I installed an O/R hpipe on my 2000 mustang gt. After a couple days the car began to idle very erratically, dipping down to 200 rpms and sometimes dying, and ran very rich, and using a lot of gas. I plugged in steeda mil eliminators, this got rid of the check engine light but did nothing to solve the idle problem. I took the car to a garage and they said it was the IAC and it needed to be replaced. I believed it was the o2 sensors and the fact that they may be faulty would cause the rich mixture. Am i right or is the shop right?

 

It might be the iac, try cleaing it first to see if thats the problem. To clean it buy you a can of brake cleaner, or caburetor cleaner, remove the iac and spray till its soak, it should remove the build up carbon build up. Let it dry which shouldn't take long, 10 min. tops and then re-install, if you are still having problems after that, its not your iac...

 

Possibly yes.

However, some systems work differently. Most OBDI cars didnt use the o2 sensors until the engine hit a certain temperature, then they would become active, and thats when air and fuel mixture will go to hell.

On OBDII cars, they usually kick in 30-45 seconds after the car has started.

I dont know if ford still uses the o2 sensor setup from OBDI on their OBDII cars. Some companys switched, some didnt. However a faulty engine temp. sensor would cause the o2's to start reading information incorrectly and at a wrong temperature that what the computer thinks it is, causing irratic idle and such.

Since you are running a exhaust with no cats, its obvious that the MIL came on, but it is possible the o2's are bad, seems to be a common problem with fords.

 

The IAC is clean and isnt the problem, we replaced it with a good one. I believe that I have narrowed it to the o2 sensors or the throttle position sensor. The o2 sensors are the ones that are upstream and determine your air/fuel ratio. Correct me if I am wrong.

 

Your rear o2 sensors shouldn't cause your car to run rich, they detect whether or not you have cats basically. Mine are even turned off because I'm running an O/R X-pipe and didn't want to mess with MIL Eliminators.