With this update I finally complete the installation of the MegaSquirt ECU into the car. This mainly involves wiring up a throttle sensor, rebuilding the intake pipes for an air temp sensor, and installing a Wideband O² sensor.
When finished, the engine bay looked remarkably similar as before, with the notable exception being the shiny customised intake pipes that replaced the air-flow-meter. With the flappy MAF meter removed I’m now 100% reliant on manifold vacuum to regulate fuel injection volume, along with air temperature and throttle position.
A manifold vacuum hose was taken from the existing fuel regulator tap, and using a Y splitter, a new line was brought into the cabin area and plugged into the ECU. The MegaSquirt contains a MAP sensor that measures air pressure and is used to regulate fuel delivery.
It is recommended if possible to use a Throttle Position Sensor (TPS) when using a MegaSquirt ECU – these tend to improve throttle response with acceleration enrichment, but my model MX5 only came with a Throttle Switch (simply closed/open and no measurement in between). Fortunately, the 3spd Automatic MX5′s did come standard with a TPS and as they are a simple bolt on, I acquired a 2nd hand throttle body and soldered some wires to the connector. 
To connect into the stock harness (and to avoid cutting it) I fashioned a plug using some plastic and stiff wire – the result is something that can be disconnected and yet be firmly secured to the existing stock plug.
The Air Intake Temperature (AIT) sensor needs to be mounted in the stream of incoming air and the first logical place seemed to be where the intake pipe would need to bolt onto the air-filter box. Unfortunately this required mounting the sensor right behind the radiator, and this resulted in unacceptable heat-soak (a condition where the sensor itself if heated to beyond the ambiant air temperature resulting in incorrect readings). I then had to move the sensor to inside the filter box, and this provided more regular air tempatures without heat-soak problems.
The final task was the installation and wiring of the Wideband O² sensor and this required installing a controller module within the engine bay (seen next to the washer reservoir). To enable the sensor, you need to supply 12V for heating of the sensor along with wires back to the ECU. The Innovate LC1 sensor (the type used here) provides a simple harness that I adapted to a DB15 connector that plugs into the DB15 port on the DIYPNP case. This provided a fairly simple place to mount the LC1 calibration switch and diagnostic LED – both of which are now mounted as part of my DIYPNP ECU case.
So, the final step is to connect the ECU to the stock harness, removing the original computer and neatly rest the new ECU into the protected cubby hole. 
After uploading a tune file (obtained from a fellow MX5 owner with a similar MegaSquirt ECU) I fired over the engine and it kicked into life – running a little roughly and spluttering to a stall when any throttle was provided, but it would start and idle reasonably OK. After a little tuning of the fuel maps, adjusting for acceleration enrichment, the car seemed to be running fairly well. Last step was to provide a method to easily connect to the DB9 communications port on the back of the ECU – so I added a ribbon DB9 extended assembled from $6 worth of parts from Jaycar.
Tuning the ECU is a subject I’m not going to cover here, but suffice to say I’m still doing it several weeks later and feel I’ve got a bit more to do to get it to behave 100% like stock… so you can have respect for the OEMs and the complexity that goes into modern computer engine management systems.
You can download my tune file for my car that would be a reasonable starting position for any similar NA6 owner who is keen to start at known good position with a recent firmware from MS2/Extra – at this time its based on the 3.2.0 release.
