A lot of us have experienced the annoying idle surge in our Foxbody Mustangs. Before the engine is warmed up, the engine control computer is running in an open-loop with predefined settings which means it isn’t using much of the sensor data. After it gets warmed up, it switches modes to closed-loop which uses all sensor data to manage the engine including idle. At that point, if you have a bad sensor or faulty sensor wiring, the computer can’t manage the engine or idle properly.

The obvious first step is to obtain the codes and see what the computer says is wrong. Codes may be present on the computer even if the Check Engine light isn’t on.

Should Get a Code 11

This says that the computer’s internal workings are functioning normally. No code 11 and you could have some wiring problems. This is crucial: The wire that provides the ground to dump the codes also provides the signal ground for the TPS, EGR, ACT and MAP sensors. If it fails, you will have poor performance, economy and drivability problems.

Simple Diagnostics Process

If you only get a code 11, chances are you have a mechanical problem but to iron out the possibility that it’s still a computer problem, you can follow these three steps to help get to the root cause.

  • Disconnect the IAC (Idle Air Controller): Does the engine keep sputtering or does it smooth out? Keep in mind you may have to give it some throttle to manually control the TB. If it still sputters, plug it back in and move to step 2.
  • Disconnect the MAF (if equipped): does the idle smooth out? If so, you may have a MAF problem, if not, plug it back in and move to step 3.
  • Remove the SPOUT near the distributor: If it continues to idle poorly, you may have a mechanical problem like a vacuum leak or even a sticking valve.

What’s Next?

Follow the common problems related to the idle surge issue some Fox Mustang experience from time to time.