2018 Lexus RX350 F Sport with 80k miles. I recently had the thermostat replaced at a Toyota dealership about 3k miles ago. I am not sure if that work could have contributed to my current issue or not. Otherwise everything has been good for the past year. Fuel is consistently from Costco and Exxon.

I drove ~200 miles from home, took an exit, and noticed the RPMs drop to ~600 when I came to a full stop at the light - a little lower than usual. The RPMs bounced around little bit. The light turned green and I went on with the rest of the trip. 50 miles later when I came to a stop in a parking lot, the RPMs fluctuated between 600 and 1200. I parked and turned the car off. 10 minutes later I tried starting the car, but it only cranked and would not fire. No CEL. After about 15 minutes of sitting and thinking, I tried again and it started with a rough idle. Being far from home without my scanner or tools, I revved the engine to 2k RPM for a min, which seemed to then stabilize the idle. I drove 20 mins to the closest AutoZone to borrow their scan tool. The idle was surging between 600-1200 RPM while scanning, and it revealed p0172 p0175 (rich banks 1 and 2). The CEL was not on. I looked for loose or broken hoses but found none. I unplugged the PCV hose while it was running, and the engine idle stopped surging for the moment. I drove back to the hotel and parked for the night. I thought maybe the EVAP canister failed open causing a rich condition causing the engine to flood with fuel or vapors when it wouldn't start. I wasn't sure of anything that night. The next day the car started with the same rough idle. I drove around locally and noticed the surging idle at traffic lights. About mid-day, the CEL finally triggered, and it disabled cruise control and AWD. The car ran well at highway speeds and I was able to cover the 200 miles back home without any issue. Once I was home, scanning with my LAUNCH Creader Elite V3.0 revealed:

• P0170 – System too lean bank 1
• P1170 – Fuel performance / port injector
• P0174 – System too lean bank 2
• P1604 – Startability malfunction
• P1605 – Rough Idle
• ID:0X00C1170 – not defined
• C1A40 – Powertrain system failure
• C1280 – Engine control system malfunction

What I’ve done:

• I checked the EVAP canister. The impedance was 25 ohms, and it behaves properly when cold (closed) and warm (open). It is original.
• I had recently replaced the PCV valve about 5k miles ago. I can blow through it one way only. I sprayed it with carb cleaner.
• The PCV hose appears to be fine. It is original, but I can see no cracks or tears.
• I pulled the intake, cleaned the MAF with MAF cleaner, checked all the surrounding hoses to front and back of intake, and found nothing loose or broken.
• I sprayed carb cleaner around the front and back of the intake listening for a change in idle. Observed nothing.

Fuel pressures seem fine:

• Fuel Rail A = 8,700kPa (range is 4k – 13k)
• Fuel Rail B = 420kPa (range is 300-500k)

I monitored the Short Term and Long Term Fuel Trims and observed the following yesterday morning:
B1 LTFT B2 LTFT B1 STFT B2 STFT
Idle: +4.688% +8.594% -0.781% -1.562%
1,500 rpm +2.344% +3.906% -1.562% -1.562%
2,500 rpm -1.562% +2.344% -0.781% 0%
Back to Idle +4.688% +8.594% -5.469% -3.906%

30 minutes later, same test with slightly different STFT readings but consistent LTFT readings showing lean conditions:
B1 LTFT B2 LTFT B1 STFT B2 STFT
Idle +4.688% +8.594% +1.562% +1.562%
1,500 rpm +3.125% +4.688% 0% 0%
2,500 rpm -1.562% +2.344% +1.562% -1.562%
Back to idle +4.688% +8.594%- 4.688% -2.344%

I drove 30 miles and rechecked the LTFTs. They showed consistent positive (rich) corrections to the lean conditions:
B1 LTFT B2 LTFT B1 STFT B2 STFT
This morning idle +4.688% +8.594% +1.562% +1.562%
Tonight idle +9.375% +11.719% 0% +2.344%
Change +4.7% +3.1% — —

Any recommendations on what to check next? I thought I would monitor fuel trims while spraying carb cleaner around the intake and the hoses to see if either bank changes.
Does the varying lean/rich conditions on the STFT readings imply an inconsistent vacuum leak that fluctuates with RPM and vacuum levels? Does it imply anything?
To me, it seems things are narrowed to too much air (vacuum leak at hoses or intake) or too little fuel.
O2 sensor(s)? Fuel pump or injectors?
Any other revealing test functions in the Launch CREADER?
Anything related to the dealership dropping the engine a few inches to replace the thermostat? That was a few thousand miles ago, so I assume it would have triggered the lean condition before now.
Thanks for reading to the end and for any suggestions!

EDIT: sorry about the table formatting. It won't let me save a better format for the fuel trim tables.

 

Ok running lean means not enough gas is getting in the combustion chamber, so that is something going on with injectors. These generation RXs have direct injection as well as port (if you have the 2GR-FKS like we have in our 2019).

I would start with a high quality fuel treatment. Also, ensure you are using toptier gasoline (e.g. Exxon, Shell, Costco, etc...). I didn't see either mentioned in your post. At 80K miles, if you are going to have issues, that's about the mileage I would expect them to show up. Not sure how this could have something to do with the thermostat and you didn't mention why it was replaced in the first place. Hope they used an OEM part, regardless.

High quality fuel treatments are those with Redline and Amsoil.

This is actually a great reminder I need to run some through ours. We almost exclusively use toptier gas, but still a good treatment once every 5K miles would be good to do given the negative impacts of direct injection in terms of carbon build up.

Lastly, ensure to get the RPMs up once in a while doing an "Italian" tune-up. This isn't just some old wives-tale. It is actually good for your engine to get out on the road and get accelerated hard once in a while.

 

I run Techron every ~7k miles, and I only use top tier fuels (specifically Exxon and Costco).

Any other ideas regarding vacuum leaks?

 

Since you had it done by the dealership, I would take it back to them and see what they could offer.

Question - you had rich codes then you had lean codes? Quite odd if you ask me.

Since the fuel trims are only high at idle - that indicates a vacuum leak (either internal or external). I would plug the PCV valve hose and the EVAP purge valve and see if the fuel trim improves.

It can't be a fuel delivery issue since the fuel trims are not high under load.

The P1604 – Startability malfunction - is on because the battery was disconnected. The rough idle is possible because the throttle body was dirty and has not relearned the throttle plate position.

Keep us posted.

 

That's correct. Rich and then lean. I cleared the codes yesterday. When looking at other live data today, I noticed the two rich codes were logged in the computer, but the CEL hadn't triggered. The LTFTs were both still positive (~ +8 and +6) which suggest it's still running lean? The rich and then lean is confusing.

The MAF live data was 2.7 g/s at idle, 10.5 at 2500 rpm, and 18.6 at 4000 rpm, which is low, low, and normal. I don't know if that suggests a vacuum leak at idle and lower rpm or if it means a failing MAF sensor.

I monitored the short term fuel trims while slowly spraying a half can of carb cleaner around the front and back of intake manifold and all hoses. They never moved.

O2 live data was all in range, and both responded similarly under increased RPMs.

Anything else I should check and post here?

 

If you still have the freeze frame data that would be helpful. It tells you when the event had happened.

Currently, if the fuel trims both long term and short term is under +/-10, you are in fuel control.

Is the check engine light on now? If not, wait for it to turn on and post the freeze frame data.