Why Does My Car Start Then Dies?

For an internal combustion engine to work correctly, it needs three things: sparks, fuel, and air. If any of these elements are either missing or found in incorrect proportions, the engine will simply die right away. The combustion process is a delicate one indeed and a fine balance between all those elements must be maintained at all time. No crank/no start conditions are usually pretty easy to fix but in the case of a car that actually starts but dies right after, the diagnostic process may be a little more complicated than that. This article will outline the most common causes of engine stalls at startup and what you need to do to get your car back to normal in no time.

Sparks

No matter if the customer’s complaint is for an engine stall condition or a crank/no start condition, the first thing a mechanic will check is the ignition system. It’s, by far, the system most likely to fail and to create intermittent problems. And it’s also the easiest system to inspect so why not start with that.

To test the ignition system, you first need to locate the fuel pump relay inside the engine bay’s fuse box and remove it so you don’t flood the engine while performing the tests.

Locate the most accessible spark plug on your engine and remove either the coil-on-plug unit or the spark plug wire to allow access to the spark plug itself. Remove the spark plug and reconnect it to the coil/wire. While holding the coil/wire with a pair of insulated pliers to prevent any risks of electrical shocks, hold the anode or “the tip” of the spark plug to a metal part of the valve cover or the engine head. Ask someone to crank the car and watch the spark plug’s tip closely.

strong spark
Source: https://pixabay.com/

If the ignition system works fine, you should see a bright spark appear between the spark plug’s anode and cathode. Technically, it should be sparking alright since the problem here is that the engine starts but then dies. The fact that it started for a second or, at least tried to, means that it was, for a slight moment, actually producing a spark but it then stopped for some reason.

Now, re-install the fuel relay and retry the same thing and try to notice if the spark disappears right before the engine dies or not. Watch out for your eyes as fuel could be sprayed out of the cylinder while the spark plug is not in the hole. If it disappears before the engine stalls, you could have found the cause to your problem.

But you aren’t out of the woods yet. It’s not enough to know that your car is stalling because of an ignition problem. You still need to find what is wrong with that said ignition system.

Alternator

When you start your vehicle, the power required to make the starter turn is supplied by the battery. But, as soon as the engine starts and the starter returns to its original position, the battery circuit is turned off and the alternator takes over. If for some reason, your car’s alternator doesn’t supply the correct amount of power, the engine will slowly drain the battery and die. In some case, it could even die right away if the circuit is open somewhere inside the alternator or if the battery wasn’t in that good of a shape.

It’s not easy to test an alternator when the engine can’t be kept running since you absolutely need the engine to run for the alternator to produce electricity. A neat trick here is to hook up the battery to a 12v power supply to keep the car running as long as you can. When it’s done, take out an auto digital multimeter and test the power output directly at the alternator. Any reading under 13.5v-14v will let you know that something is not quite right with your alternator and it will need to be replaced or rebuilt.

Bad ground

If your engine has a bad ground connection to the vehicle’s body, it will cause a higher than normal resistance in the circuit which will, in turn, reduce the amount of power reaching the various electrical components. One of the first things to fail in such a situation is usually the ignition system.

It’s pretty common to see a situation where the ground is still in good enough condition to let power go through when the engine is stopped so the car will indeed start. But when the engine starts to vibrate as the pistons go up and down, the wire will start to vibrate too and may not work as well anymore, causing the engine to stutter and stop.

The easiest way to know whether or not the ground is causing your engine to suddenly die is to supply it with a known good ground alternative. Get yourself a big wire or some booster cable and connect one end of the cable to the negative side of the battery and the other end to the engine head or transmission. If the problem goes away, you know for a fact that one of the ground connections is faulty.

If you are suspecting a potential bad ground problem, locate all the main ground/body connections and inspect them for rust and wear. Ground wires often rust out because of water and snow and they’ll need to be replaced.

Faulty sensors

The spark igniting the air/fuel mixture needs to be in perfect synchronization with the pistons or a misfire will certainly happen. The powertrain control module uses all the data he can get from the various sensors installed on your car and will then inform other modules on how they should operate. Depending on the temperature, the altitude, the position of the gas pedal, and numerous other variables, the PCM may adjust the speed, length and time of the spark to stay as fuel-efficient as possible.

In order to do that, the ignition module will use, among other things, the crankshaft and camshaft sensors to follow the position of the pistons and the mass air flow sensor to evaluate the amount of oxygen in the air. If any of those sensors is faulty, it could lead to multiple misfires causing the engine to die or a preventive shut down of the ignition system also leading to a quick engine stall.

Diagnosing sensors is not an easy task for purists and may be better left to professionals but if you possess some basic auto mechanic knowledge, use a scanner to monitor the suspected sensors’ data and find out if any reading is out of threshold. If it is, replace the faulty sensor and try again.

Fuel

Now that you know that everything is working great with your ignition system, the next natural thing to inspect is the fuel system. Too much fuel or not enough could both cause your car to stall so you need to find out which one it is.

Remove one of the spark plugs. If a strong fuel smell emanates from the spark plug hole, you can be certain that your car problem is not related to fuel starvation. Too much fuel is, instead, injected in the combustion chambers which will cause a condition called flooding. Engine flooding will typically create misfires and jerks. The unburned fuel will then drip down the cylinder walls and will accumulate in the oil pan. Taking out the oil dipstick and smelling it will let you know if fuel has already contaminated your engine oil. In some of the worst situation I have seen, so much fuel had accumulated in the oil pan that it had actually made the oil level rise on the dipstick. In such a case, an oil change will obviously be required once the problem has been found and fixed.

In case of fuel flooding

Flooding can be caused by:

  • a faulty ignition system (but we know it’s not the case here)
  • a faulty fuel pressure regulator
  • faulty sensors
  • numerous subsequent misfires

To find out the faulty components causing a fuel flooding situation, the first thing to do is to monitor the pressure inside the fuel system using a specially designed gauge. If the pressure reading is too high, look towards a faulty pressure regulator or a malfunctioning sensor. A cracked or loose vacuum hose will lead the regulator to allow for a higher-than-usual fuel pressure while a faulty MAF sensor could mislead the PCM into thinking that there is more oxygen in the air than it actually has. Both situations will eventually flood the engine and bring it to a halt.

In case of fuel starvation

Fuel starvation is the opposite of fuel flooding. Instead of having too much gas sprayed into the cylinders, there’s just not enough. The air/fuel ratio won’t be correctly maintained and the engine will stall. Fuel starvation can be caused by:

  • a faulty fuel pump
  • a faulty fuel pump relay
  • faulty injectors
  • a faulty fuel pressure regulator
  • faulty sensors

90% of the times, when there’s no gas to the injectors or simply not enough to reach a correct air/fuel ratio, it’s related to the fuel pump. It’s either faulty or a component included in the fuel pump electrical circuit is not working properly. Relays are prone to work intermittently, meaning that it could work when you turn the ignition key but it opens a couple of seconds/minutes later causing the fuel pump and consequently, the engine to stop.

Faulty or clogged injectors could also limit the amount of fuel delivered into the combustion chamber and create misfires, which could, in turn, flood the engine in a jiffy. That’s a bit counter-intuitive to think that fuel starvation could create a flooding condition, but it’s still something that could happen.

Since a couple of years ago, most cars were equipped with vacuum operated fuel pressure regulators and the membrane used to regulate the pressure would become dry with time and the regulator would start to behave erratically. Unstable fuel pressure will subsequently cause the air/fuel ratio to be incorrect which could also make the engine jerk and stop. On more recent vehicles, however, car manufacturers don’t use fuel regulators anymore and will, instead, regulate the fuel pressure directly from the engine management module allowing for finer tuning of the A/F ratio and eliminating the risk of encountering fuel regulator problems.

And just as with the fuel flooding condition, any malfunctioning sensor can also send a bad reading to the PCM making it think it needs to send less fuel than it actually does, quickly leading to fuel starvation right after the engine is started.

Air

If for any reason, the air volume entering the engine is either altered or reduced, the air/fuel ratio won’t be properly maintained and the engine will stutter and could even stall. Numerous factors can have a direct impact on the airflow getting into the combustion chambers; some as basic as a clogged air filter to more intricate ones like an improperly calibrated mass air flow sensor.

MAF sensor

To reach a correct A/F ratio, the PCM relies on various sensors to efficiently estimate the air volume going through the intake as well as the oxygen concentration. Depending on the readings it receives from the sensors, the PCM will be able to adjust the injector’s spraying time and synchronize the sparks to obtain the best possible fuel combustion.

The mass air flow sensor is one of these sensors and it is, in fact, probably the most important one of them all. Two different types of MAF are used in the automotive industry, the vane model and the hot wire model. Both sensors work in different ways but aim to achieve the same result: inform the PCM of how much air is entering the engine. If it starts misbehaving and sends a false reading to the PCM, the car will most likely die right away.

MAF testing
Source: http://www.howrah.org/

The easiest way to find out if your mass air flow sensor is in good working condition is to use an OBD2 scan tool to inspect the MAF sensor’s data and make sure it’s between the normal threshold. If it’s not, something’s definitely wrong with it.

In such cases, the first thing to do is to disassemble the mass air flow sensor and inspect it visually. On hot wire models, dirt and oil fumes deposits will accumulate on the said hot wire and will prevent it from doing its job. With vane models, on the other hand, the most common problem is rust building up on the vane door. One thing you could try to fix this up is to spray the inside of the sensor with carburetor cleaner to clean it up a bit.

Watch out not to damage the vane door or the hot wire or you’ll need to replace the whole thing.

Once it’s done, re-install it on the vehicle and give it a try. If the reading is still off the chart, the mass air flow will need to be replaced.

Air leaks

Even a well calibrated MAF sensor can’t make up for an air leak happening later in the air intake system. The mass air flow will do its job and send its measurements to the PCM but the extra air entering through a crack in a hose somewhere will distort the end A/F ratio.

Lookout for dried out hoses and air tubes, loose hose clips as well as damaged air intake gaskets. Spraying suspected components with brake cleaner while the engine is running will help you find any cracks or leaks.

Idle air control actuator

The IAC is the component allowing your car to stay at idle when you don’t have the foot on the gas pedal. When the engine is started, the idle air control actuator slightly opens the throttle to let some air in. When it becomes faulty, it won’t open the throttle anymore, preventing air from getting into the combustion chambers. Without air, no oxygen and no oxygen means no explosion.

A neat trick to quickly verify if the IAC is causing your vehicle to stall at startup is to slightly hold the gas pedal when cranking. If the vehicle starts or, at least, tries to, you know you may have found your problem.

Clogged air filter

An obstructed air filter could also cause the engine to stall but it’s definitely less frequent than most other possible causes on this list. The air filter would need to be completely clogged in order to be problematic to the point where it would make the engine stall. A dirty or partially obstructed air filter could cause other problems like a higher fuel consumption but it wouldn’t kill the engine altogether.

Watch out for bird and small rodents bringing organic material such as grass and leaves to build nests inside the opening of the air intake.

PCM

Last but not least, and also the worst case scenario is a faulty PCM. Never forget that the PCM is what is essentially holding everything together. It’s his job to interpret all the incoming data and to react accordingly. If the mapping or program is just plain wrong, it won’t be able to adjust the A/F ratio, simple as that. It could also decide to either refrain from telling the IAC to open the throttle, or the spark plugs to spark, or the injectors to inject fuel, etc. You get the point.

But powertrain control modules tend to be super expensive and really tedious to test and inspect. Furthermore, they are pretty reliable and faulty PCMs are definitely a rare occurrence.

As auto mechanics often say, when you reach the “replace the PCM” point in your troubleshooting chart, you should probably start all over again and re-test everything first.

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