Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.
What Is Wrong With My Car?
When your car engine starts but stops afterwards, whether immediately or after a few minutes, there could be one or more specific systems or components behind the failure. For example:
- the ignition or injection system;
- a low idle speed in need of adjustment;
- a maladjusted carburetor;
- vacuum leaks;
- one or more bad sensors.
To make matters worse, stalling may happen under one or more operating conditions. For example, the engine may:
- stall as soon as it starts,
- stall during idle,
- stall when warm, or
- stall intermittently under any condition.
The conditions associated with your stalling problem will give you clues as to what systems or components are causing your problem.
So the best way to start your diagnostic is to identify, when possible, the conditions under which the stalling occurs, and focus your attention on those systems or components associated with the failure under that particular condition.
How to Investigate and Diagnose Your Problem
The following sections discuss different operating conditions associated with stalling, and which systems and components are most likely to cause stalling under those conditions. The last section focuses on what to do when stalling happens intermittently and seemingly randomly.
So, start by checking those components or systems in the section that best describes your particular engine problem, and then, if necessary, move on to systems or components described in other sections, especially if you suspect these systems need maintenance or have given you problems in the past. This approach will make it easier for you to diagnose the problem much faster and will work in most cases.
Check for Codes
Also, be sure to check the computer system for any diagnostic trouble codes (DTCs) that might be stored in memory, whether the "check engine" light has come on or not. One or more sensors may have triggered a pending code. If you don't have a scanner tool, you may be able to take your vehicle to a local auto parts store to get the codes retrieved, or the store may lend you the scanner. Better yet, buy a relatively inexpensive but quality automotive scanner from your local auto parts store or online.
1. If Your Engine Stops Immediately After It Starts
2. If the Engine Stalls at Idle
3. If the Engine Stalls at Idle Unless You Press the Accelerator
4. If the Engine Starts When Cold but Stops When Warm
5. If the Engine Stalls Intermittently
1. If Your Engine Stops Immediately After it Starts
Your engine may stop as soon as you let the key return from the Start to the Run position. Most of the time, this condition points to a badly clogged fuel filter or an ignition switch in need of adjustment or with worn/burned contacts.
A quick test to see if you have a problem with the fuel system is by using carburetor cleaner or starting fluid:
- Carefully disconnect the air cleaner outlet tube from the throttle body.
- Ask an assistant to start the engine.
- As soon as the engine catches and the ignition key returns to the Run position, spray some carburetor cleaner into the throttle body.
- If the engine idles momentarily with the carburetor cleaner, there may be a problem with fuel delivery.
The problem can be with the fuel filter, fuel pump, ballast resistor, or fuel pressure regulator.
Start with the fuel filter. When was the last time you replaced the filter? Check the fuel filter service interval in your car owner's manual or repair manual. If the fuel system uses an in-line filter:
- Remove the filter.
- Blow through the filter in the direction of fuel flow.
- If air goes through with difficulty, or not at all, there's your problem
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When necessary, use a fuel pressure gauge to check system pressure. Follow the instructions in your vehicle repair manual. If you don't have the manual, you can buy an inexpensive, aftermarket copy at your local auto parts store or online.
Other components that can cause you trouble are:
- fuel pump
- fuel injectors
- engine coolant temperature (ECT) sensor (see the next section)
- fuel pressure regulator
- crankshaft position (CKP) sensor
Usually, a bad CKP will trigger the check engine light (CEL). So scan the computer for DTCs if you haven't done so yet.
To do a quick check for potential CKP problems:
- While, cranking the engine, pay attention to the tachometer on your dashboard.
- If the RPMs remain at zero, this may point to a problem with the CKP sensor.
Check your vehicle repair manual to help you test the crankshaft sensor, if necessary. Even if the sensor itself is good, there could be an electrical open or short in the circuit.
Check the ignition switch:
If you seem to be getting good fuel supply, and the previous tests didn't yield any clues, try checking the ignition switch. Ignition switch contacts wear out over time.
Sometimes, you can detect problems with the contacts by holding the key forward in the Start position when cranking the engine. The engine may catch in the Start position, but as soon as you let the key return to the Run position, the engine dies.
- First gain access to the ignition switch. Consult your vehicle repair manual.
- Make sure the switch is properly connected to the electrical connector.
- Squeeze the switch and electrical connector together with your hand as you try to start the engine.
- If the engine starts and remains idling while holding the switch and connector together, you need to properly connect the switch.
Even if this quick test doesn't yield results, you will still want to troubleshoot the ignition switch. Your vehicle repair manual may help you here.
Check for throttle body and throttle plate buildup:
Also, check your throttle body and throttle plate for coking. Carbon buildup will prevent proper operation of the throttle valve. If the valve doesn't have free movement, it can cause a rough idle and stalling with light buildup. Coking usually causes the engine to die immediately after the engine starts.
You can use carburetor cleaner to remove carbon buildup from the throttle bore and valve, if necessary.
2. If the Engine Stalls at Idle
Various components and maintenance issues may cause an engine to stall when idling. The source of the problem may differ with the type of engine model or configuration.
Ignition System Problems
The system may have a weak spark or the spark may fail to reach the spark plugs.
- Start by checking the service schedule for the spark plugs and spark plug wires.
- Remove and check one spark plug at a time.
- Check the electrodes' gap using a wire gauge and compare to specs in your vehicle repair manual.
- A plug that burns the air-fuel mixture correctly has a brown to grayish-tan color. Other colors may indicate problems with the fuel, ignition or even mechanical problems.
- A wet plug may indicate too much fuel reaching the cylinder, or a mechanical problem that is leaking oil into the combustion chamber like worn-out rings, cylinders or valve stem seals.
- Then, check the spark plug wires and ignition coils with the help of your vehicle repair manual.
- If your engine uses a distributor, check the cap as well for cracks, carbon traces, and oil contamination.
Bad canister vent valve:
The evaporative emissions (EVAP) control system routes fuel vapors into a canister and then to the intake manifold where they are burned along with the air fuel mixture.
But a system failure, especially a failure of the canister vent valve, can cause the engine to stall at idle or during acceleration. Consult your vehicle repair manual for valve testing procedures in your particular vehicle model.
Other system components to check, depending on your particular model:
- purge solenoid or valve
- relief valve
Checking the PCV Valve:
The positive crankcase ventilation (PCV) system purges blowby gases from the engine. Most PCV systems make use of a small valve to route blowby gases back into the intake manifold to be re-burned. Over time, the valve may get stuck open, causing too much air to flow into the intake manifold during idle or acceleration, which can cause the engine to stall.
You can easily remove the valve and check it by shaking the valve. If the valve doesn't rattle, probably the valve is stuck. But this common test doesn't work for all models. Consult your vehicle repair manual, if necessary.
A Bad Throttle Position Sensor (TPS)
The TPS monitors the angle opening of the throttle valve and sends this information to the electronic control module (car computer). The computer uses this information, along with signals from other sensors, to adjust air-fuel ratio, ignition timing and other outputs.
Most TPS units use a variable resistor with several contact points that can wear out and fail after miles of service. When the contacts open, it may cause the computer to lean the mixture too much and stall the engine. A bad TP sensor usually triggers a diagnostic trouble code (DTC). So you can retrieve the code from the computer using a scanner tool.
Whether or not you find a trouble code for the TPS, it's a good idea to test the sensor using a digital multimeter:
- Unplug the sensor electrical connector.
- With the engine off, measure resistance across the sensor terminals with the throttle in closed position, at half throttle, and at full throttle.
- Compare your resistance measurements to the specifications listed in your vehicle repair manual.
Checking the IAC (for Engines With Multiport Injection Systems, Most Modern Engines):
In most cases, the problem here is associated with the idle air control (IAC) valve.
The IAC valve is controlled by the computer and allows air to bypass the throttle valve under idle or cold temperature conditions to prevent a rough idle or keep the engine from stalling. You can test the IAC valve using a digital multimeter:
- First, unplug the IAC electrical connector.
- Turn the ignition key to the On (run) position but don't start the engine.
- Set your voltmeter to 20 Volts range (or above 15 Volts) on the DC (direct current) scale.
- Connect your black meter lead to ground (engine metal surface or battery negative post, for example) and with the red meter lead probe the terminals on the harness side.
- You should get around 10 to 12 volts, approximately, from one of the terminals. This is the incoming signal from the engine control module (car computer). If not, there are problems in that side of the circuit.
Now measure the condition of the valve by measuring resistance across the IAC connector terminals.
- Set your voltmeter to ohms for this measurement.
- You should get anywhere from 6 to 13 ohms of resistance.
To check for a possible internal short:
- Check for resistance from each IAC connector terminal and the IAC valve body with your voltmeter.
- You should get around 10,000 ohms of resistance.
Checking the internal physical condition of the IAC valve:
A blocked IAC may cause the engine to die as soon as it starts if the valve sticks close. IAC valves may fill with carbon deposits overtime, preventing the pintle from moving, or blocking valve passages.
On most vehicle models, the IAC valve is readily accessible and can be removed easily by unscrewing a couple of mounting bolts.
- First, turn the ignition key off and remove the key.
- Unplug the IAC valve electrical connector.
- Unscrew the valve mounting bolts and remove the valve from the vehicle.
- Check the valve's pintle and housing for carbon buildup. If necessary, clean the valve with carburetor cleaner and a soft rag to remove deposits.
- Also, check the valve O-ring. It should be soft and in good condition. Otherwise, replace it.
Consult your vehicle repair manual for the recommended testing procedures for your particular vehicle model, if necessary.
Your particular fuel injection system may have other necessary adjustments. For example, you may need to adjust
- the idle speed
- the throttle cable
- the throttle plate stop
- the idle air-fuel mixture
Consult your vehicle repair manual, if necessary.
Troubleshooting the ECT (Engine Coolant Temperature) Sensor
The ECT measures engine operating temperature and sends this information as electrical data to the computer. The car computer uses this information, and information from other sensors, to adjust ignition timing for better engine efficiency.
Thus, a bad ECT sensor can have a direct impact on engine performance. If the sensor causes a lean air-fuel mixture the engine may idle rough or stall.
Most ECT sensors use a thermistor to vary the voltage signal sent to the computer according to engine coolant temperature. The higher the coolant temperature, the lower the sensor resistance and higher the signal voltage.
To test the ECT sensor:
- Locate the sensor. It's usually close to the thermostat housing.
- With the engine turned off and cool, unplug the ECT sensor electrical connector.
- Set your digital multimeter (DMM) to the ohms scale and measure resistance across the sensor terminal. Make a note of the resistance value.
- Plug in the sensor electrical connector and start the engine. Let the engine idle for about 15 or 20 minutes so that it reaches operating temperature.
- Shut off the engine.
- Unplug the ECT electrical connector and measure resistance across the sensor terminals as you did in step 3.
- This time, sensor resistance should be considerably lower. For example, depending on your particular sensor configuration and engine model, the sensor may have 40K ohms when the engine is cold but 2K ohms when hot.
- Compare your results to the specifications listed in your vehicle repair manual.
A failing ECT sensor can trigger the check engine light. If your sensor checks Okay during your tests, but the computer says there's a problem with it, check the sensors circuit between the ECT and the computer for bad connections or damaged wire.
Also, check that the computer is sending the correct reference voltage to the sensor.
- Unplug the ECT sensor electrical connector.
- Turn the ignition key to the On position, but don't start the engine.
- Measure the voltage at the harness connector (the one leading to the computer) with your voltmeter.
- Usually, you should read about 5 volts at the connector. If not, there's a problem in the circuit.
Checking for a Stuck EGR Valve
The exhaust gas recirculation (EGR) systems reintroduces exhaust gases into the combustion chambers to reduce engine temperature and poisonous emissions.
It is common for the EGR valve passages to get blocked from carbon buildup. Carbon deposits may prevent the valve from closing, causing the engine to stall at idle. Also, an EGR valve may leak through the base, causing the engine to stall at idle, at deceleration, or during a quick stop.
If you've never checked under the valve, or it's been a few years since you last checked, it might be a good idea to take a look now. But make sure to have on hand a gasket replacement. You may need to install a new one. If you can't find one for your particular vehicle model, you still can buy gasket paper at your local auto parts store and make the gasket yourself.
- Most EGR valves are accessible and not difficult to remove. Check the EGR valve configuration for your particular model, though, to make sure you have all the tools necessary for the job. Usually, all you need is a couple of wrenches, a ratchet, the appropriate socket sizes and, sometimes, a pipe wrench to disconnect the valve from the exhaust pipe. Consult your vehicle repair manual, if necessary.
- Remove carbon deposits from under the valve and intake ports using carburetor cleaner. Make sure not to allow carburetor cleaner to reach the valve diaphragm or any electrical circuits, depending on your particular model, or you may ruin the valve.
- Reinstall the valve using a new gasket, if necessary.
Checking the EGR Valve:
Depending on your particular EGR valve configuration, you probably can stick your finger through the underside of the valve and try to move the internal diaphragm. If the diaphragm doesn't move, it may be stuck.
Here are a couple of alternative methods to check for diaphragm movement:
- Operate the accelerator linkage with your hand to increase engine speed. You may be able to see the diaphragm moving using a mirror placed under the EGR valve. If you can see the diaphragm with the mirror, but you can't see it moving as you operate the accelerator linkage, the diaphragm might be stuck.
- You can also use a hand-held vacuum pump. You can do this test with the engine off and using a mirror to watch diaphragm movement. Disconnect the vacuum hose form the valve and connect the vacuum pump instead. Apply about 15 in-Hg of vacuum to the EGR valve while watching for diaphragm movement with the mirror. If the diaphragm doesn't move, it might be stuck.
Another cause for the valve diaphragm to fail is wear or damage that causes it to leak exhaust gases. A quick way to test for a leaking diaphragm is using carburetor cleaner.
- Block the wheels using wooden blocks.
- Set your transmission to Park (automatic) or Neutral (standard).
- Start the engine and pop the hood open.
- Spray a quick burst of carburetor cleaner under the valve, towards the diaphragm.
- If the engine RPMs increase when you spray, the diaphragm is leaking and you need to replace the valve.
Bad Catalytic Converter
A bad catalytic converter can also cause you problems at idle. Depending on the particular problem, it may show other symptoms as well:
- a smell of rotten eggs
- rattling sounds
- poor engine power
One of the most common problems with a catalytic converter is a plugged unit. The internal substrate becomes partially restricted or disintegrates.
Usually, problems with the catalytic converter trigger the check engine light (CEL). The computer may store a P0420 through P0423 code. If you find a catalytic-converter-related code, do troubleshoot the catalytic converter before replacing it, to confirm that the cat is in fact bad. Catalytic converters are expensive and you need to make sure that some other components is not involved in the problem. If necessary, consult your vehicle repair manual.
Checking the IAC on Vehicle Models With Throttle Body Injection (TBI) Systems:
Many vehicles equipped with TBI systems also have an IAC valve controlled by the computer.
- You can use the test described in the previous section, if your vehicle has an IAC valve.
- Besides, some of these systems may have an idle speed adjustment (consult your repair manual),
Usually, you can adjust idle speed by hooking up a tachometer (some digital multimeters have this feature) to the engine. Then, you can turn the idle speed screw on the throttle body or carburetor until the RPMs are set according to specifications.
Diagnostics on Vehicles With a Carburetor
A stalling issue usually points to a maladjusted idle mixture screw. To reset the screw:
- Locate the idle mixture screw.
- With the engine off, bottom the screw.
- Back out the screw 2 1/2 turns.
- Start and idle the engine.
- Make the final adjustments until you get a smooth idle.