In this, the third part of our series on Fuel Trim, we will discuss the feedback components that report what is happening at the end of the line.
When checking for fuel mixture issues and fuel trim faults, always perform the same test drive. Build a strategy that is reliable and you can trust. Remember to check fuel trim under all driving conditions.
My test drive is always the following. From a cold start allow engine to isle until warm, record short and long term fuel trim. Then with vehicl ein park, raise engine RPM to 2500 RPM and record short and long term fuel trim. Next, go for a test drive, reach a steady cruise, medium load, record short and long term fuel trim.
Confirm engine is warm and warms up in a normal amount of time. If an engine cannot reach operating temp, the DME may richen the mixture to compensate for a cold engine.
To quickly test the fuel delivery system, perform a wide-open throttle acceleration. Be sure to obey speed limits and drive only in safe areas with no traffic. Your o2 sensor should peg rich and remain rich the entire time you have the throttle wide-open. if it can not reach and hold at least 800mv the entire time, there may be an issue in your fuel delivery system. Perform a fuel system test, pressure. volume and quality.
During your test drive, confirm proper operation of the oxygen sensors. Are they switching (if narrow band). Are they within expected voltage ranges?
If you suspect an oxygen sensor is faulty or bias, you can force it to reach max range in each direction. For example, you can create a large vacuum leak and the signal should go and stay lean. You can richen the mixture using propane and the signal should go and stay rich. See the below image for details.
The example shown in a narrow-band oxygen snesor, Zirconia type. This test will also work with a wide-band sensor.
How does a standard zirconia oxygen sensor work?
Oxygen from the ambient air is fed into the sensor via a hole in the sensor body or the wires. The oxygen content on the exhaust is then pulled across the Zirconia thimble. Oxygen ions travel through the thimble, the direction depending on the exhaust gas, when this happens a voltage is created.
The problem with standard zirconia type oxygen sensors is their narrow band.
An oxygen sensor only has so much adjustment range due to the surface area of the thimble. It can quickly become saturated with oxygen. It has a range of about 0.9 Lambda to 1.1 Lambda.
An oxygen sensor has to have a center range to know where the switch from rich to lean happens. In most sensors the center is about 450mv. The DME not knowing how rich or lean the exhuast gas is (with Zirconia type sensors), it can only tell if it is rich or lean; +/- 450mv.
When testing look for a 0-1 volt waveform as shown below. If the signal seems to be bias or stuck, use the method mentioned earlier to force it rich or lean. If the sensor is swithcing and there are no fault codes, don't chase a bad sensor. Most times a faulty o2 will be picked up by the o2 monitor.
Narrow-band sensors worked well for a long time. Why did we switch to wide-band?
- Lower system costs
- Better fuel economI
- Improved drivability
- Compliance with emission standards of today and future
- Fault diagnosis regulations
- Individual cylinder fuel control
- Immediate fuel control – adapts before emission can effect catalyst
- 0.7 – 1.4 Lambda range
In a wide-band sensor, the Nernst cell is compared to the reference cell. The Pump cell is controlled to match Nernst to reference reading. The DME then looks at what it takes to reach the reference reading calculation. This is how the DME determines fuel mixture.
Wide band sensors light-off / become active 15 seconds faster than Zirconia type. Allowing fuel control to begin shortly after engine starting, usually 5-10 seconds after the engine starts.
When it comes to tesing wide-band sensors, we want to make it as easy as possible.
Step one, break the wiring diagram into two parts. One side is the heater circuit, once side in the oxygen sensor. When testing sensor signal, forget about the heater circuit wiring. The heater circuit is stroked in red.
We will ignore th eheater circuit in this article. As it is a simple 12 volt and TTL control circuit.
Use the wiring diagram for the vehicle you are working on and identify the sensor wire colors.
When wide-band sensors first started hitting the repair bays, magazine articles and case studies were telling us to monitor current in order to test these sensors. I have never done this as you have to break into the circuit. Personally, any sensor that has gold plated terminals certainly shouldn’t be disturbed by cutting or piercing wires. With that said, you all know that you do what you have to do.
Current works like this:
- Pump out: lean positive
- Pump in: rich negative
5 – 10 mA is normal range
While we discuss this wiring diagram, keep in mind the terminals will vary and this only applies to the MINI I pulled the wiring for.
The wide-band sensor shown uses a 2.5 volt floating ground. This is done to reduce noise and give a specific reference point for "zero".
Below are the voltages you will see when backprobing the sensor connector, engine on and idling, when warmed up.
You will see the higher voltages 2.5+ when connected to battery negative for testing. if you connect your meter ground to the sensor ground, you will see what you may be more familiar with, 0.450mv.
When testing with a scope, expect to see the same voltages as shown above. The patterns below show the connection terminal with arrow color to match waveform color. tester ground is connected to battery negative.
When connected to the sensor ground, the waveform we see is similar to a standard narrow-band oxygen sensor. With a range of 0-1volt.
This is the same test I run with narrow band oxygen sensors. WOT – the air fuel ratio sensor stays rich. This is still a good test.
You can also force them lean and rich as we did earlier with narrow-band oxygen sensors.
In the next part of this series, Diagnosing Common Fuel Trim Faults, we will discuss the common causes, how they affect fuel trim and fuel control and how to use DME live data in your scan tool to diagnose them.