1991 Bronco XLT
1996 Ford Racing Motors 5.0 HO Roller
E40D, BW 1356, Manual Hubs, EGR Rear Disk Brakes
Dual MagnaFlow, 33x17 Nitto, 4" Rough Country Lift
Original TFI Push Start Ignition, Original ECM

TDC 12 info needed please

First the good news...

The KOEO passed with no codes


Now for the bad news...

The KOER gave a code 12

I have two charts which give two different definitions for DTC #12:
1) Pre 1992 Bronco, DTC 12 = Cannot control RPM during high RPM test
2) DTC 12 = Cylinder No. 3 low Cylinder balance test

Any help would be greatly appreciated.

 

I found a third DTC 12 definition while doing some research:

DTC 12 = Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too low) – ISC

 

What issue are you trying to resolve?

As you found out Code 12 indicates the PCM cannot control the idle RPM during the KOER tests. (Cannot control RPM during KOER Self-Test high RPM check.)

 

I am restoring a 1991 Bronco XLT. It has been passed around the family for almost 25 years now and it needs some work.

What we are working on at present was a hunting for idle and stall condition.
We cleaned the throttle body and replaced the gasket. I now know that a TB with orifices in the plates and a smooth bore finish are not supposed to be cleaned. I even found a sticker and put one on it. What is done is done.

We fixed a few vacuum leaks. The Brake Power Booster, EGR solenoid, some HVAC switch, and the Speed Control Module.

We eliminated or replaced bad wiring, grounds, and switches. We have replaced several bad connectors and some either faulty or questionable components: Ie.; the IAC, TFI Ignition module and connector, rotor, plugs, Distributor cap, wires, TPS sensor and connector, ECT and connector, etc... We are trying to get to all known good Motorcraft parts with good sealed connectors. This is about done.

We have eliminated most TDC's and we finally passed our emissions test today. We still have an unstable idle and the last code found is TDC 12.

Regarding the IAC/ISC: After replacing it we were still intermittently hunting for idle so we decided to do some updates along with troubleshooting to narrow the scope of potential problems.

We were convinced that an IAC spacer would be wise as well as remote mounting our TFI ignition module. The IAC spacer made things worse. We recently removed it to eliminate some variables. We also have not moved the TFI module from the distributor.


Please feel free to correct me where I err.

From what I have recently learned the PCM can alter the idle through at least two devices. The two devices are: 1) the IAC or ISC and 2) SPOUT/ignition module.

The ISC limits airflow though the throttle body at idle and the SPOUT can be used to advance timing. Therefore we disconnect both when trying to establish a correct curb idle.

Much of this seems to be from an idle screw that was erroneously adjusted in the past. The screw was also bent so adjusting it was impossible. I removed it and put an new bolt in its place. We have been going through the process for re-establishing a stable curb idle for a few days and a stated we are down to a TDC of 12.

Can you give me more details about TDC 12 and ISC operation? What should I check or measure to know precisely what is causing this error?

One more note: I suspect my TB return spring may be weak. However I would like to adequately test the ISC and either blame it and fix it or eliminate it and move on to the Throttle Body plates and/or return spring.

I do have a Lyman Trigger gauge that I could rig up and use to measure the spring, if someone knows the spring specs.

I am looking for a source of TB plate plugs and TB return springs just in case we need to go this far. If you know of any such sources, I would be grateful.