay...red wire, circuit 490, fuel pump prime connector, looks like it would send power to the pump if the relay switch is open.

    Guess it's in the underhood fuse box.

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  10. TahoeInFC

    TahoeInFC Member

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    Thanks east302, I also found that same diagram surfing around this weekend and came to the same conclusion. I spent awhile under the hood trying to find that damned red wire, but it's eluding me. Maybe it's wrapped up in the harness nearby the fuse box, but I didn't get serious about peeling things back.
    I've seen pictures of that wire on S10 harnesses, and it's just sticking out there in space for the whole world to see, but it must be hiding in these Tahoes. Or maybe I need new glasses.



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