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.
