Ωhm

Question. Was this voltage reading steady, or did it follow the actions of the fuel pump relay (ON for 2-3 seconds)?

That 'proved out' its not your heater circuit giving you your problem.

During a WOT condition, exhaust manifold is acting like a giant blow torch. More fuel. No need for the O2 heater circuit, only more fuel. Now is when the O2 heater relay is energized (heater circuit OFF) and B+ is supplied to the normally open (NO) contact. This contact bypasses the fuel pump ballast resistor applying B+ directly to the fuel pump, where its needed most.

O2 heater relay doesn't turn O2 heater circuit ON (de-energized), it turns it OFF (energized).

Yes When relay coil is OFF (de-energized), relay pins 1(30) and 3(87a) make contact. These are the pins used for O2 Heater circuit. No These types of relays are all structured and function the same.

One thing I'm having problems with (88 Elect Manual pg16) is O2 Heater Relay pins 1(30) and 5(86). Check color code, maybe swapped when compared to Elect Manual.

That proved it out.

Disconnect O2S. Measure ohms across C229_A and C229_B at the component. Looking for ≈6ohms. On the connector harness side measure C229_B for a path to ground and check voltage at C229_A (voltage will follow the actions of the fuel pump relay) so with KEY ON, voltage will only show for 2-3 seconds. Retry if necessary. O2 sensor (Input side) receives power from the ECU, not the heater relay. Both heater and input share a common ground.

Now I'm sure you guys have 'proved out' all that heater circuit stuff.

You've got two (2) horns and a horn relay. Run B+ to each horn and see if its truly the horns.

Maybe the heater core bypass valve is stuck open. Does the lever move between HEAT and A/C modes?

Your call.

Don't complain about loss of A/C cooling then.

True, but my response was for those that do use MPI.

Even if the SYNC sensor is self-correcting, it still serves a purpose. Fix it, make it work. Don't force the ECU to go through its self-correcting phase every time you start the engine. Prevent longer crank times, better HOT starts and any other thing caused by NO SYNC sensor, until it self-corrects. Granted, INJ's put fuel into the manifold and the engine will start, but SYNC INJ's puts fuel where and when its needed most.

Which fuse?

Clock back light is HOT (Radio fuse) with the KEY in the ON or ACC position. Clock back light is also HOT (Park fuse; Headlight SW ON (dimmer circuit); Inst Lps fuse). This function (one way or the other way) is made possible through the Radio/LCD Illumination Relay.

Now you're going to need to keep an eye out for traffic cops.

RENIX ECU knows when EGR is requested and cuts back on INJ PW during that time. So actually without EGR engine goes lean (these are very short time durations). ST/LT fuel trims should compensate for this.

That harness you have is not factory. It's a 'someone added me on harness'. Harness needs backtracking or if you're feeling lucky, just connect it to your loose wires one at a time.

From that connector in your photo, backtrack the harness to wherever it goes and see where it splice into the MJ harness. Could be a second plate of spaghetti wires waiting for you. Maybe fuel gauge wire live there or can tell you which wire goes to the gauge.

Fuel pump running 'fine' and 'as intended' can be two different things. Does fuel pump cycle ON, then OFF when KEY is turned to the ON position?

What year???

Just a thought here. Are all these overheats running in closed loop? HO's run selftest, RENIX owners without REM, don't know what to tell you. Lean=Heat.