Engine Stalling? Need Help Now?
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
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
After living with this problem for several months, it began to occur more frequently. Now the car was not starting every couple of days. We had lost all confidence in the car's reliability, so I loaned her my car to drive and started driving her car to see if it would misbehave for me.
My initial hunch was that the car had a failing fuel pump (or pump relay), or a quirky ignition module. Finally one day it refused to start for me. Fortunately, I had some tools with me just in case. When the engine refused to start, I pulled a plug wire off and hooked up a spark plug tester to see if there was any spark. Sure enough, when I cranked it there was no spark. That was good news because I didn't want to drop the fuel tank and install a new $300 fuel pump!
When I cranked the engine again, it suddenly started. Now I knew the problem was ignition-related and was probably a bad ignition module.
The Saturn 1.9L engine has a "waste spark" distributorless ignition system. A pair of ignition coils are mounted on a module block that is bolted to the front of the engine. Coil #1 fires cylinders 1 and 4 simultaneously, while coil #2 fires cylinders 2 and 3. I used a DVOM to check the primary and secondary resistance of both coils, voltage to the module (key on, engine off and at idle), and the ground circuit. I also checked for a loose or corroded connector at the module. All the voltage readings were right on the money. No problem there. So I concluded the problem was inside the ignition module. I bought a new $180 ignition module from Saturn, installed it and considered the problem fixed.
Guess what? The next day the car wouldn't start again. The problem was the same as before, no spark. Could the new module I just installed be faulty? It was possible but unlikely. Obviously, I had misdiagnosed the fault and bought a module I didn't need (sorry, no returns on electronic modules once they are installed).
I checked the car again with my scan tool and again found no codes or clues that would tell me what was wrong. Then I did what I should have done in the first place and went online to search for any technical service bulletins that might pertain to this problem. I found one (TSB 98-T-49A) that seemed to fit, but the diagnostic charts in the TSB only lead to dead ends because everything tested within specifications.
I then posted my problem on the international Automotive Technician's Network (www.iatn.net). I received back about a dozen responses from other technicians who had run into similar problems with Saturns, and they all offered the same advice: replace the crank position sensor.
But how could it be the crank sensor? I had checked it already and it tested fine - or so I thought. The problem was that I was testing the sensor at room temperature, not when it was hot. The spec said the crank sensor should read between 700 and 900 ohms. It read 780 ohms.
I removed the crank sensor, placed it in a pan of hot water and hooked up my DVOM to watch the sensor's resistance as it warmed up. Sure enough, when the sensor got hot the resistance shot up and it suddenly went open. It remained open until it cooled back down, then it began to read normally again. That would explain the no-start problem when the engine was hot. The crank sensor would absorb heat when the engine was shut off, go open and fail to produce a signal when the engine was cranked. That explained the no-start after a short drive.
The problem should have set a crank sensor code, but it did not. So had it not been for the shared expertise of other technicians who had encountered the same problem, I'd still be trying to figure out the problem. Thanks iATN. 
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