2005 GX470 (2UZ-FE VVT-i) Mid-RPM Power Surge (1.9k–2.9k RPM) — No Codes

[jimbroo]

I am chasing a specific and frustrating engine issue on my 2005 Lexus GX470 (4.7L V8 2UZ-FE with VVT-i). I am hoping some of the 2UZ experts here can point me in the right direction.

The Symptom:
The truck experiences a distinct power surge/hesitation/hunting sensation under acceleration, but it only happens within a narrow window from 1,900 RPM to 2,900 RPM.

• Below 1,900 RPM: The engine runs perfectly smooth. Low-end torque is great, and idle is steady.
• Inside 1,900–2,900 RPM: The power fluctuates rhythmically, almost like the engine timing or intake air path is constantly switching back and forth or fighting itself.
• Above 2,900 RPM: The surge completely disappears, and the truck pulls hard all the way to redline.

What I’ve Noted So Far:

• No Check Engine Light (CEL): The dashboard is clean, and there are currently no active or pending diagnostic trouble codes (DTCs).
• Not a Total Component Failure: Because it clears up completely past 3,000 RPM, I am fairly confident I can rule out global failures like a dying fuel pump, failing ignition coils, bad spark plugs, or a skipped timing belt.

My Leading Theories:
Given that this 1,000-RPM window is exactly where the VVT-i engine transitions its variable components, I suspect the issue lies with one of the following:

1. VVT-i Oil Control Valves (OCV): Sticky solenoids or clogged micro-mesh filters inside the cylinder heads causing the cam timing to "hunt" under load.
2. ACIS (Acoustic Control Induction System): A sticky intake manifold butterfly valve, leaking vacuum pod, or failing Vacuum Switching Valve (VSV) malfunctioning during the short/long runner transition.
3. MAF Sensor Load Miscalculation: A dirty Mass Air Flow sensor failing to accurately read sudden air velocity changes during mid-range RPM transitions.
4. Torque Converter Shudder: A slipping clutch in the A750F 5-speed automatic right around cruising speeds, which can mimic an engine surge.

Has anyone dealt with this exact mid-range RPM surge on a VVT-i 2UZ-FE? If you have solved this, did cleaning the OCV filters or looking into the ACIS valve fix it for you? Any advice on what specific live data parameters (like VVT advance or fuel trims) I should log on an OBD scanner to isolate this would be hugely appreciated.
Thanks in advance!

 

[Rednexus]

Are those perhaps AI-generated theories ? They VVTI/ACIS is a new one for me, I've never heard of anyone having VVTI/ACIS issues with a GX, and to my knowledge those systems don't kick in until much higher RPMS.

In all seriousness, the computer is the right way to solve this, but not using an algorithm. I would suggest recording live sensor data during the behavior to see what's going on. This can be done with an OBD2 scanner app that allows you to record data and either live plot it or export it to Excel so you can plot it yourself. You should plot engine RPM, MAF reading in g/sec, long term fuel trim on both banks, short term fuel trim on both banks, measured air fuel ratio, commanded air fuel ratio, and applied throttle %, and actual throttle %. If you can post that here it will help narrow down the possible issues. My guess is it's vacuum leak or MAF sensor related, but I also have not expeirnced or heard of this specific issue before. GX's just seem to have a lot of MAF sensor issues though.