Idle issue, tested everything. wtf..

Do you run an oiled filter? I've been having trouble with my MAF getting dirty and causing all kinds of funky idle problems that aren't consistent. Every time it starts to do this too me, I clean the MAF and it clears up. I've been looking around at retrofitting an airbox back onto the car so I can run a panel paper filter again.

Do the "Surging Idle Checklist set mechanical base idle proceedure and see if the idle will stabilize for the test. If it doesn't stabilize, you have mechanical problems. That means vacuum leaks, faulty injectors, weak cylinders or bad valves/valve springs.

Idle does stablize when you do the set mechanical base idle proceedure. That means you have electronic sensor or actuator problems.

Disconnect the IAC/IAB: does it still surge?
Yes - reconnect the IAC/IAB and check the timing advance with a timing light. Remember to remove the SPOUT when you check the timing. If the engine stops surging when you remove the SPOUT, your timing is definitely off.
No - clean or replace the IAC/IAB.

Check the TPS sensor - see http://www.stangnet.com/mustang-forums/825424-adjusting-your-tps-0-98v-not-necessary.html for help.

Check grounds:
2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges.

Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just a much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972


Correct negative battery ground cable.


3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.


4.) All the sensors have a common separate ground. This includes the TPS, ACT, EGE, BAP, & VSS sensors.

5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.

It sounds like an intermittent bad electrical connection. Start jiggling wires and see if anything changes.