When an engine is idling freely in neutral or park, it has a negligible load on it resisting the motion. As soon as any gear is selected, the load is put onto the engine. The fuel and air delivery, spark plugs, and transmission need to be adequately functional to drive the car further.
If the vehicle’s engine stalls as soon as it’s put into reverse or drive, it could be due to the following listed possible causes.
- Faulty Idle Control Valve
- Clogged Fuel Delivery
- Faulty O2 Sensor
- Torque Converter
- Vacuum Leaks
- Corroded Spark Plugs
1. Faulty Idle Control Valve
The idle control valve controls the ratio of air-fuel mixture sent to the engine when the throttle is at the default position. It is usually controlled by a solenoid spring mechanism on a valve.
Over time the valve can develop soot, carbon, and dirt build-up, which can cause the valve to not open fully to its designated position under load.
Insufficient supply of air-fuel mixture as soon as a load is applied on the vehicle, by putting it in reverse gear, without any throttle input can cause the engine to stall.
Stuck or clogged idle control valves can easily be cleaned and reused. Changing or cleaning the idle control valve takes around 30-60 mins and can be done on your own by watching some guides on the internet.
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2. Clogged Fuel Delivery
Another factor that might cause your car to stall when put in reverse is a clogged fuel delivery system. A clogged fuel filter or fuel injector would not allow adequate fuel to be pumped into the cylinders.
Hence whenever the car is put under sudden load, there is a chance of your car stalling down. A fuel injector can get clogged due to unfiltered impurities in the fuel over time. Injectors are housed on the fuel rail, usually located on the engine near the intake manifold.
A clogged fuel injector can easily be diagnosed by taking it out and turning the key to pump fuel out from it. A weaker fuel stream than the rest indicates a clogged injector or two.
These can be easily replaced on your own or cleaned if possible.
A choked fuel filter can be diagnosed by monitoring the fuel rail pressure. A low fuel rail pressure indicates a bad fuel pump or fuel filter.
A clogged filter can also cause the fuel pump to go bad if the issue has not been taken care of for a long time.
The fuel filter and the pump require professionals for trouble-free replacement and can be a bit harder if done on your own.
3. Faulty O2 Sensor
An O2 sensor reads the exhaust air constituents and ensures that most fuel is being burned in the cylinder.
It decides the air-fuel ratio depending on the amount of unburned fuel inside the exhaust gas. A faulty O2 sensor can cause the mixture to be extremely lean or rich, which results in improper combustion.
If sudden load, such as shifting the gear in reverse, is applied on an engine with a faulty O2 sensor, it is likely to stall as well.
Cleaning the O2 sensor thoroughly or changing it can mostly fix the issue if it is the culprit.
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4. Torque Converter
A faulty torque converter can cause the engine to stall if it’s not operating normally. A faulty lockup clutch can usually cause the torque converter to lock in reverse gear as soon as it’s applied.
It’s just like dumping the clutch in a manual. Torque converters are supposed to engage clutch lockup at higher speed in 3rd gear or above.
But a faulty transmission can engage lockup in reverse, which can cause the engine to stall due to the sudden load at idle position.
Low transmission fluid or problematic solenoid or stuck valve can cause this issue. Check your transmission fluid and ensure it is adequately filled and is in good condition.
To replace or clean the lockup valves and solenoids will mainly require disassembly of the tranny and is better done by professional mechanics.
5. Vacuum Leaks
A vacuum leak is a leak anywhere along the intake side of the car that allows your vehicle to breathe air in from a passage other than the confined intake manifold passage.
Mass air flow sensor measures the amount of air coming through the intake manifold, and the ECU then releases an appropriate amount of fuel to form a good air-fuel mixture for complete combustion.
Such leaks draw more air in than what is reported in the ECU, which results in a leaner Air-Fuel ratio. The engine is unable to perform complete combustion with this leaner Air-Fuel mixture and loses power.
It can also cause the vehicle to stall when under load, such as when the reverse gear is selected.
Vacuum leaks can be located in the engine bay and mainly develop near the intake manifold. They can easily be detected when the engine is idling by spraying water or propane on multiple locations near the intake manifold.
Spraying water will cause the engine RPMs to drop, whereas propane will cause the engine RPMs to rise if sprayed on the leak. The leak can be fixed using epoxy or welding.
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6. Corroded Spark Plugs
Bad spark plugs can also cause the engine to stall under load by putting the car in drive or reverse. Spark plugs produce spark necessary for igniting the air-fuel mixture in the cylinder.
A corroded spark plug might not produce a noticeable spark required for complete combustion or might cause delayed combustion.
Checking the vehicle’s spark plugs condition by taking it out is fairly easy and can be done by yourself.
Spark plugs do have a long life and can be reused after cleaning using sandpaper or wire brush if they are not older than 25,000 – 40,000 miles.
If you have an automatic car that stalls when put in gear, diagnosing is not an easy job that can be done on your driveway. I recommend having a professional car mechanic inspect the car for a proper diagnosis.