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Is this a 2.5L? Poster needs a signature with vehicle information.

That will work. Looking for the 5vdc Supply Voltage (C218_C) to drop out when vacuum is connected to MAP.

Not sure exactly what I'm trying to do here, but with C218 connected, backprobe C218_C with (+) and use battery negative for (-). With KEY ON check for 5.0vdc. If you have ≈5vdc on C218_C, START ENGINE with MAP vacuum hose disconnected. Watch DVOM. Should hold near ≈5vdc. Now plug in MAP hose, watch DVOM, does ≈5vdc reading drop to 0vdc?

https://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Isn't C218_B=MAP Output and C218_A=MAP Ground?

Is this 2.1L the same diesel covered in the 87 Electrical Manual?

a.k.a. ECT - factory location

I took it as wire was under the dash. Both YELLOW or ORANGE wire can be tap at the Fuel Pump Relay, if needed.

More options. The closest thing we have to an ENGINE RUNNING circuit is the Fuel Pump Motor circuit. Circuit is HOT with KEY ON in RUN position but will time out if ECU see NO CRANK or NO ENGINE RUNNING. Orange wire down by the driver's side kick panel.

System Sentry (SS) only comes ON with ENGINE RUNNING. So, you don't have ENGINE RUNNING only position on your IGN SW. I guess it's a great circuit for ENGINE HEATER Relays. Question becomes when do you want the SS ON or OFF in this case? IGN SW set to RUN will be HOT with ENGINE RUNNING or ENGINE NOT RUNNING. SS will be on with both of these conditions. Okay. So don't leave your KEY ON without the ENGINE RUNNING, could drain your battery. Tap YELLOW wire off the IGN SW (C267_I1).

First, if I'm understanding this at all. Just how were the choke heater relays controlled (I1_I3_A)? Meaning when did that wire go HOT? Ignition SW has different contacts for different KEY positions (ACC_OFF_RUN_START). This would be considered an IGN SW I1 Feed (RUN_START(HOT)).

Just as a quick check look for fuel at the vacuum hose between the Fuel Pressure Regulator (FPR) and intake manifold. Leaky FPR.

Yes and Yes. Resets LTFT back to 128 with each battery disconnect. 88 FSM Computer Relearn Procedures.pdf Basically, it says just keep driving the vehicle after each START UP.

Yes Use EX=RICH/LEAN instead of A/F. See if LTFT changes? Note from REM readouts: Note: This is an REM only reading and is very approximate since narrow band o2 sensors are not absolutely accurate or consistent. This is only used to put a “Real” number to the otherwise meaningless o2 voltage, and should not be used for real tuning needs.

Most O2S heaters fail OPEN circuit. Your ohms reading was good. Sorry if I miss led you on this being good or bad. Does the switch rate with the new O2S (REM set; EX=RICH/LEAN) change at a faster rate than the old O2S did? Does vehicle still stall on deceleration? I see you just posted a video, let me check it out.

If/when it happens again set AC Mode Select Switch to VENT or HEAT. See if the problem goes away. Quick check.

If you're talking about the 88 and if equipped, could you tell if the AC Clutch engaged with the fan ON? ECU will kick idle up if it sees AC SELECT/AC REQUEST signals.

Not sure if this has been posted up before or not. Comanche Trackhawk

Disconnect O2S. On the O2S connector (component side) look for ≈6 ohms between C229_A and C229_B (heater). Engine Control Harness side shown below:

I've seen VAC=15"Hg before and @NickInTimeFilms mentions "greater than 15"Hg is good. But I've always like 18"Hg of vacuum for Intake Manifold vacuum, so OFF TOPIC here, what is a good reading for Intake Manifold vacuum ("Hg)? I don't think this is your problem. What I did notice is your O2S is sluggish. It does switch enough to keep ECU in CL, but should switch between RICH/LEAN closer to the speed of light (very fast). Suspect O2S. Thanks for the video.

STFT is trying to take away fuel (sees RICH). Don’t know why LTFT stays at 128. Not sure if driving vehicle is necessary, let’s see how Fuel Trim’s act at IDLE. Watch the Fuel Trim's. Also, At IDLE does O2S signal switch rapidly between RICH/LEAN during Closed Loop (CL)? What is your INJ PW time (mS)? Did you mention before VAC (not MAP reading) was ≈16”Hg, or was that the MAP reading. If so, what is VAC (“Hg) @ IDLE? Just as a quick check look for fuel at the vacuum hose between the Fuel Pressure Regulator (FPR) and intake manifold. Leaky FPR.

Let's see what the ECU thinks. Using your REM, what are your STFT and LTFT values?

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