Engine Code 94 and 13

Hey guys, first time poster...

Got myself a 1991 Mustang GT. I ran KOEO codes, and everything was fine. 11-10-11.

Ran KOER codes and got a 94 and 13.

Code 94: Thermactor Air System – Air Diverter (TAD) System Fault (Driver’s Side).
Code 13: Idle Speed Control – Cannot Control Idle at Engine RPM.

After doing a fair amount of research, I think code 94 indicates a blockage before the driver side heads from the TAD valve. It's typically accompanied by a code 44 (meaning the passenger side is blocked, too). Getting BOTH of these codes would be common, I think, if the smog pump or TAD valve wasn't working correctly. It means it's not pumping air to either of the heads.

The fact I'm NOT getting a code 44 might indicate an actual clog in the air crossover tube. I think the air goes from the smog pump, to the TAD valve, to the crossover tube, which first goes to the passenger side heads, then finally to the driver side heads. The fact that it seems to be going to the passenger side, but NOT the driver side would maybe indicate a clog in the Air Crossover Tube, right?

Btw, I live in California so removing this stuff (permanently) isn't an option for me.

My car has 70,000 original miles. Is it really possible that the Air Crossover Tube would be plugged up with so few miles?

So my questions: is this sound logic? Does this indicate my crossover tube must be clogged (with only 70k miles)? Or could this code 94 (and lack of 44) indicate something else?

And any thoughts on the code 13? Could this code be related to the 94?

I'm asking here first, because taking the Air Crossover Tube off is a PITA.

Thanks guys!

Thank you!

The Code 13 info you sent me is very interesting (tbh I haven't looked much into that issue yet). It sounds like Code 13 will trigger if my base idle is above 725, which it probably is. Although the idle is stable (no bursts or stalling), I can't really get it below 750-800 with the throttle stop screw, it closes the throttle plate completely at about 750. Also, when the car is warmed up, the base idle will sit around 900.

Would this indicate a vacuum leak? I've smoked tested it from the throttle body and I haven't seen any smoke leaking


Regarding the Code 94:

Could this be caused by a bad O2 sensor? I ended up ordering a couple. I think any problem before the crossover tube would cause a Code 44, too.

The only similar scenario I've seen is where this person was getting a Code 44 without a Code 94 (so the opposite of my problem, I guess):