Something I found searching the net that puts out some good info. Thought everyone might want to read it. Might be the answer your looking for.
More On Intermittent Engine Stalling Problems
By Larry Carley c2007
Diagnosing an intermittent engine stalling problem can be a challenge, especially if the engine only quits occasionally. And if there are no pending trouble codes, hard codes or history codes in the vehicle computer to provide some direction, you may find yourself guessing at a diagnosis.
Every engine needs three things to run and idle smoothly without stalling: a correct air/fuel ratio, sufficient idle speed for the idle load, and a good spark. If any of these is lacking, the engine may quit running.
Cold stalling problems are the most common because the engine needs a richer fuel mixture to maintain idle speed until it warms up. Intermittent cold stalling problems are almost always fuel-related.
Carburetor Stalling Problems on Older Vehicles
On older, carbureted engines, cold stalling (and hard starting) is most often due to an automatic choke that is sticking, misadjusted or broken. Cleaning the choke mechanism with aerosol carburetor cleaner may free up the choke allowing it to work properly again. If the choke housing as an electrical heating element, the element may not be receiving voltage when the key is on, or the element may have burned out (check resistance with an ohmmeter).
Other causes of stalling with a carburetor include an idle speed adjustment screw that is set too low (turn screw to increase idle speed rpm). The engine may stall if the idle fuel mixture screw(s) are not adjusted correctly or the idle mixture port(s) are dirty or clogged with fuel varnish deposits (clean the carburetor and readjust the idle mixture screws for smoothest idle). Stalling can also occur is there are vacuum leaks in the carburetor, under the carburetor (bad base gasket), or any vacuum hose connections to the carburetor or intake manifold.
Fuel Injection Stalling Problems
On fuel injected engines, stalling can be caused by anything that upsets the air/fuel mixture. This includes vacuum leaks or unmetered air entering the intake manifold downstream of the airflow sensor, a faulty throttle position, MAP or oxygen sensor, dirty fuel injectors, or low fuel pressure to the injectors (weak fuel pump, faulty fuel pressure regulator or restricted fuel filter). Like older carbureted engines, a defective thermostat may be preventing the engine from warming up quickly or reaching normal operating temperature. Or, a defective coolant sensor may be telling the PCM the engine is colder (or warmer) than it really is. Any of these conditions can upset the fuel calibration of the engine and cause a problem.
Idle Speed Control Circuit
One of the most common causes of stalling on fuel injected engines is the idle air control (IAC) solenoid or idle speed control (ISC) motor. If the idle speed control device fails to provide the correct idle speed, the engine may die when you slow down or come to a stop. In many cases, the idle control solenoid or motor is gummed up with carbon and fuel varnish deposits. Cleaning the idle port in the throttle body, and the IAC or ISC valve with aerosol throttle cleaner can often solve the stalling problem. If the situation is not improved after cleaning, however, the IAC solenoid or ISC motor may have to be replaced. Check the connector to the device to make sure the connector is not loose or corroded.
Engine Control Issues
Sometimes stalling is the fault of the Powertrain Control Module (PCM) or the inputs to the PCM. The factory programming may not provide enough idle speed when the A/C is on, when the alternator is under high load or when the temperature is unusually hot or cold. The fix here may be to reflash the PCM with the latest OEM update.
A faulty MAP sensor can sometimes mislead the PCM into thinking the engine is under a greater or lesser load than it actually is. The MAP sensor senses intake vacuum, which the PCM uses to estimate load so it can adjust the air/fuel mixture accordingly. If the MAP sensor isn't reading right, the PCM will receive bad information and possibly add or subtract more fuel than it should causing the engine to stall.
The same thing can happen if the throttle position sensor on a speed/density EFI system (no airflow sensor) is out of calibration or had a dead spot. The PCM may not realize the throttle is at idle, and may give the engine too much or not enough fuel causing it to stall.
When attempting to diagnose an intermittent stalling problem, therefore, it's important to always use a scan tool to first check for any trouble codes that might shed light on the condition, and secondly, to look at all the essential sensor inputs to see if they are within range and are supplying accurate information to the PCM.
Intermittent stalls that seem to happen at random are often ignition-related. A sudden loss of spark will kill the engine cold and prevent it from restarting. The most common causes for loss of spark include hot shorts/opens in ignition coils, ignition modules and crankshaft position sensors. Loose or corroded wiring connectors that cause a sudden loss of voltage in the ignition circuit will also stop an engine dead in its tracks.
More Causes
One of the more unusual causes of intermittent stalling we've heard about is a defective vehicle speed sensor. When the vehicle stops moving, the PCM may not engage the idle speed motor (carbureted engines) or idle air bypass motor (fuel injected engines) so the engine will idle normally.
We've also heard of faulty automatic shut down (ASD) relays on some older Chrysler products suddenly killing the ignition and fuel pump for no apparent reason. Replacing the relay usually cures the problem.
Another cause of stalling that is often overlooked is bad gas. Moisture sometimes finds its way into underground fuel tanks, and engines don't run very well on water. Sometimes alcohol additives may not be properly mixed, or may separate from the gasoline in the presence of water. Bad gas can cause an engine to run rough or stall.
If you suspect bad gas, drain the tank and refill it with fresh gas. If the problem goes away, your diagnosis was correct.
Sometimes an engine will experience a stalling problem when the A/C compressor is engaged. Normally, the PCM should increase the idle speed to compensate for the added load on the engine when the A/C is on. But if the A/C signal fails to reach the PCM because of a communications glitch between the A/C module and PCM, the PCM may fail to increase idle speed causing the engine to lug or stall.
According to Ford TSB 04-21-13, a cold stalling problem with 2003-'04 Ford Focus models with 2.0L SPI engines during extremely cold weather can be caused by a faulty PCV valve that sticks open, allowing too much air to be sucked into the intake manifold.
Ford also has issued a recall (04S13) for 2001-'03 Ford Escape sport utility vehicles with 3.0L V6 engines for an intermittent stalling problem. The stalling typically occurs while decelerating at speeds below 40 miles per hour. The problem is caused by the calibration of the idle air control valve and evaporative emissions system. The fix here is to reflash the PCM with updated information.
The National Highway Traffic Safety Administration (NHTSA) investigated reports of sudden stalling with Toyota Prius hybrid electric cars. The cars were suddenly shutting down at speeds of 35 to 65 mph. In some cases the vehicle could still be driven in the electric mode, but in others everything went dead and the car had to be towed. The fault turned out to be a computer glitch, and the fix was a PCM reflash.
Diagnostic Strategies
One strategy for diagnosing a stalling problem that only occurs intermittently is to wait until the problem gets worse before you attempt to diagnose it. It's always easier to find a part that has failed than to find one that works most of the time and only acts up occasionally.
One time-saving step that may allow you to zero right in on the cause is to check for any Technical Service Bulletins (TSBs) that might have been published by the vehicle manufacturer. It may be a situation where there's a pattern failure and the manufacturer has already figured out the problem and posted a fix. The few minutes you invest in doing a TSB search can save you hours of frustration and wasted diagnostic time. And with many late-model vehicles, the cure is often a PCM reflash rather than replacing something.
The next thing you should always do is hook up a scan tool and check for codes even if the Malfunction Indicator Lamp (MIL) is not on (the lamp may be defective). Look for history codes or pending codes that may shed light on the problem. Also, look at the sensor inputs to the PCM when the engine is idling (both after a cold start and when it is warm). You should also look at short-term and long-term fuel trim. Is the engine running unusually rich or lean? That would tell you something is amiss.
Another basic check that should always be made is to check battery voltage and charging voltage. A low battery, weak alternator or overvoltage condition can all play havoc with onboard electronics. Solenoids and relays all require minimum voltages to function properly, so if the battery or charging system are not within normal specifications you may have found the root cause of the problem.
If a problem has left no tracks (no trouble codes or odd readings to steer you in a particular direction), is there a pattern? Does the engine only stall when it is cold or hot? Does it only occur during wet weather (might be bad spark plug wires)?
