Ford IAC valve replacement:

Below are some pictures of the original idle air control valve from a 1998 ford crown victoria. In this case, one day the vehicle mysteriously refused to start when the ignition key was turned. The starter cranked, but the engine would "flood out" and the exhaust had a slight gasoline odor to it. Pushing the gas pedal down a little while turning the ignition key started the engine and kept it running, but the vehicle would stall out as soon as the accelerator pedal was released.

The symptoms were consistent with a defective idle air control (IAC) valve. Base idle on this vehicle should be around 800 RPM in park,
but engine crankshaft speed appeared to be dropping so low that the engine stalled out. Since this vehicle did not have a factory installed tachometer, a scantool was connected to the obd2 diagnostic port under the dashboard and revealed that no error codes were present and the idle was very smooth if engine speed were manually raised to around 800 RPM via the accelerator pedal. After hitting the IAC with the plastic handle of a screwdriver a couple times, the low idle speed problem "fixed" itself. But to prevent repeat occurences of this problem in the future, the IAC was replaced with a new service assembly.

Some other points of intrest about the IAC:

• Police use often involves idling the vehicle's engine for several hours a day. This gives the IAC valve a real workout and may cause the device to fail with relatively little mileage accumulated on the odometer. Civilian "remote starters" and getting "stuck in traffic" frequently will also increase the rate at which this device "wears out".
  
• The IAC valve is a pulse width modulated device and cannot accurately be tested by simply applying +12V and ground
• A defective IAC valve can cause intermittent stalling problems when the vehicle operator releases their foot from the accelerator pedal. If the vehicle is stalling or misfiring under medium throttle application, there are other problems present besides a defective IAC valve though.
• The old backyard mechanic method of raising the base idle speed via the throttle stop screw on the throttle body is highly discouraged and likely to cause other problems. In fact, the screw in question has it's head removed by the manufacturer to deter adjustment. And if the goverment in your area mandates vehicle smog testing, your vehicle is likely to fail the test if this fastener has been tampered with.
  
• Installation of a "pre-owned" IAC valve from a salvage yard vehicle is discouraged. First, you may not get the "right one" for your car. Since there are a handful of different iac valves released for the hundreds of different crown victoria pcm calibrations that have been released over the years. And these devices are also wear items, so you're somewhat likely to get a "bad one" that won't "fix" your problem.
  
• Malfunction of the IAC valve is sometimes dependent on the ambient temperature. e.g. warming the IAC valve with a warm hair dryer after the vehicle has sat outside in cold weather for an extended period might cause the IAC valve to function properly until it cools off again.
• All 1996 and later crown vics IAC valves look very similar in appearance and have the valve located in the same location on the back of the throttle body. 1992-1995 crown vics have the valve mounted in a different location though.
• If the cosmetic engine cover is still present over the throttle body, it will be necessary to remove it prior to replacing the IAC valve. It is retained to the throttle body using a single stud and nut.
• It may be useful to lubricate the bolts which retain the IAC to the throttle body with some penetrant oil such as WD40 or PB Blaster prior to attempting removal of the bolts. But the top area of the engine is usually relatively dry and these bolts very often turn freely without use of pentrant oil or heat from a torch.
  
• IAC valves are sealed non-serviceable items and not designed to be "cleaned" with solvents such as carberator cleaner. Any results that may be realized as a result of "cleaning" are likely to be short lived.

Another point of intrest, the IAC valve basically creates an electrically controlled air leak on the throttle body. So if you have a vacuum leak elsewhere, it can exhibit symptoms similar to a bad IAC valve. When looking for vacuum leaks, don't overlook:

• The vacuum controlled automatic release system for the parking brake (the control valve is on top of the steering column shifter tube, the release actuator on the foot pedal assembly). Note: This system has been "decontended" from 1999+ police interceptors and 2003+ civilian vehicles.
  
• The mode doors for the air conditioning & heater system are vacuum controlled. If air sporadically flows from the defroster vents when the controls are set to the floor or dashboard vent positions, then you've definetly got a vacuum leak somewhere. Some pictures of the interior passenger cabin vacuum hoses are avaliable by clicking here.
• The vacuum based brake booster diaphram.
• The vaccum based positive crankcase ventilation (PCV) system
• The evaporative emissions system (controls gas tank hydrocarbon vapors)
• The exhaust gas recirculation (EGR) system. The DPFE valve is located on the of the back of the engine block in front of the firewall and can be difficult to find if you haven't seen one before.
• The vaccum based fuel pressure regulator system. You'd likely notice some driveability systems if this were malfunctioning. Note: 2003 and later crown vics use a returnless fuel system that no longer has a fuel rail mounted pressure regulator.
  
• The vacuum distribution block on the firewall. After enough poweron hours & resulting engine bay heat, the vacuum lines get brittle and develop cracks. This area is very easy to access with the engine removed. But with it in place, you may want to remove the winshield wiper transmission housing assembly to get better access to the area.
  
• The throttle body itself could be leaking significant amounts of air around where the throttle plate shaft passes through the throttle body housing. The throttle shaft wears a miniscule amount of aluminum away each time the gas pedal is moved. After a bunch of movement, the circular hole in the throttle body starts to become eliptical and the throttle shaft a little skinnier than it should be. Air leaks are the least of your worries here, as a severly worn throttle shaft can cause the accelerator linkage to bind (a.k.a. gas pedal stuck to floor and won't pop back up after you release your foot).
  

Do keep in mind that the larger the vacuum leak is, the more it will impact engine idle quality. For example pulling the vacuum hose off the brake booster or pcv valve will cause the engine to run really lean or to stall out immediatetly. But you'd likely need to monitor fuel trim and other block learn values with an electronic scantool to notice a vacuum hose that was pulled off a climate control system mode door actuator.

Also keep in mind that model year 2005 and later crownvics have electronic throttle control (ETC) and no longer use an idle air control valve anymore. The drive-by-wire powertrain control module (PCM) can command the throttle body butterfly plate to any position desired. So a valve to allow a little air to bypass the throttle body butterfly valve at idle speeds is no longer needed.

 

Note: IAC valves are sealed non-serviceable items and not designed to be "cleaned" with solvents such as carberator cleaner. So any results that may be realized as a result of "cleaning" are likely to be short lived.

Additional pictures of a crown victoria throttle body, an iac valve, and an air intake plenum are avaliable by clicking here.