Air/Fuel? Vacuum Leak? Help..

[jussagt]

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!

 

[wmburns]

The hanging RPM's are for SMOG reasons. They all do it. But if your are sure there is something wrong, change the TPS (since you already have a new IAC). Never hurts to look for a vacuum leak.

Also, if you adjusted the Idle set screw, it is most likely letting in too much air. This will cause the idle to be too high. Esp after a change in weather.

There are tons of threads on how to trouble shoot an IAC.

Post #3:
http://www.stangnet.com/mustang-forums/810729-starts-but-dies-idle-help.html

 

[jussagt]

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 ?

 

[wmburns]

Yes for bad IAC (or wiring to/from IAC).

Orrrrrr the idle set screw is set so high that the IAC duty percent is ZERO.

I am presuming that you do not have an ODB2 scanner and a method to monitor IAC duty percent. Disconnect the IAC connector. Close the TB set screw down slowly until the car almost stalls.

Plug the IAC back in. Does the idle go up? If yes, the IAC is working. If no, replace the IAC.

In any case, that is a better adjustment point for the set screw. The RPM's must drop when the connector is removed. Otherwise, the idle will never be correct.

 

[jussagt]

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...

 

[wmburns]

Why didn't you mention the non-function speedo in the 1st post?

Note, the PCM uses car speed and brake=on to activate slow idle. If the speedo reads ZERO, then one would expect the PCM to activate slow idle too quickly.

Now, if the PCM thinks the car is MOVING all of the time, then the PCM will delay activation of the slow idle process.

If you were truely trying to diagnose this problem correctly, you would look at IAC percent, TP-mode (for closed throttle determination), as well as car speed. Because this problem could be TPS related as well as vacuum leak related.

In any case, the fact idle does not slow when the IAC is disconnected means that the IAC is not actually controlling idle.

OBTW, your car does not have a VSS. It has an Output Shaft Sensor (OSS). The signal output of a VSS is not compatible with the OSS. The difference matters when swapping transmission from different MY cars.

 

[jussagt]

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]

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

TB set screw is allowing too much air to enter. The IAC can only ADD air. It can not REMOVE air.

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.

It is an purely an academic issue which sensor you actually have. The autoparts store is not going to diferencate between the two sensors. If you get the correct sensor for your MY car, life will be good. The difference only matters if swapping transmission across MY.

If you really have to know, look at the wiring diagrams. The VSS is powered with +12 volts. The OSS is an hall effect sensor that creates it's own power.

 

[jussagt]

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.

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?

 

[wmburns]

If you are expecting the RPM's to free fall back down to an idle, you will be disappointed. The stock PCM slows the idle in steps for SMOG reasons. They all do it.

The PCM enables a fast idle when the car is moving. The RPM's will not drop below say 1100-1200 as long as the car is moving (stock set up). Further, any decrease in RPM's will be at a slower rate as long as the car is moving.

Once the car's speed is zero, the PCM will enable a slow idle. The IAC will adjust the idle in a series of steps down to the 600-650 level (or what ever your PCM's base idle speed is set to).

Again, to get the idle to quickly return to slow idle all the time will require an IAC restrictor gasket (sold on ebay) as well as a custom tune. In any case, the TB set screw must be set correctly for the IAC to actually control the idle.

 

[jussagt]

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.