99 C1500 Suburban P0300

Discussion in 'Engine Performance + Maintenance' started by needlenose, Aug 31, 2019.

Car Parts
  1. needlenose

    needlenose Newbie

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    My suburban is throwing the P0300 and sounds like it's running on about 4cyl. I have already replaced the fuel pump with a new Delco unit about 1.5 years ago. About 2 years ago I did the MPFI upgrade with a new Delco unit. I just replaced the fuel filter today.

    Pressure starts at about 52, then raises to about 60 under almost WOT load. From my research, I understand this is too low. I have pinched off the return line with the key on and pressure spikes to almost 90. Also from my research, I understand this indicates that the fuel pump is ok. At this point I *think* it's the regulator. Given that the regulator was new with the new spider it seems a little strange, but....

    Does anyone have anything else I can try to check the regulator before I strip it down and replace it?

    Thanks!
     
  2. Schurkey

    Schurkey I'm Awesome

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    WHAT SCAN TOOL are you using to find the P0300 code?

    Cheap-junk consumer-grade crap tools often provide generic codes instead of the specific codes. I'm wondering if this is a "real" P0300, or if the real code is cylinder-specific and the scan tool can't display it.

    You've already checked the regulator, and you've found that it isn't working properly.
     
  3. needlenose

    needlenose Newbie

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    I used a Blue Driver. I got a cyl 5 misfire a few months ago and replaced the cap/rotor. The misfire went away. I should also add that I put wires and Delco plugs in about a year ago whenI found that one of the wires was arcing against the manifold shield.

    I just wanted to see if there was anything I could have missed to test the regulator before I dismantle.

    Thanks for the response!
     
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  4. needlenose

    needlenose Newbie

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    So I got back in town from a work trip. I found out how to jump the fuel pump relay to test the WOT fuel pressure without driving down the road with the gauge poking out the hood cowl and my head at 90 degrees. It's 62lbs continuous. A fuel leakdown test shows that it takes about 30 minutest to get to 40lbs from 55lbs.

    Does anyone know how to activate each injector using the pins? I think if I do so with the key off, I should see a pressure drop. I'm really hoping that the MPFI isn't bad.
     
  5. Schurkey

    Schurkey I'm Awesome

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    How does jumping the fuel pump relay test WOT fuel pressure? You haven't applied any load to the system, so fuel flow is at minimum, not maximum.
     
  6. Schurkey

    Schurkey I'm Awesome

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    How does jumping the fuel pump relay test WOT fuel pressure? You haven't applied any load to the system, so fuel flow is at minimum, not maximum.
     
  7. needlenose

    needlenose Newbie

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    Sorry, wrong term. The fuel pump is running continuously and is being regulated at 62lbs. If I just key-on, it only runs for 2s then shuts off and drops to 55lbs. I've read about 1000 posts and this seems to be expected behavior? 62lbs seems to be acceptable to fuel the engine. At least, it seems like it should.

    I also watched a youtube video from ScannerDanner where he troubleshoots a 4.3 with a leaking regulator and the ST fuel trims are in -60s. Mine starts around -25 - -30 when cold and corrects to -3 - +3 when warmed. I'm looking for info on what's normal.

    I'm still just trying to make sure I don't waste $100 on a new Delphi regulator and 3-4 hours of time stripping the top of the engine only to find I didn't accomplish anything.

    I did put a spark plug tester on each plug. It seems like the spark color wavers brighter/dimmer some on every plug. I also replaced the coil and cap with spares that I had just to confirm that it doesn't change anything. My other truck is a 12valve cummins, so I don't really have something to compare the tool to.
     
  8. east302

    east302 I'm Awesome

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    Here is the GM test procedure for a 98, if you’re at 52-psi KOEO (no vacuum at the regulator) then it’s out of spec (60-66 psi). Maybe it will help.

    A leaking injector would show as a drop when you turned off the ignition. The table goes over how to test for that *if* you were within pressure specifications with ignition on/engine off to begin with. Essentially, you’d pressurize the system and then block the feed and return lines to see if it holds.

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    Link to some service manuals:

    http://67-72chevytrucks.com/vboard/showthread.php?t=700011

    Someone also posted them on this forum but I cannot find the thread. It may be the same person who did the ones in the above link.

    Here is the P0300 table from a 98 service manual. No guarantee that it is exactly the same for a 99, but it’s probably close enough for government work.

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    Last edited: Sep 14, 2019 at 1:13 AM

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