Ωhm

No ballast OK. But I would suggest you swap the wires (1(30) with 5(86)) on your O2 Heater Relay. Currently your O2 heater circuit current flow through your IGN SW (12vdc/6ohms=2amps) with KEY ON. This 2amps is above everything else that flow through the IGN SW. One upon a time, when MS1700 was the tester of choice, vehicle would have failed and the fix was this swap. Also if ballast resistor was installed, part of the instructions was to include this swap. Suggestion only, feel free to ignore.

That a good starting point.

Okay. They had a kit (fuel pump noise) out there where one could add a ballast resistor. This included the O2 heater relay update.

This show O2S has slow response time. Should flicker LEAN/RICH at the speed of light. Less than 128 takes fuel away (decreased INJ PW time). O2 sensor see a RICH condition.

Your wired according to the 88 Elect Manual schematic. That is IGN feed (YEL) to pin 1(30) and FP relay feed to 5(86). Do you have a ballast resistor?

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.