A Stalling problem most of the time is the inadequate of fuel and/or too much air. A cold engine needs a rich mixture to start and run cold, and to idle. Any of the following can cause or contribute to a hard start and/or stalling problem:
Possible problems which might affect these conditions are:
Note: Do the basics first. Visual checks such as checking the air filter and perform a fuel pump pressure test. And checking all vacuum line connections.
A vacuum leak from gaskets and/or vacuum hoses. Check for loose or broken vacuum hoses, leaks around the intake manifold gasket or throttle body, leaks around the PCV valve and EGR valve, and around brake booster.
A dirty or defective airflow (Mass Air Flow) sensor. Mass air flow 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.
A coolant temperature sensor could be out of range. Computer needs in input from the (CTS) coolant temp sensor signal for (ECM) Electronic Control Module to effectively supply the correct amount of fuel to injectors. Take a scanner which can which can read the Live data and verify the temperature reading of sensor compared to engine temp. If reading, let say, is 198 degrees and the engine is cold, say, 50 degrees, then you know the temp sensor is in accurate. Replace the temp sensor.
Note: Even though you do not have any codes does not mean one of the sensors is out of range. A code simply means you have an electrical problem with either a wire to sensor connector or a bad sensor.
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.
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.
Low engine compression. If your engine has a lot of miles on it and compression is low because the piston rings and/or cylinders are worn out it has one or more leaky valves, it may not have enough oomph to keep idling. A compression check will tell you if this is a problem or not, and if it is there is no easy fix other than to overhaul or replace the engine.
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.
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 you’re last fill-up, suspect bad gas. The cure is to drain the tank and refill it with fresh gas from another filling station, or add some isopropyl to gas and 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.
Problem: Your engine stalls when you stop for a traffic light or when idling:
A stop light or idle stall often means the engine is not idling fast enough (idle speed too low), or the engine is being pulled down by a load on it created by the air conditioning compressor and/or alternator- it could also mean the fuel mixture is too rich or too lean, causing the engine to run poorly, or a bad or clogged Exhausts Gas Recirculation (EGR).
Possible causes that may contribute to this kind of stalling include:
Exhaust Gas Recirculation. This part known as EGR for short is a well know problem. When coming to a stop, the egr hangs open, due to carbon deposits or clogged ports or tube. This part will cause vacuum leaks as well, so check the EGR first.
A Bad A/C compressor. If the compressor is binding up, possibly due to a lack of lubrication, internal wear or an over-charged system (too much refrigerant), it may be lugging down the engine when it is engaged. If the problem only occurs when the NC is on, there is an issue with the air-conditioning compressor.
Unusually high electrical load on the charging system. If the battery is run down and the alternator is working hard to recharge it, the increases load on the engine may pull down the idle rpm to the point where it causes the engine to stall. Check the battery state of charge to see if the battery is run down or failing. Alternator average should charge at a rate of 13.3 to 14.8 volts at idle. If the battery is low, use a battery charger to recharge it. If the battery is failing and is not holding a charge, time to buy a new battery.
NOTE: Using a volt meter, while engine is running, at idle you can test the alternator output. – Red lead of meter to the positive and Black lead of meter to negative side of battery. A reading between 13.3 to 14.8 volts is a good range for charging. Make sure all accessories are off. In addition, to Load test the battery you will need a load tester or go to your local parts store. They should be able to perform the load test for free. Remember testing the battery engine off with a volt meter is not a true way to load test. With that said, any battery you test with a meter, dead or charged will read the batteries voltage until a load is applied. A battery is only as good as its weakest one.
Problem: Your engine just stalls unexpectedly while driving:
Stalls like this are often ignition-related and happen when the engine loses spark. The underlying cause is often a bad crankshaft position sensor, or sometimes a failing ignition coil (if the engine has only one coil). A faulty ignition switch that loses contact intermittently may also cause the engine to suddenly die for no reason.
When this happens, open the hood and check for spark. This can be done by pulling off a plug wire (if the engine has plug wires), and placing the end near the block while a helper cranks the engine. DO NOT hold the wire as it may shock you if the ignition system is working. If you do not see a spark or hear the plug wire snapping when the engine is cranking, the fault is in the ignition system.
If the engine has spark, it may have died due to a loss of fuel pressure. When fuel pumps fail, they usually just quit with little or no warning. The engine usually won’t restart and the vehicle has to be towed in for repairs. Listen for a buzz from the vicinity of the fuel tank when the ignition is turned on. No buzz means the fuel pump isn’t running. It might just be a blown fuse or a bad relay, but on a high mileage vehicle is often a bad fuel pump.
Another possibility is a bad ECM (engine computer) relay. The power supply to the PCM is often routed through one or two main power relays. If one of these relays loses contact momentarily, it’s like pulling the plug on the PCM. The ECM shuts down and turns off the ignition and fuel injectors, causing the engine to stall. One way to see if this is a possibility is to switch or replace the ECM power relay(s). If the problem goes away, the cause was a bad relay. If it continues, the fault is something else (possible a wiring fault in the ECM relay or ECM power circuit).
Yet another possibility is low system voltage, loss of voltage, or overcharging. The PCM and other control modules require a steady 12 volts to operate correctly. If the supply voltage suddenly drops below 9 volts, or surges about 14 volts, or cuts out, the Electronic Control Module may temporarily kill the injectors or ignition circuit. The underlying cause may be an intermittent short somewhere in the electrical system or charging system that causes a momentary drop or surge in voltage. These can be very difficult to find, and often require hooking up a scan tool that can capture snapshot data when the stall occurs. By looking at the data, a technician can see the chain of events that caused the stall, and hopefully identify, isolate, and repair the fault.
By Master Tech Lee Davidian, Sr. | Auto Repair Help.US