Afterfire/muffler explosions bad mpg

[Alang]

Stock 7mgte in 85 MK2 auto, 86-88 ECU, 35k mi on rebuild, DDP, no cat, 3” exhaust, new plugs 2k mi
EARLY TSRM http://www.supra.co.nz/87tsrm/EFI/efi31.htm
Long term issues:
Code 51 (over 2-3 years) that has survived TPS resets. Testing the disconnected TPS harness connector still gets the code 51. AND also occasionally have a neutral start switch issue where I have to use neutral position not park to start.
Often on warm restarts, would have a miss lasting a few minutes or would go away if I did a couple of 3k revs.
Cold engine starts were not the single kick, but were a few spins.

STORY: Parked overnight no start and code 12 only. My pro wrench, Tony Orta, called an internal CPS wiring issue. He redid the internal CPS wires and dropped it back in. Showed only the 51. Seemed to start quicker and a bit more responsive.
A few days later, after a no start I found a broken CPS harness side wire and a broken O2 sensor wire probably from from the CPS R&R. I cut cleaned and twisted them together. I reset the TPS and I replaced the FPU valve under the intake manifold with a brand new one. Reset the timing. I also opened the longtime plugged charcoal canister line from the gas tank to the air (no canister). Cleaned up the engine grounds.
BUT the restart miss was replaced with low speed pedal off coasting afterfire/muffler explosions, and my MPG went from a poor 17-18 down to a 10-11 mpg although driveability seemed ok. It had been 19-20 for my driving before the plug replacement brought on by the MPG drop to 17mpg. There had never ever been afterfire/muffler explosions

NOW: My reconnected CPS wires started to show some poor connect/disconnect. The MPG is bad with the afterfires. Codes 12 and 51 are both showing.
Need some guidance as I currently have limited physical resources and time and Orta is not readily available.

Q1. Thought code 12 blocked other codes until cleared. What is the distinction when it shows with other codes?
What does 12 with another code# do to ECU/EFI management?
Q2. Would a poorly connected O2 sensor wire send the ECU into a default mode? If so, what would that be?
Q3. When the 51 refers to a NSW issue and not a TPS issue, what does the 51 cause the ECU to do or not?
Q4. Is there a cold start injector issue that overpumps gas after engine starts? Could I test by disconnecting that injector?
Q5. Would the poor CPS wire connect create the code 12 but still let the engine run??
Q6. Does shorting the disconnected TPS harness connector and getting the code 51 mean that the TPS wire circuit is the issue or does it rule out the NSW circuit/switch or not?

Have spent countless hours on searches which helped refine the questions but haven’t completed the answers.

 

[Nick M]

Stock 7mgte in 85 MK2 auto, 86-88 ECU, 35k mi on rebuild, DDP, no cat, 3” exhaust, new plugs 2k mi
EARLY TSRM http://www.supra.co.nz/87tsrm/EFI/efi31.htm
Long term issues:
Code 51 (over 2-3 years) that has survived TPS resets. Testing the disconnected TPS harness connector still gets the code 51.

The IDL contact pulls to ground. That is what the computer sees. If you have it unplugged, it will never see ground.

AND also occasionally have a neutral start switch issue where I have to use neutral position not park to start.

Park and neutral are the same. The contacts are probably getting worn.

Often on warm restarts, would have a miss lasting a few minutes or would go away if I did a couple of 3k revs.

Heat causing vapor issues.

Cold engine starts were not the single kick, but were a few spins.

What?

STORY: Parked overnight no start and code 12 only. My pro wrench, Tony Orta, called an internal CPS wiring issue. He redid the internal CPS wires and dropped it back in. Showed only the 51.

You might check that the TPS has a good connection to E2 ground. 12 will can keep you from starting. After you are no longer cranking and the engine has fired, the CPS and airflow meter provide the input to run.

A few days later, after a no start I found a broken CPS harness side wire and a broken O2 sensor wire probably from from the CPS R&R. I cut cleaned and twisted them together.

What do you say you get a high quality crimp, then fill it with solder, then heat shrink it.

I reset the TPS and I replaced the FPU valve under the intake manifold with a brand new one. Reset the timing. I also opened the longtime plugged charcoal canister line from the gas tank to the air (no canister). Cleaned up the engine grounds.
BUT the restart miss was replaced with low speed pedal off coasting afterfire/muffler explosions, and my MPG went from a poor 17-18 down to a 10-11 mpg although driveability seemed ok.

Well, you unintentionaly did something besides what was listed here. I don't know what you mean "reset TPS".

NOW: My reconnected CPS wires started to show some poor connect/disconnect. The MPG is bad with the afterfires. Codes 12

12 is the same as before. It means the computer isn't seeing the expected signal from the CPS. The muffler explosion or backfire is just unburned fuel. Usually that is from an engine that is out of time, or the ignition event.

Q1. Thought code 12 blocked other codes until cleared. What is the distinction when it shows with other codes?
What does 12 with another code# do to ECU/EFI management?

51 is independent. I am going to get the book out for the protocol with 12. Meaning a TCCS book.

Q2. Would a poorly connected O2 sensor wire send the ECU into a default mode? If so, what would that be?

Oxygen sensor is the least important input the computer in terms of driving normal. It keeps the cat in its effeciency range when in cruise or moderate acceleration. Having a problem with it won't put it in a limp home mode or anything of the sort. I think that is what you are asking.

Q3. When the 51 refers to a NSW issue and not a TPS issue, what does the 51 cause the ECU to do or not?

NSW? 51 doesn't do much. It is from the A/C being on, or the IDL contacts not being closed, evidence from no ground.

Q4. Is there a cold start injector issue that overpumps gas after engine starts? Could I test by disconnecting that injector?

Not without rewiring the circuit. It only can be turned on during cranking. And then only when temps allow.

Q5. Would the poor CPS wire connect create the code 12 but still let the engine run??

If you have 12 and your car runs then isn't the answer yes? I think you need 13 and 14 before shutdown occurs. Like I said, I am going to get the book out.

Q6. Does shorting the disconnected TPS harness connector and getting the code 51 mean that the TPS wire circuit is the issue or does it rule out the NSW circuit/switch or not?

If you short the IDL to E2, then yes, you have a wiring problem. But you would have other problems if E2 doesn't go to ground.

 

[Alang]

Is it OK to use butt connectors to connect the cps wires to the harness wires. Soldering would be better but it has been 50yrs since I last held a soldering iron.

 

[Nick M]

Crimped connection, and then filled with solder. Solder alone is not a repair. Sort of like twisting wires together. Sure it changes things, but that isn't repaired. If you use a generic butt connector, get the kind that shrinks with heat.