An exhaust gas oxygen sensor (EGO) is very useful for setting up the MegaSquirt volumetric efficiency table, and while it is highly recommended, it is not essential.
MegaSquirt can read from just one oxygen sensor. People who have engines with separate cylinder banks (V6, V8, etc.) will have to make a choice:
They can use one sensor in the crossover pipe between the exhaust pipes (which they may have to add), however the sensor will be a long way from the heat of combustion and you may require a heated sensor (see below),
They can put a sensor in just one bank, as close as possible to the point where the exhaust gases from that cylinder merge, and assume the other bank is the same. Many manufacturers did this in the early days of electronic fuel injection,
They can put in two oxygen sensors, one in each bank, with a switch on the lead to MegaSquirt for the driver to choose which bank of cylinders to read.
It is very important that there be no exhaust gas leaks upstream of the oxygen sensor. Certain conditions can draw ambient air into the exhaust, causing MegaSquirt to compensate for an apparently lean condition. This will falsely create a rich mixture in your system that can be difficult to diagnose. Those who have converted late-model “emission” engines, should be careful to understand and modify the operation of any air injection systems they have to be sure air is not being pumped into the exhaust ahead of the oxygen sensor(s) during “closed-loop” operation.
Closed loop refers to those times when an EFI computer is using the feedback on the mixture provided by the oxygen sensor to effectively control the injected amounts. For MegaSquirt, this is when the engine:
has been run for 30 seconds,
the engine RPM is above the “EGO Active Above RPM”,
the coolant temperature is above the “Coolant Temp Activation”, and
the TPS is below 70%.
See the Settings/Enrichments section of MegaTune. “Open Loop” refers to those times when MegaSquirt ignores the feedback from the oxygen sensor. Note that MegaSquirt also allows you to set limits on how much (EGO + Limit () and how fast (EGO Step ( and Ignition Events per Step) the oxygen sensor feedback can influence the injected amount.
One, three and four wire narrow band O2 sensors [NB], and two wide band sensors [WB] are currently available on the market. MSD offers a heated sensor under part number 2330.
MegaSquirt was originally designed with an interface to a basic narrow band 02 just for cruise. Bruce, Al and others are working on options for wide band [WB] EGO sensing and tuning, and the current tuning software accommodates both the narrow band and wide band stoichiometric and voltage slope characteristics.
Narrow band O2 sensors are designed to measure stoichiometric [chemically correct] air/fuel mixtures [A/F] of 14.7:1 to allow catalytic converters to work efficiently. Narrow band sensors always have one wire for the sensing function. Additional wires are for the heater and its ground (3 wire sensor), and possibly an additional wire to ground the sensor itself (4 wire). The sensor needs to be quite hot to operate. The heater keeps the sensor at operating temperature under more conditions.
Standard Motor Products SG5 (~$18) is a one-wire sensor for a mid-eighties Chevrolet; the mating connector is S554
Bosch 12014 is a one-wire sensor for Buick Grand Nationals
AC DELCO AFS75 is a four-wire sensor; PT368 is the matching “pigtail” connector for splicing into your harness (from a 95 Corvette 5.7l V8). You can also get this as a Walker 4-wire OEM replacement pn# 250-24012, priced ~$50 (black connector end), or the Walker 4-wire UNIVERSAL replacement is pn# 250-24000, priced ~$49 (solder/crimp wire ends)
Bosch 13942 is a heated sensor for Ford 5.0L V8 Mustangs
The difference between the heated (3 or 4 wire) O2 sensor and a non-heated (one wire) sensor is the A/F ratio sensing of warm up and low load conditions. The heated sensor uses an internal coil to heat the ceramic element to the desired 400° Celsius in 30 or 40 seconds. This temperature is also maintained when the car is at idle for extended periods of time or is under low load conditions where the exhaust gas temperatures fall below 400° C.
Under other operating conditions the exhaust gas temperature will be much greater than 400° C. and heating is not necessary. The non-heated sensor relies on the exhaust gas heat to keep it at its operating temperature. This works most of the time but there is still times that it might drop below its desired operating temperature and show a leaner than actual mixture as its output drops to zero.
A 1-wire sensor is as good as a 3-wire provided that it is always at operating temperature. If you cruise around for a bit with the engine at low load, the O2 sensor COULD cool down. If you do not have exhaust gas temperature [EGT] monitoring then you cannot be sure. Once warm, a 3-wire O2 sensor will stay warm. For most of us the one wire will prove to be adequate. A 4-wire has a shielded cable. You only need to ground the shield at one end. In many installations there is not enough voltage drop from the manifold to ground to make shielding worth the bother, but every little helps. So the more wires the O2 sensor has, the more situations in which the sensor will be active and accurate, but you are still stuck with knowing whether you are rich or lean, but not by how much.
MegaSquirt software has some support for Wide Band (WB) EGO sensors. Such sensors are made by:
NTK (L1H1), also sold as Bosch 13246, and
Bosch (the LSU4).
These sensors (with an appropriate controller) have a different trigger point for stoichiometric compared to a narrow band sensor, and the opposite “slope” to the voltage curve. They require a separate driver board to operate, such as the:
Tech Edge controllers, or
See the links for details.
If you are using a wide band sensor and controller with MegaSquirt, select WB on the PC Configurator or MegaTune Enrichments screen and set the EGO switch point to 2.500 volts (for stoichiometric AFR) to take the WB characteristics into account.
Connecting the DIY-WB controller to MegaSquirt is really easy. The DIY-WB has a number of wires going to the sensor (you can cut the connector off the sensor, and run the wires all the way to the DIY-WB case, and install an equivalent calibration resistor there.) The DIY-WB board also has power and ground wires (two sets, which you can combine). These connections are detailed on the DIY-WB site. To connect the DIY-WB board to MegaSquirt, the output signal from the DIY-WB (J8) goes directly to the EGO sensor pin on MegaSquirt (pin #23 on the DB37 connector)
The DIY-WB can be calibrated to a "free air" reading of 4.00 volts.