By Larry Carley
Technical Editor

Intermittent driveability problems of any kind are always a challenge to diagnose. When a customer brings you a vehicle and complains of a stalling problem, you may not have much to go on other than their description of what is happening and when it happens. The more information you can get out of them the better, because you’ll likely need all the details you can get to narrow down the list of possible causes.

Many times, there will be no check engine light and no fault codes to give you a clue as to what might be causing the engine to stall. So the trick to diagnosing an intermittent stalling problem is to catch the vehicle when it is acting up — and that rarely happens when you have the vehicle in your shop. You may have to keep the car and drive it for a few days to see if it stalls. Or, if you have a flight recorder, you can hook it up to the car and send your customer home with instructions on how to catch a snapshot of the problem when it happens. But even then, you may not capture any usable data.

Common Causes
Common causes of intermittent stalling may include a bad idle speed control (ISC) system, low fuel pressure, loss of ignition, vacuum or EGR leaks, or other problems we’ll get to later in this article.

The engine’s idle speed is controlled by how much air is allowed to bypass the throttle via the idle bypass circuit in the throttle body. The PCM monitors idle speed and operates a solenoid or small electric motor to move a valve that regulates airflow through the air bypass circuit. If the ISC valve is gummed up with fuel varnish or dirt, it may stick preventing the engine from maintaining the proper idle speed when the throttle closes. A faulty idle speed control circuit may also allow the engine to stall when idling if it fails to compensate for idle loads placed on the engine by the charging system or A/C compressor. This can lug down the engine causing it to stall.

A problem with the idle speed control circuit would be the most likely cause of stalling if the problem only occurs when the engine is idling or when the car comes to a stop after driving.

The idle speed control circuit can often be diagnosed with a professional-grade scan tool that has the proper software to command an idle speed control test. If such a test is not available on the vehicle, then obviously you can’t check it this way. But if it is, and you have a scan tool that can run the test, a good place to start would be to command an increase in idle speed to see if the ISC system responds as it should. No response? Then you know the ISC system has a problem. But what?

If you don’t have a scan tool that can do an idle speed test, or the vehicle does not have this capability, you can still test the response of the ISC system by turning on the A/C to max. If the ISC system is working correctly, the PCM should adjust idle speed to keep the engine from lugging down when the A/C compressor engages. If idle speed drops down and does not come back up, the ISC system is not responding the way it should.

Don’t jump to conclusions — even though the most likely cause would be a bad idle speed control solenoid or motor. Don’t replace anything until you do the following:

1. Clean out the idle air bypass circuit with several shots of aerosol throttle cleaner. Aim for the little air bypass holes in the throttle opening. Use the small plastic tube that attaches to the nozzle on the aerosol can to aim the cleaner directly into the bypass port(s). Let the cleaner soak for at least 10 minutes before giving it another blast of cleaner. Then start the engine and repeat the idle speed test.

If the engine now idles normally, and you can command idle speed changes, you’ve fixed the problem (a dirty idle bypass port or valve). But if the ISC system still fails to respond to your command to increase idle speed, or fails to maintain normal idle speed, there’s a problem that will require further diagnosis before you replace any parts.

2. Check the idle speed control system wiring harness and connector to make sure it is tight and corrosion-free. Problems here can prevent the PCM command signal from reaching the ISC solenoid or motor. Look up the ISC circuit wiring diagram for the vehicle, and check for voltage and ground at the appropriate terminals. Replacing a perfectly good ISC motor or solenoid won’t fix the problem if the fault is in the wiring.

3. If the wiring harness appears to be good, and the ISC solenoid or motor is receiving a signal from the PCM — but is not responding — you can assume the fault is a bad ISC motor or solenoid. Unfortunately, some technicians just replace the ISC motor or solenoid before checking these other possibilities, and then discover it didn’t fix the fault.

Another possible cause of ISC-related stalling problems would be a bad ISC driver circuit in the PCM. This doesn’t happen very often, but the driver circuit may be damaged if one of the wires that carries voltage in the ISC control circuit shorts or grounds out. The driver circuit can be tested with a noid light or a digital oscilloscope to see if the PCM is sending a digital control signal to the ISC motor or solenoid. No signal would tell you the driver circuit is dead and your customer needs a new PCM.

Stalling Due To Low Fuel Pressure
If the engine stalls while idling or driving, the engine may be running out of fuel or not receiving enough fuel pressure to keep it running.

The most likely cause of this kind of stalling would be a fuel pump that is not spinning fast enough or is cutting out intermittently. If the vehicle is more than seven or eight years old and has a lot of miles on it, the fuel pump would certainly be suspect. But as with the idle speed control circuit, don’t replace anything until you’ve run some diagnostic tests.

The first thing to check would be fuel pressure. Look up the specifications for the year, make and model of the vehicle, then connect a gauge to the fuel system and measure the pressure with the key on, engine off (KOEO), then again with the engine idling. Fuel pressure should be within specifications when the key is on, then drop 4 to 6 psi once the engine starts and is idling.

Note: Some vehicle manufacturers list pressure specifications for both KOEO and idle, while others only list a fuel pressure specification for one or the other. Make sure you are using the correct specification.

If fuel pressure is low (less than the minimum specification), check the fuel filter. When was the last time it was replaced? Never?

Next, check the fuel pressure regulator to see if it is leaking. This can be done by pulling the vacuum hose off the regulator. Look in the hose to see if there is any fuel inside (there should be none). If the inside of the vacuum hose is wet with fuel, the diaphragm inside the fuel pressure regulator is leaking. Replace the regulator.

Fuel pressure should drop 4 to 6 psi or more when the vacuum hose is removed from the regulator. No change in the fuel pressure gauge reading would tell you the regulator is not doing anything.

You can also pinch off the regulator return hose to see if fuel pressure goes up (it should). Little or no change in fuel pressure would indicate a weak fuel pump, not a bad regulator.

You should also check the fuel pump’s ability to deliver an adequate supply of fuel by measuring fuel volume. A good fuel pump is usually capable of delivering at least 750 ml (3/4 quart) of fuel in 30 seconds. If it can’t, there’s a problem. It might be a weak pump, or maybe the pump isn’t spinning at normal speed because of low voltage or a bad ground connection. Loose or corroded wiring connections anywhere in the fuel pump electrical circuit, or bad ground connections may be affecting the operation of the pump. Even low charging voltage can affect fuel pressure. So you may have to make some additional voltage checks to figure out if the problem is the wiring or the pump itself.

Even if the pump is generating normal pressure at idle, it may not deliver adequate flow at higher engine speeds. The engine may start and idle fine, but run out of power or even stall at highway speeds. If the engine starts right back up after stalling, low fuel volume delivery because of a weak pump is likely the cause.

Intermittent Stalling Caused By Ignition Faults
If the engine is running normally one minute, but suddenly quits, loss of ignition may be the problem. This can be caused by anything that disrupts the ignition system. Possible causes include an intermittent short or open in a crankshaft position sensor or the sensor’s wiring connector or harness, an intermittent short or open in an ignition control module, or an intermittent short or open in the coil primary voltage supply circuit (bad ignition switch or wiring).

Magnetic crankshaft position sensors are fairly simple and reliable, but the internal windings can sometimes open and kill the crank signal if the sensor gets too hot. If the engine has a magnetic crank sensor, you can test it with an ohmmeter to see if the resistance is within specifications. But even if the sensor tests “good,” it may still cause trouble. We’ve seen Saturn crank sensors that measure within specifications when tested cold with an ohmmeter, but suddenly go wide open if the sensor gets hotter than 200 to 220° F. One minute it’s working fine producing a good signal, and the next it goes dead and causes the engine to stall. If an overheated crank sensor is causing a stalling problem, the engine may not restart until it sits awhile and the sensor cools down.

Hall effect crank position sensors are more troublesome because they are vulnerable to electrical problems in the wiring and internal circuit components. Hall effect crank sensors typically have three wires: voltage supply, ground and signal return. They produce a digital on-off signal rather than a sine wave AC signal like a magnetic sensor. If a Hall effect crank sensor does not receive the required supply voltage from the PCM, or it has a bad ground, or its internal circuitry is not working correctly, it won’t produce a crank signal. So be sure to check the wiring connector and harness for problems before replacing the sensor.

Intermittent faults in ignition modules can also cause an engine to suddenly stop running, and are often heat-related. When the module overheats, it quits working — but starts working again after it cools down (say 15 to 30 minutes later). When a bad module is responsible for an intermittent stalling problem, the engine will usually cold start just fine and run OK for short distances, but stalls after driving more than a few miles or when driving at highway speeds (especially during hot weather). It’s as if somebody suddenly turned off the ignition.

Ignition modules produce internal heat and require a good heat sink to prevent them from overheating. Modules that are mounted in or on distributors or the engine often require a layer of dielectric grease underneath to conduct heat away from the electronics. If somebody replaced the module and forgot to apply the grease, it may cause the module to run too hot and shut down.

An intermittent open that occurs in the primary ignition circuit can be yet another cause of engine stalling. A worn ignition switch in a high mileage vehicle is often the culprit. Many people tote around huge key rings with multiple keys, fobs and trinkets dangling from the ring. When all of this weight is suspended from the ignition key in the switch, the jingling back and forth as the vehicle travels down the road can wear out the ignition switch and lock over time. Replacing the ignition switch is the fix here. But your customer should also be advised to lighten up their key ring.

What about a bad ignition coil? A bad coil on an engine with a distributor and a single coil will typically cause a no-start, or misfire due to a weak spark. But if the coil has an intermittent open, it could cause a momentary loss of spark that would kill the engine.

If you suspect a bad coil, measure the primary and secondary resistance with an ohmmeter — first with the coil at normal room temperature, then after heating it up with a hot air gun. Primary resistance is usually very low, maybe a couple of ohms or less, while secondary resistance is usually high, say 8,000 to 15,000 ohms or more. If you see a significant difference in the resistance readings hot versus cold, or a reading goes open, replace the coil.

On engines with distributorless ignition systems or coil-on-plug ignitions, a single coil failure will cause the engine to misfire and lose power. But the misfire usually isn’t bad enough to cause it to stall unless the engine is idling and can’t maintain a high enough idle speed.

More Possibilities
Vacuum leaks on engines with airflow sensors can lean out the fuel mixture and may cause enough lean misfire at idle to kill the engine. Vacuum leaks usually don’t have much effect at higher engine speeds because there’s enough airflow coming in through the throttle. But at idle when airflow is restricted, any leakage downstream of the airflow sensor can disrupt the air/fuel mixture. Common leak points include any vacuum hose connection to the intake manifold (including the PCV valve, EGR valve and brake vacuum booster), intake manifold gaskets, the throttle body gasket, and the air inlet hose between the airflow sensor and throttle.

If the check engine light is on and you find a lean code (P0171 or P0174), or a random misfire code (P0300), check for vacuum leaks. You can also look at fuel trim with your scan tool to see if it is higher than normal. Fuel trim should usually be plus or minus 8, 10 or higher indicates a lean air/fuel mixture, and 20 will usually set a lean code.

An EGR valve that fails to close at idle is another common cause of stalling. If the EGR valve is stuck open or can’t seat because of carbon buildup under the valve, it will allow too much exhaust to be sucked back into the intake manifold. This can make the engine idle rough, misfire and/or stall. Inspecting and cleaning the EGR valve (and replacing it if necessary), and cleaning the EGR port in the intake manifold should cure the problem.

If an engine starts and runs OK for a few minutes, then stalls, the problem is likely a buildup of exhaust backpressure that is strangling the engine. The cause is usually a plugged catalytic converter, but you should also inspect the pipes to see if any are crushed or kinked.

A quick check for excessive exhaust backpressure is to hook up a vacuum gauge to the intake manifold and start the engine. Normal intake vacuum at idle will usually be 18 to 22 inches of Mercury (Hg). If intake vacuum is low, and continues to drop as the engine idles, it would tell you pressure is backing up in the exhaust system.

Bad gas can be yet another cause of intermittent stalling. If gasoline has been contaminated with water, the water can settle to the bottom of the fuel tank and cause the engine to stall when it is sucked up by the fuel pump. Adding a bottle or two of alcohol fuel drier to the gas tank can help absorb and dissipate the water. But if the fuel contains a lot of water, you may have to drain the fuel tank and dispose of the bad gas.