We know some of you must be thinking; “How are we going to cover this in one blog post?“ OK – time to get honest. We’re not going to go into great detail, just cover the basics. Some of the points covered here will have to be diagnosed by your dealership or trusty local mechanic. The point is, is to try and give you (and the person who is going to fix it) some basic direction as to which direction to proceed in.
First off, the kind of stalling problem were going to refer to often means the engine is not getting enough fuel and/or too much air. A cold engine needs a fairly rich fuel mixture to start, and to idle smoothly while it warms up. Any of the following could cause or contribute to this kind of stalling problem:
1). An engine vacuum leak. Check for loose or broken vacuum hoses, leaks around the intake manifold gasket or throttle body, leaks around the PCV valve and EGR valve.
2). A dirty or defective airflow sensor. A sensor that has been contaminated by fuel varnish or dirt will under report airflow and be slow to react to changes in airflow. This can upset the air/fuel mixture causing idle, stalling and hesitation problems. Cleaning the airflow sensor wire with aerosol electronics cleaner can often restore normal operation and cure the problem.
3). A defective idle speed control system. Idle speed on a fuel injected engine is controlled by allowing a small amount of air to bypass the throttle. If the idle air bypass circuit is plugged with dirt or fuel varnish, or the solenoid valve is sticking or broken, the engine may not get enough air to idle normally causing it to stall. Cleaning the idle air bypass circuit in the throttle body with aerosol throttle cleaner will often remove the gunk and solve your stalling problem. If a good soaking with cleaner fails to fix the stalling problem, check the wiring connector. It might be loose or corroded. If no wiring faults are found, you may have to replace the idle speed control solenoid.
4). A faulty coolant sensor. If the coolant sensor is bad and tells the PCM the engine is colder or warmer than it really is, that can screw up the fuel mixture, too. If the coolant sensor reads colder than normal, or cold all the time, the engine will run rich. This won’t cause cold stalling but it can make for a rough idle once the engine warms up, and it kills fuel economy. If the coolant sensor reads warmer than normal, or reads hot all the time, the PCM will lean out the fuel mixture too much, causing the engine to stall when it is cold. See the article on coolant sensors for how to test the sensor. Replacing a defective coolant sensor will cure this cause of stalling.
5). A faulty air temperature sensor. This sensor tells the PCM the temperature of the air entering the intake manifold. The PCM needs an accurate input so it can balance the air/fuel mixture properly. Just like a bad coolant sensor, a bad air temperature sensor can upset the fuel mixture causing stalling problems.
6). A bad Manifold Absolute Pressure (MAP) sensor. This sensor monitors intake vacuum, which the PCM uses to determine engine load. If the MAP sensor is not reading accurately, the PCM may add too much fuel or not enough, causing the engine to stall. See the article on MAP sensors for how to diagnose this sensor.
7). Worn or fouled spark plugs. Ignition misfire can make any engine stall at idle. When the engine is running slowly, there is less momentum to keep it going, so a bad misfire may cause it to stall. If the spark plugs have not been changed in a long time, a new set of plugs and/or plug wires can restore a good hot spark and eliminate the misfire. A weak ignition coil or a faulty crankshaft position sensor may also cause a stalling problem.
8). Bad gas. Gasoline that contains too much alcohol (more than 10%), or gasoline that has been contaminated with water or some other substance may not burn well and cause your engine to stall. If the stalling started to occur shortly after your last fill-up, suspect bad gas. The cure is to drain the tank and refill it with fresh gas from another filling station, or just use up the bad fuel (if the engine runs okay at highway speeds), then refill at another station when the tank is near empty.