Ok guys, I need a complete rundown of all the functions of what the stock ECU does in my '87.5 RX.

 

Fuel injection.. Fairly obvious it does, what else?

Tex, fuel injection covers cold start enrichment, fuel pump, boost enrichment, boost control via the wastegate solenoid (it can if it deems nes. increase the max boost pressure) and fast idle speed

 

The other major system is spark control, timing advance and knock retard are controlled solely by the hot wire MAF ECU with it's optical crank angle sensor distributor (unlike the vane style pre 87 systems whose dist has vacuum and mechanical controls and a stand alone knock control unit)

 

There are other minor systems which have connections to the ECU (AC and transmission)

Hope this answers your question somewhat.

hmmm, ok

 

I'm contemplating building a Megasquirt fuel injection computer, and I'm wondering if I need to keep the stock ecu. Looks I would have to.

 

Wish there was a simple way to isolate the system... any ideas?

isolate? as in just run your fuel off of megasquirt? you could do that with a carb, or a pre 87 mpfi disty.