99 RX300 Engine code keeps coming up lean

Are you getting a P0171 code too or is it just the P0174? Since you're getting a lean code, if it's not accompanied with a P0171, your MAF sensor may be just fine, especially since you've cleaned it.

Has the fuel filter been changed lately and have you checked fuel pressure to make sure it's within spec? That would be the next thing I'd look at, since low fuel pressure will cause lean conditions.

Is fuel pressure good at idle, under load and is holding after turning off the engine? Another thing to consider...a weak fuel pump can work just fine at idle and low speeds but not pump sufficient fuel volume at higher speeds, causing a lean condition.

I'd move onto checking vacuum lines and intake for leaks if the fuel pump checks out good, then onto testing the 02 sensor at closed loop.

What make vehicle do you have ? This code doesn't necessarily mean the sensor is bad ! That is why these thing's need to be tested before replacing , check with a scan tool that shows engine data . This code indicates a lean condition ( vacuum leak ) or a ( fuel problem ) P1131 Lack Of HO2S Switch Sensor Indicates Lean Fixed Lean O2 Is the sensor lying

It could be caused by many things, but generally it means that your engine is running too lean - usually indicating a vacuum leak. It could also be that the O2 sensor is bad and giving a false reading.

before 1996 they use onboard diagnosis system for jeep.

when you turn on key you need to count check engine light blinks.

here are codes taken from internet.

11* No crank reference signal detected during engine cranking.
12* Direct battery input to PCM was disconnected within the last 50 key-on cycles.
13** No difference recognized between the engine MAP reading and the barometric (atmosphereic) pressure reading at start-up
14** MAP sensor input below minimum acceptable voltage.
or
MAP sensor input above maximum acceptable voltage.
15** No vehicle distance (speed) sensor signal detected during road load conditions.
17* Engine coolant temperature remains below normal operating temperatures during vehicle travel (thermostat).
21** Neither rich nor lean condition detected from the oxygen sensor input.
or
Oxygen sensor input voltage maintained above the normal operating range.
22** Engine coolant temperature sensor input above maximum acceptable voltage.
or
Engine coolant temperature sensor input below minimum acceptable voltage.
23** Intake manifold air temperature sensor input above maximum acceptable voltage.
or
Intake manifold air temperature sensor input below minimum acceptable voltage.
24** Throttle position sensor input above maximum acceptable voltage.
or
Throttle position sensor input below minimum acceptable voltage.
25** A shorted condition detected in one or more of the idle air control motor circuits.
27* Injector #1 output driver does not respond properly to the control signal.
or
Injector #2 output driver does not respond properly to the control signal.
or
Injector #3 output driver does not respond properly to the control signal.
or
Injector #4 output driver does not respond properly to the control signal.
or
Injector #5 output driver does not respond properly to the control signal.
or
Injector #6 output driver does not respond properly to the control signal.
33* An open or shorted condition detected in the A/C clutch relay circuit.
34* An open or shorted condition detected in the speed control vacuum or vent solenoid circuits.
or
Speed control switch input below the minimum acceptable voltage.
or
Speed control switch input above the maximum acceptable voltage.
35* An open or shorted condition detected in the radiator fan relay circuit.
41* An open or shorted condition detected in the generator field control circuit.
42* An open or shorted condition detected in the auto shutdown relay circuit.
44* An open or shorted condition exists in the engine coolant temperature sensor circuit or a problem exists in the PCM's battery temperature voltage circuit.
46** Battery voltage sense input above target charging voltage during engine operation.
47** Battery voltage sense input below target charging voltage during engine operation. Also, no significant change detected in battery voltage during active test of generator output.
51** Oxygen sensor signal input indicates lean air/fuel ratio condition during engine operation.
52** Oxygen sensor signal input indicates rich air/fuel ratio condition during engine operation.
53* PCM internal fault condition detected.
54* No fuel sync (camshaft signal) detected during engine cranking.
55* Completion of diagnostic trouble code display on the malfunction indicator lamp (check engine lamp).
62* Unsuccessful attempt to update SRI (service reminder indicator) miles in the PCM EEPROM.
63* Unsuccessful attempt to write to an EEPROM location by the PCM.

* - Check engine lamp will not illuminate at all times if this code was recorded. Cycle ignition key as described and observe code flashed by check engine lamp.

** - Check engine lamp will illuminate during engine operation if this code was recorded.

PCM = Powertrain Control Module (the engine computer)
MAP = Manifold Absolute Pressure (the sensor that measures how hard the engine is sucking air through the intake manifold
EEPROM = Electrically Erasable Programmable Read Only Memory (a type of computer chip used inside the PCM)

POSSIBLE CAUSES TO YOUR ENGINE STALLING OUT BECAUSE OF LEAN FUEL CONDITIONS LIKE STOPPED UP FUEL FILTER,FAULTY TPS SENSOR,FAULTY MASS AIR FLOW SENSOR,FAULTY MAP SENSOR,FAULTY SPARK PLUGS AND WIRES.LOOK FOR AIR INTAKE DUCT LEAK.CHECK FUEL PUMP PRESSURE.SHOULD BE 60 - 66 PSI. DO A CODE SCAN ON VECHICLE TO CHECK FOR ANY FAULT CODES.THAT WILL KEEP YOU FROM SPENDING TOO MUCH MONEY BUY A LOT OF UNNECESSARY PARTS.

DTC - 34 MAP Sensor Circuit Error (Signal Low Indicating High Vacuum)

The ECM uses the Manifold Absolute Pressure (MAP) sensor as an indication of engine load. High voltage would indicate low engine vacuum and higher engine load. Low voltage would indicate high engine vacuum and lower engine load. The ECM uses the information from this sensor to help calculate fuel delivery and spark timing.

Fault Code Description:
* 34 - MAP sensor circuit error (signal low indicating high vacuum)

Symptoms:
* Poor engine performance
* Engine running lean (not enough fuel, hard starting, lack of power)

Common Problems:
* GM MAP sensors do not commonly fail
* Due to erratic engine vacuum, other engine performance problems can cause a code 34 to set


Good luck (remember to rate this answer).

If you haven't had the idle air control (air bypass valve) replaced or cleaned I would. The valves have a history of sticking cold. If you send more info like engine size and what fault codes are in the computer I could assist more.

LEAN TROUBLE CODES (P0171 or P0174) A code P0171 or P0174 indicate the engine is running lean. This means there is too much air and/or not enough fuel. You can confirm the engine is running lean by looking at Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). Normally, STFT and LTFT should be plus or minus 8 from zero. If STFT and/or LTFT are more than about +10 or higher, it indicates the engine is running LEAN. Negative numbers (-10 or more) indicates the engine is running RICH. A lean fuel condition can be caused by:

* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (use a fuel pressure gauge to check fuel pressure at idle)

* Dirty fuel injectors. (try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR valve)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires)

______________________________-- Causes A code P0171 may mean that one or more of the following has happened:

• The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
• There could be a vacuum leak downstream of the MAF sensor.

Possible Solutions Possible solutions include:

• In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
• Inspect all vacuum and PCV hoses, replace if necessary
• Check for a dirty fuel filter and proper fuel pressure

Back firing under acceleration indicates that the engine is running to lean and you need to have the Montero scanned for any DTC codes that hope would indicate which sensor is causing your engine to run lean. They could be a bad O2 sensor, dirty MAF sensor, and MAP sensor, regardless you need to see if there are any codes to lead us to the lean condition to your engine. Good luck and hope this helps, Auto Zone scan free of charge and they can give you the step by step instructions to how to address the issue your engine has. 

I would look at the codes,, jumper the two upper connector holes in the diagnostic link with a paperclip,, turn the key on, and read the blinks of the check engine light,, first will come 3 12 codes then any error codes. If you get a repair manual for that vehicle it will tell you how to perform these tests.