Starts, but dies at idle. Help.

Sounds like the IAC. Is it still the original ?

How to troubleshoot IAC idle problems

Start first by checking for blown fuses. Especially fuse F2.8 in the CJB (1999-2004 MY).

Clean the Mass Airflow Sensor (MAF) using a cleaner designed for it. The sensor is delicate and easily damaged. Never use a harsh cleaner or leaves a residue. NO brake clean. NO carb/choke cleaner.

Check for other base engine problems such as low fuel pressure, clogged fuel filter, or weak ignition.

Next look for any vacuum leaks. Even a very small vacuum leak can cause drivability issues. Inspect all hoses (PCV, breather, evaporative purge). Look for cuts, chaffing, dry rot or any other defects that may allow unmetered air to enter. Also inspect all intake connections to be sure they are tight and leak free. Don’t forget the oil cap, oil dip stick, and valve covers. If your PCV valve is old, replace it.

There are two kinds of Idle Air Controllers (IAC). One has a black atmospheric vent. The other does not.

IAC with black vent: Usually seen on older Mustangs (2000 prior). The vent supplies bypass air to allow the motor to start. It contains a number of springs and diaphragms. The vent must internally close off when the engine is running.

Do not clean the black vent IAC. The cleaning will damage the internal diaphragms. Replace it. Autozone makes a lower cost non-Ford replacement that works well.

Non-vented IAC: these can be cleaned. Be advised, that cleaning may not fix the problem or it may only work for a short period of time.

The Mustang’s idle strategy relies on the idle air being supplied by the IAC. The throttle body butterfly valve is designed to allow only a very small (if any) air to pass. Resist the urge to adjust the TB set screw for low idle concerns. It may appear to work but when the weather gets cold, a high idle will result.

A bad IAC may cause a no start condition. Try cracking the throttle to see if the motor will start. If so, suspect bad IAC.

First functionality test of the IAC: Start and allow the motor to idle. Disconnect the IAC electrical connector. The RPM’s should drop (black vent) or the motor should die out right (non-vent).

If the idle does not drop as expected then:
• The IAC is bad or dirty.
• There is a vacuum leak downstream of the IAC.
• The evaporative purge value (VMV) is stuck open. This is allowing excess air to enter via the charcoal canister.
• The throttle body butterfly valve is allowing too much air to bypass.
• The throttle body linkage is loose or worn allowing air to enter.
• The Throttle Position Sensor (TPS) is loose, bad, or noisy (not giving good closed throttle indication).
• The electrical connection to the IAC is open or shorted. Inspect the wiring. Pay special attention on boosted/modified applications that often need to extend the IAC wiring.

If you have access to an ODB2 scanner, monitor the IAC duty cycle (percent). It should change as load on the motor changes. Turn on the AC and head lights. Verify the IAC duty cycle changes in response to changing load. Also monitor the TP-MODE PID and see if it reads "closed" when the throttle is actually closed.

If the IAC duty % is very low (< 20), suspect TB set screw allowing too much air or a vacuum leak.

The IAC handles a fair amount of air. Some engines have more vapors in the PCV system than others. Excessive blow-by can contribute to shorter life span of the IAC.

For intermittent high idle concerns, suspect a problem with the TPS and/or loose/worn throttle body linkage (aftermarket TB's). Confirm the throttle return spring is in good condition.

For the 2003/2004 Cobra that dies coasting to a stop, Ford has an updated PCM calibration to resolve this issue.

Very low idle problems could also be due to a weak cylinder (not pulling it's share of the load). A cylinder power balance test can help. Also look for a vacuum leak in the line between the air tube and the IAC.

The TPS sensor is NOT used the same way it was during the SPEED DENSITY strategy days. The Mass Air Flow (MAF) strategy uses the TPS mainly for closed throttle and WOT processing. Remember, the PCM already knows how much air is passing through the butterfly valve because of the MAF sensor.

The PCM needs to know when the throttle is closed so that the idle trim strategy can be enabled.

The PCM needs to know when the throttle is at WOT so that the WOT tables/strategy can be enabled.

The bottom line is that the VALUE of TPS sensor is not as important as the stability of the value/position.

Please review the following excerpt from the Ford service CD.

Idle Speed Control Closed Throttle Determination.

One of the fundamental criteria for entering rpm control is an indication of closed throttle. Throttle mode is always calculated to the lowest learned throttle position (TP) voltage seen since engine start. This lowest learned value is called "ratch," since the software acts like a one-way ratch. The ratch value (voltage) is displayed as the TPREL PID. The ratch value is relearned after every engine start. Ratch will learn the lowest, steady TP voltage seen after the engine starts. In some cases, ratch can learn higher values of TP. The time to learn the higher values is significantly longer than the time to learn the lower values. The brakes must also be applied to learn the longer values.

All PCM functions are done using this ratch voltage, including idle speed control. The PCM goes into closed throttle mode when the TP voltage is at the ratch (TPREL PID) value. Increase in TP voltage, normally less than 0.05 volts, will put the PCM in part throttle mode. Throttle mode can be viewed by looking at the TP MODE PID. With the throttle closed, the PID must read C/T (closed throttle). Slightly corrupt values of ratch can prevent the PCM from entering closed throttle mode. An incorrect part throttle indication at idle will prevent entry into closed throttle rpm control, and could result in a high idle. Ratch can be corrupted by a throttle position sensor or circuit that "drops out" or is noisy, or by loose/worn throttle plates that close tight during a decel and spring back at a normal engine vacuum.

A vacuum leak between the air tube and input of the IAC will cause the RPM's to drop too low. Again, even a tiny leak will cause problems.

Try cycling the key on/off serveral times without starting. Does this improve the starting? If so, suspect fuel pressure leak down.

Next try cracking the throttle halfway open while cranking. Does this improve starting?

Sounds like your car is angry for having to be put up and not driven all winter. Though the car appreciates you not ruining her in the snow and ice, it is still upset that you don't care enough to move to a warmer climate where winter storage is not necessary. Sit down with your car, talk to it, show it how much you care and explain that in today's economy moving is not an option. Cars are sensitive.


:woah: Car Psycology. It's a new science but we are making great strides in recent studies.

This could be fuel related. What is the fuel pressure? What about the quality of the gas in the tank? Stale? Water?

Did you clean the MAF?

Put some seafoam in the gas and go for a long highway drive to use up
the old gas..... keep the rpm's up over at least 2500.

FWIIW, there is a check valve at the end of the filler neck. It's purpose is to prevent gas from running out in the event of a rollover. It will also prevent draining the gas via siphon.

What I have done is disconnect the fuel line at the filter. There is an electrical connector that goes straight to the fuel pump that can be seen at the rear of the tank. It is very easy to inject 12 volts directly to the fuel pump (RD/BK BN/PK). This will force the pump to run making it possible to drain the tank without dropping it.

Of course if this is a gas varnish problem, the fuel injectors themselves may be gummed up.

Try these things and post the results.

Crack the throttle half way open while cranking. Does this improve the starting?

Cycle the key on/off several times without starting. Each time the key is cycled, the fuel pump will run for a few seconds. After several on/off cycles, try to start. Does this improve starting?

Note, a vacuum leak on the intake side of the IAC will cause a low idle.

Clean the MAF. Take the air tube apart and spray MAF cleaner directly inside the MAF sensor. Do not jam the hose onto the wire but spay the cleaner in all of the openings.

Confirm the vacuum reference line from the fuel pressure sensor is still connected.

The gas could still be bad. This problem could continue until the bad gas is used up and there is fresh gas in the tank.

If problem continues even with good gas, we really need to know the fuel pressure.

Change the fuel filter. Couldn't hurt. Something could be blocking your fuel to show a 0 pressure condition. After that I consider changing your fuel pump. Something causing your engine to not get efficient amount of fuel, you said the engine only starts w/ the fluid. So at least we know this must be fuel related, something is starving your engine of fuel, maybe we can rule out fuel injectors, considering the car runs fine above 1000 RPMs. I guess we now know the importance of cranking the car and letting them run for a while if we don't drive them, don't feel bad, your cars failure is going to help a lot of people!!

Drain that fuel, put in fresh fuel and run the Seafoam through your fuel system.

Center pin needs to open Schrader valve

It is not possible for a Fuel Injected car to run with ZERO fuel pressure. If the pressure truely were zero, the car would start (after spraying) and run for only a few seconds before quitting. Your post states that once started, it will run as long as the idle is kept up.

More likely the pressure guage center pin is not opening the Schrader valve. This is the reason the guage reads zero.

well im having about the same probem but mine is running so i went to advance and had them hook it up and it was th IAC so i replace it and it ile just fine except with the air and heat on. it idle up and down it goes from running about 900 to 600. is this normal?

i dont think i need to replace some hoses on there next. does anyone know if i can just go to a part store and get the hoses i need