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Please help me figure out if low RPMs on my daughter's 2005 Suburban 5.3 FlexFuel is the cause or a just symptom of problems with oil and fuel pressure. The problem only happens when the engine is warm and I think that’s because it runs 200-300 extra RPMs while the engine is cold. I am totally stumped and this ruined my Memorial Day weekend and I would love to read any ideas about what the problem might be, what is the cause vs the effect, or what I could try next.

Background & Symptoms: Last Thursday evening my daughter said her truck was making a terrible noise and had no "get-up-and-go." I started it but it sounded fine idling in Park. I put it in gear (foot on the brake) and heard what sounded like a bad engine knock or maybe shaking a can of spray paint except the can has multiple marbles. The sound stopped when I let my foot off the brake. I braked and the sound quickly returned. I put it in Park and the sound instantly disappeared. I rolled down the windows and turned off the AC to hear better. I put it in gear again and, to my surprise, the sound was much quieter. I experimented with the windows and AC with the truck in gear and foot on the brake. I found that the noise was much louder with the AC on and while the electric windows were operating but completely disappeared in Park or Neutral even with all the accessories on. It has serious hesitation trying to launch from a complete stop or goosing it while rolling under 10 mph but above 10mph it would pull hard and sounded fine. I stopped on a steep hill and could not resume climbing from a dead stop so I backed down in reverse. Oil pressure is normally steady at 40 psi but drops to 20-25 when the knocking noise happens. I don't know if oil pressure in the low 20s is enough to cause engine knock. The oil pump is inside the pan and I haven’t messed with it (yet.)

Troubleshooting: My cheap scan tool said it was running lean, and the short-term and long-term fuel trim numbers were maxed. I hadn’t noticed on the tachometer, but the scanner also showed the RPMs were 550 +/- 20 in Park or Neutral but only 500 +/- 35 when knocking in Drive or Reverse with the brake on. Other differences revealed by the scanner include that the Spark Advance ignition timing drops from 18-22 degrees above top-dead-center to the single digits and sometimes to 0 or -2, the throttle position sensor opens from 5% to 15%, and the MAF reading goes from ~5.5 g/s to ~12.5 g/s. All four 02 sensors have the expected readings so I think the extra air is burning extra fuel at the proper ratio but I don’t know if the computer is lowering the ignition timing because of low RPM .. or because of knocking ... or it's just keeping the injectors open longer. A fuel pressure gauge showed normal and steady with the key on and engine off but with the engine on the needle shakes +/- 3 psi so fast it’s literally a blur. A good push on the accelerator drops the fuel psi by 10 for not even 1 second before it returns to the fast blur bouncing. With the engine off it loses about 10 psi in 10 minutes so that's not great. The fuel pressure needle is only steady when free reving or accelerating hard from 20 to 50 mph. The fuel pressure regulator and filter are on the fuel pump, which is inside the 32 gallon gas tank which must be dropped for access. I haven’t done that (yet.)

I don’t really know if the knocking and hesitation are separate problems or both symptoms of one problem. I used smoke in the intake to find and fix vacuum leaks in the EVAP purge line and valve cover gasket. It has a hydraulic brake booster so no vacuum lines. I also cleaned the throttle body and followed some manual-relearn steps I found online because my scan tool doesn’t have the option to just tell the computer to do a relearn. The RPMs are now 600 +/- 10 at idle in Park and 550 +/- 15 in gear foot on brake. The problems remain so that means it is now knocking and hesitating in Drive/Reverse at the same RPM levels where it was previously not in Park/Neutral. After that repair, the fuel trim numbers dropped to less than 10 and are mostly close to 0 and I can’t hear any other vacuum leaks. The lights on the instrument cluster no longer dim when the engine knocks and although the AC blower speed still drops, it is less obvious than before the fix. I think these electrical improvements result from the alternator spinning faster but the increased RPMs didn’t seem to help the oil pump, fuel pump, or ignition timing.

Other Troubleshooting: I thought maybe she got some bad gasoline but octane booster didn’t help. I thought maybe the oil filter was defective but a new one didn’t help either. Next, I wondered if the transmission flexplate was cracked and rattling near where the torque converter bolts but I crawled under it with a stethoscope and the knock was definitely coming from the engine. I thought maybe the knock sensors were causing the computer to retard the timing but unplugging them didn’t do anything.

The engine revs freely in Park/Neutral but very slowly raises the RPMs if I goose while in gear with foot on brake. It refuses to rev past 650 even with the throttle position reading 99%... although doing this does stop the noise and raises the ignition timing back above 10 degrees. The very slow rise in RPM in this test mirrors the hesitation trying to launch from a dead stop... except the 650 RPM limit which might be just the computer not allowing that.

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    Kudos on a very well written question. I don't have an answer for you, but it's true some vehicles cut engine power by several different means when the brake is applied. (See Audi unintended acceleration and Toyota floor mat recalls.) Suggest you check that the brake light switch is not sticking, as that could account for many of these symptoms.
    – MTA
    Commented May 28 at 18:06
  • @MTA I removed the brake bulbs and nothing changed but I appreciate the clever idea... if a brake wire had been shorted to the body that could cause all sorts of problems. Also, you confirmation that some computers do this on purpose helps me sort the real clues from the red herrings.
    – AlexPace
    Commented May 28 at 21:08
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    I'm talking about the brake light switch (physically activated by the brake pedal at knee level under the dash) being wired to the ECU or the body control module so the ECU knows that you're stepping on the brake. This inhibits engine power to prevent runaway cars if the accelerator is stuck to the floor. Back up to a wall at night and step on the brake, observing brake light. Light should go out immediately upon releasing pedal. Note that a defective brake booster could keep the pedal depressed enough to hit the brake switch but not slow the car or drag the brakes.
    – MTA
    Commented May 28 at 22:09
  • Ever see a car ahead of you whose brake lights flash every time it hits a bump? I have. The brake pedal is too low and just on the verge of hitting the brake light switch. Such cars must suffer mysterious power losses from time to time!
    – MTA
    Commented May 28 at 22:12

2 Answers 2

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The oil pressure thing is quite normal for the LS based 5.3L LM7 which is in her Suburban. This stems from the pump's pressure spring wearing out and/or the plunger it backs getting sticky in its bore. Best solution for this is to get a new oil pump and install it. While the oil pul isn't technically in the pan, I'm pretty sure you have to take the pan off in order to get to the bolt which holds the pickup tube on (I've only ever removed them with the engine out of the vehicle and oil pan off).

As for the issues, check to make sure the battery is fully charged (at least 13+ VDC with engine off) and the alternator is charging correctly (~14.1 VDC running). If either of these are low, it could be your issue.

The fuel pressure should be ~58psi. If the fuel pressure is low, it may be causing the noise you are (or were) hearing. Knock doesn't usually show itself unless there is a torque demand put on the engine, like it being in gear. Usually you wouldn't hear it unless you actually step on the gas, but if bad enough (as you were suggesting) it could be causing it to appear while at idle in drive/reverse. A very lean state can cause knock, which if the fuel pressure is low, there you go.

Whether or not the engine is knocking, it sounds like the computer is picking it up as such considering what you were saying about it retarding the timing. While this isn't the real issue (retarded timing) it sounds like it's the issue which is causing it not wanting to accelerate. The computer can pull enough timing which will absolutely kill power. Since you aren't getting the noise in park/neutral, it will free rev without issues.

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    And a well written question gets a well-founded answer! I know off-topic comments are . . . off-topic, but this question and answer should be a poster child for how this site works best.
    – MTA
    Commented May 28 at 19:50
  • @MTA - And the best thing you can do to show that is to vote. Not that I need points, but showing how you feel about posts (whether good or bad) is how we make this site better. Commented May 28 at 19:59
  • Wow, thanks @Paulster2 for reading all that and taking the time to respond. Your answer is exactly the kind of information I need! It sounds like both pumps are suspect but it has 335K miles so I guess that's to be expected.
    – AlexPace
    Commented May 28 at 21:17
  • @AlexPace - If you've got 335k on the LM7 and are going to do an oil pump, you might want to consider doing the lifters as well. Those are another sore point with these engines as they start making more noise when older. 335k is nothing to scoff at ... I wouldn't be surprised if you've already replaced the transmission in it (at least once). Commented May 28 at 21:52
  • @Pᴀᴜʟsᴛᴇʀ2 Could the large fuel trim offsets be a sign of a vacuum leak, for example in the rocker cover ventilation hose? I'd expect the hoses to be decaying at that sort of mileage. Commented May 29 at 7:40
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Excessive knocking will cause the knock sensors to retard the timing. These trucks had problems with the knock sensors getting wet and corroded (they are under the intake manifold) But that would cause low power, not a knock. I think your 20 psi oil pressure is OK at idle, you should have at least 40 at 2000 rpm. Good trim numbers now, so I'd say once you find the cause of knocking, your timing and power will return.

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