What causes unstable idling?

Many of the issues that cause unstable idling are based on fuel or air intake or the ignition system. Erratic idling could be down to fuel injectors, spark plugs, dirty filters, faulty sensors, or even from being set to the wrong idle rate in the first place.

Unstable or rough idling might not seem like much of a problem at first, even with an occasional stall here and there, but beware, it can soon develop into far more serious issues making your car dangerous to drive or even causing your engine to fail completely.

What are the most common causes of unstable idling?

Dirty fuel injectors

Risk level – Medium

What to do – Have the injectors cleaned, or if they’re beyond saving, replaced.

Fuel injectors inject fuel into the engine cylinders. It’s at this point it combines with air creating the idle mix to ignite, burn, and power your car. Clogged, dirty or failing fuel injectors prevent the precise measure of fuel from entering the cylinders, and with such tiny nozzles, it doesn’t have to be much to prevent a full measure from being delivered.

Defective spark plugs, plug leads, and ignition coils

Risk level – Medium

What to do – Replace the faulty components.

There are plenty of other components that affect fuel and air delivery into the engine’s cylinders—all of which can cause rough or unstable idling. Spark plugs, leads, and ignition coils, where faulty, worn or damaged, can cause the juddering and jolting of an unstable idling engine.

Faulty filters, valves, and sensors

Risk level – Medium

What to do – Replace the faulty components.

There are other components that, if left to clog, become dirty or contaminated, can affect the air/fuel mix and your engine idling.

An air filter, when clogged over prolonged use, prevents the correct mix of fuel and air.

The PCV valve (positive crankcase ventilation) is responsible for returning unburned fuel back into the engine and, over time, can be congested with sludge and dirt.

An EGR valve (exhaust gas recirculation) sends exhaust gases back through the intake system to lower emissions. But, once again, over time, the EGR valve can seize—open or closed—causing problems with the fuel mix leading to more idling.

Then there are the sensors that send information to your ECU. The oxygen sensor can be affected by high temperatures or clog with carbon deposits. If the level is misread, a too-lean mix of fuel and air will result in unstable engine idling.

Engine set at the wrong idle speed

Risk level – Medium

What to do – Have the idle speed set to the manufacturer setting.

It sounds far too simple, but sometimes an engine is merely set to the wrong speed for that particular make and model; alternatively, it can change due to wear and tear. A quick tune-up will rectify this simple problem.

Faulty fuel pump

Risk level – High

What to do – Replace it immediately.

Another essential component possible of wear over time is the fuel pump. If this component is slowly failing, the mix won’t include the correct amount of fuel for a steady idle.

Leaking vacuum system

Risk level – High

What to do – Locate the leak and repair or replace the offending part.

There are plenty of places your engine system can develop a leak, and because its performance demands a sealed vacuum, they’re always going to cause problems and performance issues.

What should I do if my engine is idling rough or unstable?

Have your nearest Fixter mechanic locate the problem with a diagnostic test. They’re easy to organise and can uncover all kinds of issues long before they get out of hand.

What are other common symptoms related to unstable idling?

When your engine isn’t performing correctly, you may also notice poor acceleration, lower fuel economy, a lack of power, regular stalling and difficulty starting the car. In any of these cases, it’s time to have the root of the fault diagnosed and rectified.

What is unstable idling exactly?

When you run your car’s engine while stationary (i.e., not driving or pressing the accelerator), the speed of your engine at that point is known as idling. Measured in revolutions per minute, it's usually set somewhere between 600rpm and 1000rpm depending on your car make and model. If the RPM varies and changes erratically, you’ll feel little jumps and judders—this is unstable idling.

How hard is it to diagnose an unstable idling engine?

In modern cars, it’s easier than ever. With so many sensors and monitors in the fuel, combustion and exhaust systems, a simple diagnostic test will soon spot any faults.

Keep an eye out for the ‘check engine’ warning light on your dashboard. That’s the first visible notice you’re likely to see.

What are other less common causes of unstable idling?

All kinds of problems can cause unstable idling, as well as those we’ve already mentioned. For example, head gaskets leaking, a worn throttle valve, or a problem in the EEC system (evaporative emissions control) can all deliver the same problem. Failing or faulty MAF sensors (mass air flow), ECT sensors (engine coolant temperature), and IAC valves (idle air control) are also occasional causes of unstable engine idling.

How much does it cost to fix an unstable idling engine?

Depending on the severity of the issue, it can cost a few pounds or a few hundred (if not more) to get your car running as smooth as its usual silky idle rate.

If your engine bounces and jumps while idling, then it’s ready for an electronic once-over to get the full diagnosis. Book in with Fixter, and we’ll run a diagnostic test to uncover the issue. Once we know where the problem is, we’ll talk you through the solution what it will cost.