[meangreen92]

Question:

Can an exhaust leak at the header cause this strange behavior with the idle?

I had both headers leaking near the H-pipe, (they had holes from rust) while I don't remember it making the idle really erratic, it did cause a slight hunt at times. What happens is, the exhaust stream pulls outside air into the exhaust, and past the O2s. The O2s smell this outside air with it's oxygen molecules intact, and reports to EEC that the motor must be running lean. EEC responds by injecting more fuel. So you'll have a car whose exhaust smells richer than Bill Gates, yet EEC tells you it's running lean.
This problem is analogous to a leak between the MAF sensor and the throttle body - they are both "false air" scenarios.

 

[meangreen92]

Scratch that whole thing about exhaust leaks and idle. I ran mine for a whole day on open headers and no problems, no check engine lights all day. Neighbors loved it.

True, no illumination of the check engine light, but, were you to have scanned EEC, you would have pulled some codes. Also, were your O2s disconnected when you did this? That would make a difference, although, I would think that you woulda gotten a light.
A leak at the head will only affect that cylinder's worth of exhaust. But a leak down by the collector, affects all four cylinders on that bank.

 

[meangreen92]

93BlackMustangGT, I would go ahead and fix the exhaust, do any other tuneup stuff you haven't done (use the checklist on this site as a guide), and then, lastly, throw in a new set of O2s and disconnect the battery for thirty minutes, reconnect and start the car and let it idle for a few minutes. If it still acts up, put your old O2s back in, and repost.
In response to what '95.0 stang said, if I understand Charles O. Probst correctly, the MAF foxes will let you set the speed - as long as it's between 650 and 700 RPM.

 

[meangreen92]

Charles O. Probst wrote: How To Understand, Service and Modify Ford Fuel Injection & Electronic Engine Control. The idle setting procedure he gives (only for the Mass Air fox bodies) is complex, and I don't think that I really understand it. But I think that it's somewhat tailored to the first startup and run in of the engine. I bought that book knowing that it would be alot more help than the Haynes manual in that area. My car came with a removed "check engine" light bulb - or MIL.
Once I resolved all my issues, all I did, was adjust my throttle body so that the blade wouldn't bottom out, and went with it.
Another thing we're neglecting here, is battery cables. I've noticed my '92 will idle poorly and roll, when the battery terminals start to get cruddy.