Nad1370

Hey guys,

Just want to share some knowledge if you are having problems diagnosing a Lean/Rich code.

Fuel trim explained:
Fuel trim refers to the feedback compensation value compared to the basic injection time. Fuel trim includes short−term fuel trim and long−term fuel trim.
Short−term fuel trim is the short−term fuel compensation used to maintain the air−fuel ratio at its ideal theoretical value.
The signal from the A/F sensor is approximately proportional to the existing air−fuel ratio, and ECM comparing it with the ideal theoretical value, the ECM reduces fuel volume immediately if the air−fuel ratio is rich and increases fuel volume if it is lean.
Long−term fuel trim compensates the deviation from the central value of the short−term fuel trim stored up by each engine tolerance, and the deviation from the central value due to the passage of time and changes of using environment.
If both the short−term fuel trim and long−term fuel trim exceed a certain value, it is detected as a malfunction and the MIL lights up.

P0171--Lean--Bank 1(firewall side)
P0174--Lean--Bank 2(radiator side)
When air−fuel ratio feedback is stable after engine warming up,
fuel trim is considerably in error on rich side
(2 trip detection logic).

Possible causes:
Air induction system
Injector blockage
Mass air flow meter
Engine coolant temp. sensor
Fuel pressure
Gas leakage on exhaust system
Open or short in A/F sensor (bank 1, 2 sensor 1) circuit
A/F sensor (bank 1, 2 sensor 1)
ECM

P0172--Rich-Bank 1
P0175--Rich Bank 2
When air−fuel ratio feedback is stable after engine warming up,
fuel trim is considerably in error on lean side
(2 trip detection logic)

Possible causes:
Injector leak, blockage
Mass air flow meter
Engine coolant temp. sensor
Ignition system
Fuel pressure
Gas leakage on exhaust system
Open or short in A/F sensor (bank 1, 2 sensor 1) circuit
A/F sensor (bank 1, 2 sensor 1)
ECM

^^ I should add fuel quality to this.

In order for you to properly see the fuel trims and diagnose a problem, you need a Diagnostic Tester that will read the full data list and see the output signals of the Air Fuel sensors.

I'll give you an example:
Code is a P0171--Lean condition. Which bank? Usually B1 right?..not sure since some models don't have a P0174(bank 2)(My bro's.99 RX) to distinguish which bank has the problem.
Sure you do your usual vacuum leak test, clean the MAF, spray around the intake hoses etc. w/ T-body or carb cleaner and see if you can find the problem.
But if you own or have access to a Diagnostic tester( I got mine used for $160), this is a simple problem to solve and taking out the guess work by replacing unecessary parts.

First pic is a normal values of fuel trim at idle.
Second pic is a Lean condition on Bank 2<--Long term reading is high(rich) to compensate for the lean condition.
Now this is at idle.
If the total of the short−term fuel trim value and long−term fuel trim value is within ±35 % (80°C (176°F) or more), the system the system is functioning normally.
The ECU can still compensate and still fall w/in spec value(19.5+4.7=24.2). but for how long and value will change as soon as you drive it.

In this case. Bank 2 has the lean code. Only a few possibilities remain.
AF sensor B2S1(radiator side), wiring, ECU.
1MZ motor has a history of eating up AF sensors. Unless you have a rodent problem...I'm betting it on the AF.

Happy Diagnosing..!

blueridge

Thanks for the info. I just had P0174 pop again and same code "pending". As I stated in the other thread, happens right after a long drive after refueling. Probably has more to do with the vehicle cooling off then the refuel.

thomas1

You know you have been a very positive asset to the posts on this forum....Thank you for the fuel trim tips...

Nad1370

Thanks Thomas.
I, like yourself and many others here in CL, just like to contirbute knowledge to the community.

Nad1370

blue,
I still think you have an A/F that is just reading values at the edge of threshold.
If the code turns current and not pending, check if you can see the freeze frame on that data. Look at the value of your fuel trims.