LTRM,
gee seeing what you said about all the problems with this truck before you started i was reluctant to even say anything here, that truck sounds like its a wreck... but....i may be able to help a little bit...

this is an excerpt right out of the troubleshooting HPK doc from MCFA about troubleshooting the S15G system. Naturally i cannot post the whole document because of my position but i can give you this bit of info that may assist you in some way.

Test the oxygen sensor:
a. Observe the oxygen sensor voltage, 0.2 to 0.9 volts is the desired range.
If the oxygen sensor voltage is 0.9V or above, the dash light will come on
indicating a system problem. The error code will be for the oxygen sensor.
This is normally caused by an incorrect dwell reading with the resulting
problem being in LP mixer or the throttle body.
NOTE: Rarely is the oxygen sensor the problem.


Do you have the means to run MCFA Diagnostic program?
If so you may need to do this... if this is a OEM mixer then the dwell screw might not be readily available and you will have to remove the plug they put in place over it. Once that is removed a adjusting tool can be inserted into the hole to the adjusting screw to make the adjustment. DO not attempt to adjust this without using the Diagnostic tool or you will fubar it up worse. Keep in mind though that the plug is in place for a reason, these are pre-set during production and are only intended to be adjusted by qualified personel. ;o)

since you said you put a new mixer on i thought this may come in handy. The ideal dwell angle the S15G system runs at is between 35-45 % at idle and 80-100% at full RPM
You also may want to double check the throttle body.

I believe the new mixer diaphram part number that addresses some issues with the earlier version mixer is SE000071. Being a new mixer it should already have this updated diaphram in it.