Why Won’t My Engine Stay Running?

Problems related to the engine are never a good sign, especially when you notice trouble with the engine startup or idling. Erratic engine runs can result from various vehicle issues, and narrowing it down can be hard. Begin by inspecting the electronic control unit (ECU) using an OBD tool to find any trouble codes to narrow your search. If you don't have access to a scan tool or do not get any trouble codes, you can start by checking the following systems for signs of trouble.

There are several reasons for the engine to die immediately after it starts. However, the most common cause is due to issues with the fuel delivery system. A clogged fuel filter can restrict fuel flow, which causes the engine to stall and die. A bad fuel pump can prevent fuel from reaching the engine. This can prevent engine startup as well. A leak in the fuel system is another reason for the engine stalling, as it results in poor fuel supply to the engine. The fuel may be enough for the engine to start but not enough for it to idle or function properly. Faulty fuel injectors can also cut the fuel supply and starve the engine. As debris clogs the fuel injector nozzle, fuel does not make it to the combustion chamber which causes a few of the cylinders to not run. This eventually results in the engine dying. Dirt and debris in the injectors can in turn be caused by a bad fuel filter that fails to do its job. In a carburetted engine, a bad float valve can stop the fuel supply completely, restricting the fuel supply to the cylinders. A malfunctioning fuel pressure regulator can supply too little or too much fuel to the engine, both of which can cause engine idling and running issues. Here are other fuel delivery system failure symptoms that you will experience.

Weak spark plugs can cause the engine to idle poorly, die immediately, or misfire. In older cars with a distributor-type ignition system, the rotor contact or ignition points can wear ou easilyt, preventing current from reaching the spark plugs. In Coil-on-plug ignition systems, one or more failing ignition coils can also cause engine idling issues or cause the engine to die after startup. Check the spark plug wires for wear and tear. Remove the spark plugs and inspect the condition and gap. Replace the spark plugs if necessary, along with any failing ignition coils. Follow this guide for ignition coil replacements.

Carbon buildup on the throttle body and plate can hinder the movement of the throttle, which will adversely affect the engine performance. Since air supply is crucial for proper engine functionality, a problem with the throttle body, like a stuck throttle plate, bad idle valve, failing throttle position sensor, etc., can limit air passage to the engine and cause idling issues.

The throttle position sensor (TPS), among other sensors, assists the ECU in determining the amount of air flowing through the system, allowing it to adjust the fuel supply for optimal fuel-air ratios. Most TPS units feature a variable resistor with multiple contact points that can wear out and fail over time. When these contacts open, it causes the ECU to lean the mixture, leading to engine stalling. A faulty TPS typically triggers a trouble code, which you can check using an OBD scanner tool.

An idle air control (IAC) valve is used on most modern engines. Its function is to allow airflow to bypass the throttle during idling or cold starts to prevent rough idle and engine stalling. A bad IAC valve can stay closed, restricting air supply and suffocating the engine. Debris and carbon deposits in the idle valve passage can also restrict airflow. Similarly, an idle valve is found in carburetors that performs the same function. If you have a carbureted engine, check the idle valve and adjust it to open wider. Check the owner’s manual for more accurate information on the process. 

When a certain amount of air bypasses the usual air intake path and directly reaches the engine, it is called a vacuum leak. Normally, the airflow is directed through the throttle body so that the mass-airflow sensor (MAF)can measure the air supplied and, consequently, the ECU can supply the required amount of fuel. When a vacuum leak occurs, the airflow into the engine increases. However, the ECU only supplies enough fuel for the measured amount of air. This results in a lean air-fuel mixture, leading to poor idle and engine stalling. Vacuum leaks can be caused due to tears in the intake hose or one of the vacuum hoses connected to the intake. A Worn intake manifold gasket or throttle body gasket can also be the culprit. Check and replace any frayed vacuum or intake hoses. 

The camshaft position sensor (CMP) and crankshaft position sensor (CKP) are used to determine the position of the cam and crankshaft, which allows the ECU to time the spark and fuel. If these sensors fail, the ECU receives false readings, causing the engine to fail to start or to die shortly after starting. Check the wires leading to the sensors and the sensors for wear and tear. Usually, bad CKP and CMP sensors put out engine trouble codes along with a “check engine” light in the dashboard.

If the engine idles well in park or neutral but stops as soon as you put it in gear, then there is likely a problem with the transmission system. On the other hand, if the engine runs smoothly until you stop, you may have a problem with the torque converter (TC). If the TC sticks, it can cease the engine as the vehicle comes to a stop. You will need to get your vehicle checked by a professional to troubleshoot transmission problems. 

The engine needs fuel, air, and spark to run. Check each system thoroughly and by eliminating potential causes, you can solve engine running issues. Whether it is a mechanical issue or an electrical one, you will always find symptoms of failure for every system, which can help you isolate a problem and fix it. Refer here for more informative automotive technical articles to help you solve your vehicle’s problems.