Fuel trim malfuction - 2000 Mercedes-Benz S-Class

You don't say what engine ,does it have E85 flex fuel .assume USA vehicle ?

Fuel Trim
The engine control module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage while in Closed Loop and regulates the fuel delivery by adjusting the pulse width of the fuel injectors based on this signal. The ideal fuel trim values are around 0 percent for both short term and long term fuel trim. A positive fuel trim value indicates the ECM is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel delivery changes the short term and long term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltage. These changes fine tune the engine fueling. The long term fuel trim makes coarse adjustments to the fueling in order to re-center and restore control to short term fuel trim. A scan tool can be used to monitor the short term and long term fuel trim values. The long term fuel trim diagnostic is based on an average of several of the long term speed load learn cells. The ECM selects the cells based on the engine speed and engine load. If the ECM detects an excessive lean or rich condition, the ECM will set a fuel trim diagnostic trouble code (DTC).

code p0170 refers to two problems
fuel trim bank 1 --malfunction
heated oxygen sensor(HO2S) 3 bank1 --circuit malfunction
for the first error==causes-- intake leak --AIR system---fuel pressure /pump---injectors--EVAP canister purge valve---HO2S
for the second one ===causes=--wiring--HO2S---ECM
The explanation of fuel trim is long winded and you may get some typo errors but I think that you will get the idea
short and long term fuel trim refers to the strategy used to reduce exhaust emissions after a basic computation of injector period, using engine load as a major parameter
both front and rear oxygen sensors signals are used to fine tune the air/fuel mixture by increasing or decreasing the injection period =/- 25% above or below the basic level
any fault requiring a correction beyond this level will result in a fault code being logged ( in this case p0170)
the fuel trim will oscillate the injection period =/- 5% above or below the mean level level A
manufacturing and in-service tolerances of load sensors (MAP,MAF, VAF) and injectors in particular and faults such as intake leaks( vacuum hoses from vent controls and transmission) will affect the air/fuel mixture and cause the trim to compensate
an air intake leak for instance would increase the injection period increasing for example to 115-125%. this level will also oscillate +/- as before level B
this new short term fuel trim level will be stored in the ECM if it is established as a new basic mixture level
It will become a long term fuel trim correction and results in the correct mixture level immediately after starting, even when the HO2S has had sufficient time to heat up
Now having some idea of what trim is, you will see the necessity of repairing any problem sensors ,air leaks ,vacuum leaks
You will also probably understand the reason of why an engine after running short runs ( city driving) feels that it gets more power and better economy after 1/2 hour on the open highway
The ECM resets the trim to the new load power settings and that becomes the new trim

if its over 100k its best to replace the whole fuel pump assembly with oem or airtex and try not to run the tank below 1/4 as the pump uses the fuel to keep itself cool

DTC P2187: Fuel Trim System Lean at Idle

Fuel Trim Description
The Engine Control Module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled differently during Open and Closed Loop. During Open Loop, the ECM determines fuel delivery based on sensor signals without Heated Oxygen Sensor (HO2S) input. During Closed Loop, the HO2S inputs are added and used by the ECM to calculate short and long term fuel trim fuel delivery adjustments. If the HO2S indicate a lean condition, fuel trim values will be above 0%. If the Heated Oxygen Sensors (HO2S) indicate a rich condition, fuel trim values will be below 0%. Short term fuel trim values change rapidly in response to the HO2S signals. Long term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 14.7 : 1.

This code can only set when the engine control is in Closed Loop. The engine must be fully warmed up.

This must be diagnosed using a scan tool that can read engine data in order to determine the actual cause of the problem. It can be caused by the following things:

> Vacuum leaks
> Malfunctioning, dirty, or skewed Mass Airflow Sensor (MAF)
> Malfunctioning or skewed Manifold Absolute Pressure Sensor (MAP)
> Dirty or sticking injectors
> Malfunctioning or improperly programmed Powertrain Control Module (PCM)

If the car is running fine, then that is a possibility. The mechanic should have cleared the code. If you ever have an issue that triggers the ECU, then that is stored in the memory, and will need to be cleared.

DTC P0171 is set when fuel trim is driven rich beyond a certain value. Possible causes are:
1. Exhaust system leak.
2. Improper fuel pressure.
3. Fuel injector restricted.
4. Defective heated oxygen sensor No. 1.
5. Defective Manifold Absolute Pressure (MAP) sensor.
6. Defective Engine Coolant Temperature (ECT) sensor.
When the air fuel ratio feedback is stable after the engine is at operating temperature and the fuel trim has reached its limit of correction to the rich side. (2 trip logic).
Feedback from the HO 2 sensor influences short-term fuel trim and short-term fuel trim influences long trim fuel trim. Short-term values are temporary and not stored when the ignition key is turned off. Long-term values are stored in memory because they are part of the basic injection duration. Long term values affect injection duration in closed and open loop because they are used to calculate basic injection duration. It is important to remember that the actual fuel trim will be the opposite of the DTC. A system too lean, DTC P0171 will mean the ECM is making a + or rich correction.
read (and let me know)above data and Freeze Frame by using (example, generic) Scan Tool on # mode 2, etc.
Slowly accelerate the engine in the increments shown and record voltage,
grams/sec of MAF and SFT, LFT, Injection Pulse, Fuel Pressure, etc.
HTH,
Vehicle have MAF Sensor?

It measures the oxygen in the exhaust system, for instance, if the fuel is to rich it sends values to the computer and the computer will adjust the fuel and timing to bring it back into perimeters

Firstly, these codes are set when the fuel 'TRIMS' are out of spec (either tool LEAN - P1123, OR too RICH - P1128). The air/fuel control system, in addition to a number of sensors, includes the following components and systems: a. Intake air system. b. Exhaust system. c. Evaporative emissions control system (includes purge control solenoid valve). d. Fuel injectors. e. Fuel pressure regulator. f. Fuel pump. For the air/fuel ratio to be within limits, all of the sensors, components and systems associated with the air/fuel control system must function within normal parameters. STOP replacing parts/sensors unless you can VERIFY they are defective! The Powertrain Control Module (PCM) on Hyundai OBD II vehicles (all 1996 and later models) can adjust the engine Air to Fuel (A/F) ratio to compensate for variations in operating conditions. This adjustment only takes place during "Closed Loop" operation. The PCM uses input from the following components to determine the amount of fuel to deliver:

• Oxygen sensors
• Air flow sensor
• Coolant temperature sensor
• Intake air temperature sensor
• Throttle position sensor
• Engine speed sensor

The ability to adjust the A/F ratio enables the PCM to compensate for engine, fuel and sensor variations as well as temperature, altitude and humidity. This enables the vehicle to maintain a stoichiometric (14.7 to 1) A/F ratio that will ensure optimal engine performance and minimal emissions. Since the PCM can change the A/F ratio to compensate for a wide range of conditions, an indicator is needed to identify that a compensation has been made. Fuel Trim values shown on the HDS scan tool are the primary indicator that an adjustment in fuel delivery has taken place. The scan tool displays the PCM's A/F ratio compensation as "Short Term" Fuel Trim (ST TRIM), Long Term Fuel Trim (LT TRIM), and "Oxygen Sensor" Trim (TRIM B1 S1 and B1 S2). The most beneficial diagnostic information comes from "Short Term" Fuel Trim and "Long Term" Fuel Trim. AFTER ALL THIS, I think it's simply your Coolant Temperature Sensor giving 'FALSE' information to the engine control module/powertrain control module (ECM/PCM).

I'm not quite sure about the code in Hyundai's case but it definitely relates to fuel trim...

I could try to hash down the explanation for Short term fuel trim and Long term fuel trim in this post but I think a visit to
http://www.wellsmfgcorp.com/pdf/counterp_v6_i4_2002.pdf
would be more thorough and appropriate.

Basically, a high value of long term fuel trim indicates that your engine has had and will have to continue to inject a lot more or a lot less fuel to attain the correct air/fuel ratio for a proper burn, which mainly indicates a malfunction somewhere (could be lots of things)