Guys, I can't find the issue on my LT1 (Gen V, Corvette 2014 Z51) so I'm asking you the gurus

Symptoms:

• Not much torque on Low-mid range RPM
• Full power on high range RPM
• Strange sound coming from engine on low-mid range RPM. It's like there would be another thing rotating in the engine or something off-balance. The sound might be present on high RPM but I can't hear it because of the (stock) exhaust.
• Idle is smooth but sometimes the car will rev by itself when coming to a stop/slow down and no pedal pressure is applied. When this happens I shortly and gently apply pedal pressure and the RPMs go back to normal
• Idle is 1100 RPM on cold engine and 680 RPM on thermostat temperature
• Scared driver, I know something is wrong in the internal or engine management. I don't drive it anymore except for test drives.

What happen the night before this problem and could help find the cause:

• Always used 98 octane but for the last week I have been using 95 (with 10% ethanol crap)
• Some jackass were purposefully blocking the 2 lanes on a 2x2 road driving 30mph on a 60 mph road. I downshifted from 4th or 3rd to 2nd to pass them. That was a bit optimistic as the engine reved too high and stuttered 2 times before finally catching up. Gas pedal was to the floor. Shift to 3rd full power
• Back home it was very late and I was very tired. I park in the garage. Car kept on beeping (as when you left lights on), I didn't pay much attention to it and closed the garage. Next morning, battery to 8V. Had to jump start it.
• Yes I have been twice stupid that night

What I tried:

• Tried to locate vacuum leak, couldn't find one. Idle would have been rough I think if I have a vacuum leak
• Opened valve covers, no stuck rocker arms but I still suspect some issue in the valve train
• ChatGPT (don't laugh) suggested to do a throttle reset procedure. I don't recall exactly but it involved pushing accel pedal for 30 seconds with ignition ON / engine OFF. It didn't seem to do anything and I couldn't find official GM procedure on that.
• Have some misfires on cyl 5, solving this right now but I don't think that's the problem

Here is the HP Tuners log from last testdrive (not from the night when problem occured), is there anything unusual?

 

HP Tuners log file:

 

The fact that your battery lost voltage over night is very suspicious.

Have that battery charged and tested at a reputable auto parts store, or mechanic shop.

If it tests bad, replace it. Also, check alternator output.

Charging issues/ improper volltage will make all kinds of weird things happen on newer cars. Things that seem completely unrelated to the charging system. The computers run everything, and they don't like it when they don't get enough (or too much) voltage.

 

Hi dixie,

in the meantime I had the battery changed. I don't have any drain.

Also completed those tasks:

• intake valve cleaning
• new spark plugs
• new belt
• new belt tensionner
  
  The car is runner smoother but the original problem is still present. It might be related to the throttle body or some exhaust control?
  
  Another thing but that is probably related, when I rev to 1500 the car will be stuck at 1500. Today I could reproduce this multiple times. Nothing can bring down the RPM down apart from turning the ignition off.
  
  I noticed in HP TUners that pedal position never goes below 20% and never above 85%.
  Also, "Throttle Position (SAE)" never matches "Commanded Throttle Actuator". See attached picture.
  Bad throttle position sensor?
  
  
  
  Probably not related but I have noise coming from the back of the car. It seems from an actuator just before my passenger side muffler. It is vibrating/rattling.
  
  My traction button is stuck, somebody slammed it?!
  
  New DTC:
  0x543: U0422 - Invalid Data Received From Body Control Module (SES, Pending, Current, Old, History)
  
  
  New logs including the 1500 RPM stuck engine

 

misfire on cylinder 5, and strange engine noise .... I would compression test that cylinder, compare it to another.

what are the current CEL codes ?

 

Forgot to mention but I did a compression test already (before cleaning the intake valves) and here are the values:

On cold engine/dry cylinder:

Cyl #1: 16 bars (232 psi)
Cyl #2: 15 bars
Cyl #3: 16 bars
Cyl #4: 15.5 bars
Cyl #5: 16 bars
Cyl #6: 14 bars (203 psi)
Cyl #7: 15.5 bars
Cyl #8: 15 bars

So min value is 203 psi, max value is 232 psi.
I couldn't find expected values for a healthy LT1 but that's a 13% variance

No CEL codes on engine. I'll post other DTCs today after scan (mainly BCM related, not sure they can affect engine?)

 

13% is a tad "over" but nothing to worry about really. Odd that there are no engine codes. Only way to trigger one is to keep driving it.
"when I rev to 1500 the car will be stuck at 1500. Today I could reproduce this multiple times. Nothing can bring down the RPM down apart from turning the ignition off."
this seems to be the main problem. Usually "hanging" idles like that are caused by vacuum leaks. maybe double check with some soapy water and look for bubbles.

until then, pretty hard to have any help without codes or hearing it in person.

 

I also checked for vacuum leak and couldn't find any. Idle is very smooth too (in my experience vacuum leak gives a rough idle)

Could it be thrown rods?
Also suspecting a bad IAC or bad TPS/TB

The noise is difficult to capture on video but here is an attempt (focus on the "double" rotating sound, it sounds worse in real life than in record):

https://www.youtube.com/shorts/iENzU10_pNk

Here is a much better capture:

https://www.youtube.com/shorts/a4aboiYmInY

 

0x541: B1395 - Control Module Voltage Reference Output 1 Circuit (Pending, Current, Old, History)
0x541: B3205 - Driver Window Motor (Pending, Current, Old)
0x541: B3210 - Passenger Window Motor (Pending, Current, Old)
0x541: B3600 - Passenger Compartment Dimming Request Signal Circuit (Pending, Current, Old, History)
0x543: U0422 - Invalid Data Received From Body Control Module (SES, Pending, Current, Old, History)
0x543: U042B - Invalid Data Received from Chassis Control Module A (Pending, Old, History)
0x552: B1277 - Microphone 1 Input Signal Circuit (Current, History)

 

Sorry, but I couldn't tell much of anything from your video. Diagnosing most noises via recordings is usually a waste of time.

It IS NOT "thrown rods."
Your car does not have an IAC.

Do you have the capability to do a throttle body relearn/ reset? Does anyone in your area have that capability.

 

kind of hard to tell, but that video sounds exhaust related.

 

I did a TB relearn on HP Tuners while the car was running. It didn't help

That might be it actually. Sometime I have noise coming from the back of the car. It seems from an actuator just before my passenger side muffler. It is grinding sound like a motor trying to close or open but can't get to the desired position. I don't have NPP rpo code so I'm not sure what this device is
From my research that would be the AFM valve just before the passenger muffler. Broken or clogged by catalytic part? Building engine backpressure?

Video from that noise:

You Tube