What Causes A Car To Stall Whilst Idling?

Car Stalls at Idle

There’s nothing worse than having a car stall on you whilst idling at a set of traffic lights. Not only is it hugely embarrassing, it can also be extremely stressful. Having a huge queue of angry drivers in your rear view mirror isn’t a particularly nice feeling, not at any time of the day!

If a car stalls at idle, it’s a sure fire sign that something is broken or not operating properly within the engine compartment.

There are a number of components and factors that can cause a car to die and stall whilst idling, let’s take a look at the most common causes and how to fix them.

A Faulty Mass Air Flow Sensor

Without a shadow of a doubt, a faulty mass air flow sensor is the most likely culprit for a car that repeatedly stalls at idle.

A car engine operates on a simple of mixture of fuel and air. The weight and ratio of fuel and air mixing together is key for a car engine and is essential to maintain and steady and constant idle speed. This is primarily governed and controlled the Mass Air Flow Sensor. The MAF is a small-ish sensor which is located on the car air intake system and measures the amount of air coming into the car via the intake system. The MAF sensor passes its readings onto the car ECU (Electronic Control Unit), the ECU then requests the absolute correct amount of fuel be injected into the fuel injection system. When all working as it should, this allows for a perfect mixture of the two and creates a steady and correct idle speed.

When a Mass Air Flow sensor goes bad, it can cause major running problems. As it’s an electrical item, a whole host of different issues can arise such as incorrect readings being sent to the ECU. Incorrect readings will invariably result in the ECU sending either too much or too little fuel. Too little fuel being injected means that the car doesn’t have enough fuel to keep running and therefore stalls.

Can A Mass Air Flow Sensor Be Repaired?

Depending on the fault, in some circumstances, a Mass Air Flow sensor can be fixed simply cleaning it throughly. It’s really important that you only clean the sensor with a dedicated Mass Air Flow sensor cleaner however. Please take our advice here – DO NOT USE BRAKE CLEANER OR ANY FORM OF DEGREASED TO CLEAN YOUR MASS AIR FLOW SENSOR!!

Brake cleaner, engine degreasers and clutch cleaners can cause severe damage to the sensor and render it completely useless.

Our recommendation would be the excellent Liquid Moly Air Flow Sensor Cleaner. This is a dedicated MAF sensor cleaner and has been specifically designed to safely clean the intricate parts within the sensor. We have seen extremely positive results when using this cleaner.

A Faulty Idle Control Valve

An idle control valve is a small valve which controls and regulates a vehicle idle speed. With the aid of an actuator, the idle control valve is able to open and close upon instruction from the ECU to create a consistent idle speed. If the actuator is stuck, the ICV won’t be able to move and therefore can’t make the necessary adjustments. Depending on the severity of the problem, this can sometimes cause the engine to stall at idle.

If the actuator is sticking, it may be possible to strip it down and repair it with the aid of some lubricant. Our advice would be to use a throttle body cleaner or similar, such as Liqui Moly’s Carburetor and Valve Cleaner.

If that doesn’t work, you’ll likely need a new actuator. Some vehicle manufacturers don’t sell the actuator separately so you may need to buy a complete new Idle Control Valve.

Faulty Spark Plugs

Fully functional spark plugs are a key component of a car engine and are an essential component to ensure smooth and proper operation. As spark plugs are a replaceable, consumable item on a engine, over time they become dirty and worn out, meaning that the can’t create the necessary spark to keep the engine running at its optimum. It’s important that they are regularly checked and maintained.

Depending on the number of cylinders that the engine has, each car will have different number of spark plugs. Most common nowadays are 4 cylinder engines, they would therefore have 4 individual spark plugs. When replacing spark plugs, it’s always best practice to replace all of the plugs at once.

A diesel engine operates in a completely different way to a petrol / gasoline engine, diesel engines don’t actually have spark plugs, no need to worry about these if you’re a diesel car owner.

If your car is suffering from a lack of power, uneven idle or a stalling at idle, it would be wise to check your spark plugs and ensure that they aren’t covered in soot. If they are, it’s time for a new set of plugs. Be sure to follow the manufacturer’s torque setting specification when tightening the spark plugs down. Over tightening the spark plugs can cause the cylinder head to crack which will result in an extremely expensive repair bill. A torque wrench is an essential item for this task and our recommendation would be the excellent Teng 1/4″ Torque Wrench

Automatic Transmission Problems

If your car is fitted with an automatic transmission, stalling at idle can sometimes be an indicator of a problem within the transmission. If the transmission is the source of the stalling, it’s more than likely going to be an issue with the torque converter that’s faulty. The torque converter is the equivalent of a clutch in a manual car, it allows the engaging of drive and disengaging of drive in a smooth and controlled manner. Over time, the torque converter can become worn and result in stalling of the engine at idle. Sometimes, a simple automatic transmission fluid change can alleviate the problem but it may unfortunately be too late and require replacement. It’s really important to regularly change the fluid within an automatic transmission to prevent problems like this occurring. Interestingly, some manufacturers claim that their automatic transmission’s are “sealed for life” and don’t require any maintenance. Our advice would be too completely disregard that and stick to a schedule of changing the fluid and filter every 40-60,000 miles.

Most car manufacturers don’t actually make the transmissions themselves. For example, BMW quite often use ZF transmissions in their cars, as do Jaguar, Audi etc. Mercedes-Benz are an exception to this and build their own transmissons such as the 722.6 5 Speed or the 722.9 7 Speed 7G Tronic.

A Faulty EGR Valve

A faulty or blocked EGR Valve can often result in a stalling at idle problem. The EGR (Exhaust Gas Recirculation) Valve is a small valve which opens and closes to allow exhaust gasses to pass through it. EGR valves were introduced to reduce vehicle emissions, particularly on diesel engines. As diesel is an oily fuel, it produces a thick, black soot carbon build up which can often cause problems, particularly with EGR Valves. If a diesel car isn’t driven a high speeds (motorway for example) often enough, the engine never gets hot enough to remove some of the carbon deposits itself and eventually they can cause extreme running problems. Modern Diesel engines aren’t suited to low speed town driving unfortunately.

More often than not, a faulty EGR valve can be fixed with a simple strip down and thorough cleaning to remove all the hardened carbon deposits within the valve. Take a look at our extensive review and guide of the Best EGR Valve Cleaners for a comprehensive insight into which cleaners work best.

A Blocked DPF Filter

Ahh, the good old DPF Filter, the Achilles heel of all modern Diesel engines! Where do we start with this one? I’d hazard a guess that a very large number of running problems on a modern Diesel engine are somehow linked to a faulty DPF Filter.

The DPF (Diesel Particulate Filter) is a filter which sits in the middle of the exhaust system and catches the harmful exhaust gasses and “purifies” them before they leave the tailpipe and into the atmosphere. In terms of emissions control, they are a superb idea and principle but unfortunately have a few inherent problems. First of all, the DPF isn’t finite in size, so therefore it eventually becomes blocked with the exhaust gas ashes. When it does become blocked, the exhaust gasses can’t leave the exhaust system and cause a huge back log of pressure within the system.

The DPF filters are designed to be self cleaning and maintenance free. In order for this to be possible, the vehicle has to meet a stringent set of parameters before it can perform a DPF regeneration, a regeneration is where the ash inside the DPF filter is burnt off and safely deposited out of the exhaust system. Once complete, this then creates space within the filter for more ash and filtration. In order for a regeneration to take place, the vehicle has to be completely up to operating temperature and has to be driven at a relatively high speed for a prolonged period of time, usually around 20-40 minutes. Generally speaking, this requires a frequent motorway journey in order for this to take place. If you don’t drive your car regularly on the motorway and just use it for short, in town trips, your car will never get hot enough to perform a regeneration and eventually the DPF will become blocked and require manual intervention.

Take a look at our excellent review and buying guide of the best DPF cleaners to avoid a costly repair bill and headache.

We hope this article has been useful in diagnosing your staling at idle problem. If you have another issue that you’re unsure how to proceed with, feel free to contact us and we will be more than happy to assist you and help resolve the issue.

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