Air/Fuel? Vacuum Leak? Help..

Okay guys.. to start.. Its a 99 Stang GT. (4.6L SOHC) The car has BBK Long tube headers, X-pipe, MSD Ignition, CAI, Under-drive Pulleys, Stage II Clutch, Steeda Short-throw shifter.. everything else is believed to be stock on the car.

The car used to hesitate at idle and jump up and down and sometimes get so low that it would stall. Also, if I rev the engine up.. the engine wants to stay there for a few seconds before climbing back down.

I first tried Idle Air Control Sensor, this didn't change anything.

I changed the Air Charge Temperature Sensor & Cleaned out the entire throttle body.
-this seemed to have stopped the fluctuation at idle but..

The car still climbs rpms and stays there a few seconds before coming back down, as well as imbetween shifts.. the car wants to stay at those rpms with out free-falling. As well if your just cruising down the street and put it in neutral, the cars rpm's bounces up and down between 1000 - 1500rpms until you come to a stop and/or put it in gear.

The rear O2 Sensors have been disconnected from previous owner, but I don't think this would cause this issue.. but I could be wrong? (I plan to get those changed and plugged in ASAP.

What sounds to be the issue? Vacuum leak? Suckin air somewhere? EGR fault? Throttle Position Sensor?

I'd like to get some help where to start before spending a bunch of money on sensors that may not even need to be replaced.

Thanks in advance for you inputs!

Thanks for that information...

I did the test on the IAC, and unplugged it while the car was running. Made no difference in RPM's. Still stayed the same..

So shall I think this is a Faulty IAC, or the Black Vent between the Air Intake & IAC ?

Thanks, Ill give that a shot. I also want to note that my Speedometer does not work, and the most case scenario is that the VSS is bad or wires are.. but in either case, I have learned that the IAC recieves a signal from the VSS and if the VSS isn't working properly if at all, then the IAC won't work properly causing the RPM's to hang for a few seconds before falling (on manual cars). On Automatics, they car would shift very hard if at all.

So I'm going to start with the VSS as it is the cheaper part and go from there...

I did replace the TPS today and it had no affect on the car at all.

Once you unplug the IAC and rev the motor, the rpms do fall back normal and don't hang at all.

You claim that my car does not have a VSS and it has an OSS but in fact I can't find an OSS for my vehicle anywhere. Every place I search, including local part stores all have the VSS for my car.

So essentially it would be the OSS, Cluster Itself or Computer... for reasoning why the speedo does not work.

I should mention that my ABS light is on as well. (but i figured that was more dealing with traction control, what not - not so much the speedometer or this idle / surge issue)

wmburns said:

If the idle returns to normal levels with the IAC disconnected, the only logical conclusion is that the idle was set using the TB set screw.

I suspect that you do not fully understand how the IAC works and how the TB set screw affects idle and the behavior of the idle.

Click to expand...

I am fairly new with some of these components so you'll have to excuse my ignorance when it comes to some things. I am here for others help and troubleshooting techniques. So I do appreciate your help..

The car sits right around 800rpms at idle. I did notice that the throttle body set-screw has been tampered with. The threads look pretty boogered up; but I will try and back it off a little if I can. What is normal readings for rpms at idle? 600-700 rpms?

I won't have a chance to mess with it until tomorrow; but do you suspect that its opened too much and not closing fully when letting off the gas causing the RPMs to hang and not free fall back down?

Thanks sir. That does explain a lot to me. I guess my last question is though..

Even at a stand-still, not rolling.. when you apply gas and release the RPMs will still hang there for a few seconds before falling.. that is normal?

Thanks for the information.