Fix A Car’s RPM Dropping While Idle

It can be a concerning time when you come to a stop only to find your car’s RPM is dropping at idle. It sometimes feels like the car will turn off, making most of us anxious.

Let’s look into the individual issues a little deeper to help paint a clear picture of what can cause RPM drops when your car is idling.

1. Warm Idle Adjust Set Too Low (Carburetors only)

Idle RPM adjust screw on a carburetor.

Chances are that you are experiencing low idle once the car’s engine is warm. Engine RPM runs a little faster when the car is still cold due to the ECM allowing the fuel injection system a richer fuel mixture until it warms up.

As long as the temperature sensor is operating properly, you shouldn’t have any issues with idle speed settings with fuel-injected vehicles.

If, for some reason, you still have a carburetor. You can simply locate the warm idle adjust screw and turn it clockwise by a very small amount until you are satisfied with the idle RPM.

To find the correct adjustment screw, look at the workshop manual of your specific engine or carburetor to find it.

Important: Adjusting the warm idle screw up too much can cause the idle circuit of carburettor to be bypassed. This can cause erratic idling or other anomalies. Instead, check the ignition timing first. If the timing isn’t advanced enough, getting your warm idle stable will be extremely difficult.

Caution: Be careful not to advance your ignition timing too much. To check, ensure that your total timing doesn’t exceed the recommended amount of degrees BTDC.

2. Faulty Fuel Pump

A car's fuel pump module.

Sometimes the wiring or isolation seal for the wiring can get damaged over time and cause intermittent functionality of the fuel pump.

If the fuel pump cannot deliver enough fuel to the engine, everything from idling, the ability for the engine to rev, or being able to drive at all can be an issue.

In this case, it is best to get it checked out by a professional. You don’t want to risk the chance of leaks anywhere around the fuel tank.

Sometimes the fuel pump itself can simply go faulty, and the motor inside the component can no longer do its job due to worn-out brushes or a dirty commutator.

Another cause of the fuel pump motor going faulty is worn-out bearings. These bearings can sometimes seize up or become so badly worn that the motor cannot spin efficiently enough, causing too much current draw.

A fuel pump motor that draws too much current can cause a fuse to blow, which will need to be replaced over and above the repair on the fuel pump.

The relay that supplies power to the fuel pump can also go faulty. They sometimes become burnt out, causing intermittent, weak, or no power transfers to the pump.

Also, check all the fuel line hoses to ensure that there aren’t any cracks causing fuel to leak out. The fuel line connectors on the entrance to the fuel pump housing can also sometimes go bad.

3. A Clogged Up Air Intake Filter

An old and new car air intake filter placed side by side for comparison.

If your engine is unable to get the right amount of air, it affects the overall air-fuel ratio. Too little air means that the mixture becomes too rich and can affect the idling.

A richer mixture will affect idling and fuel economy when you are driving about.

This can also lead to something called engine oil fuel dilution, which eventually causes extra engine wear.

So make sure that you replace or service the air intake cleaner at least every 15000 to 20000 miles. Some cars specify 30000 to 45000, but keeping an eye on it a little earlier won’t hurt. After all, it is the air that your engine breathes.

Don’t leave it for symptoms to remind you to service this important component.

4. Worn Spark Plugs (Caked up, Worn, Misadjusted, or Faulty)

An old spark plug next to a worn one.

Spark plugs are very important for the engine in any vehicle to run properly. It provides the spark to ignite the fuel for the combustion stroke.

The quality of the spark can considerably alter the running of an engine, from affecting fuel economy or performance to the idle RPM and overall smooth running of the engine.

When spark plugs begin to misbehave, they cause a misfire. You can notice it if you listen when your car’s engine is idling.

While it most certainly isn’t the only cause of an engine misfiring, it starts painting a picture of how bad spark plugs can affect the running of an engine.

When they become worn out or faulty, they can cause engine RPM to drop at idle to a point where the engine might turn off.

Removing them in most vehicles is easy enough to give them a clean and re-gap until you can afford new ones.

Your vehicle’s manual should have the specification of the spark plug gap, which you set using a feeler gauge or spark plug gap tool.

Sometimes spark plugs can get old and start to arc or leak inside, providing a weaker spark than they should. At this point, replacement is inevitable.

This is all assuming that your spark plug high-tension leads are fine and aren’t malfunctioning.

5. Faulty Idle Air Control Valve

A car engine's idle air control valve.

When this component becomes faulty, you can experience a range of symptoms with regard to the operation of the engine at idle.

They can become stuck or worn and cause incorrect amounts of air to the air intake causing improper air-fuel mixture ratios when idling.

Not only will your car idle too low depending on the way the valve has gone faulty, but the engine can also be harder to start and could turn off after starting.

6. Worn Valve Guides

When valve guides become worn out, combustion gasses enter the valve cover area, forcing them out through the breather hose connected to the air filter housing.

This is bad in a few ways:

  1. Depending on the design, it can cause excessive fumes to build up under the hood, causing fumes to be drawn into the vehicle.
  2. Depending on the design, a messy build-up inside the air intake housing can start clogging the air filter or intake passage.
  3. The quality of the air being drawn into the engine is compromised, leading to poorer performance and excess smoking out the exhaust tailpipe.
The breather hose outlet on a valve cover.

You can notice this easily if you disconnect the breather hose from the valve cover and note whether there is any smoke coming out of the opening when idling or revving the engine.

If you remove one end of the hose and your car’s idling picks up, it’s a clear sign that your valve guides need replacing.

7. Clogged Fuel Filter(s)

A car fuel filter.

When a fuel filter becomes clogged up, it could cause your engine RPM to drop enough to a point where the engine shuts off, depending on how bad it is.

Normally, you won’t be able to drive very far if the fuel filter is badly blocked.

These filters are quite cheap and easy to replace, so they will be a good place to start when looking for the cause of this problem.

Just make sure that you follow the direction arrows indicated on the fuel filter to ensure proper flow direction through it.

8. Car With A Dirty Carburetor

A dirty carburetor from a car.

Not many cars on the road have carburetors in them anymore. They are basically a thing of the past. But I thought I’d better include this point in case you might have one for some reason.

There are carburetor cleaners that you spray in the air intake part while keeping the engine’s RPM up.

It tends to choke out the engine when you spray it in, so you must depress the accelerator or tug in the accelerator cable to keep the engine running when you do this.

If it helps, you may want to get your carburetor checked out and cleaned properly.

9. Faulty Fuel Injectors (Faulty, Clogged, Leaking)

A car fuel injector.

Over time fuel injectors become clogged, and you get a rougher running engine.

Amongst these symptoms:

  1. Engine misfiring.
  2. Starting issues.
  3. Gas mileage becomes heavier.
  4. Difficulty getting higher RPMs.
  5. Dancing or low RPM at idle.

Just a heads up, some fuel injectors are expensive to replace, so you might want to check it before opening your wallet.

Sometimes, professional cleaning is all they need and could cut down the cost of the repair as opposed to replacing them.

It may be better to replace the vehicle in some cases where it may only be worth $2000 or less if replacement of the fuel injectors is the only cure.

If you are feeling brave, with enough research, you can pull them out and give them a thorough clean to see if that cures the problem before rushing out to pay for an expensive repair.

Fuel injector cleaning kits are available if you choose this approach to curing the RPM drops at idle.

10. Malfunctioning Advance Unit

A vacuum advance unit on the side of a distributor unit.

A vacuum advance helps keep your engine’s timing at optimum. It works by a vacuum provided by the fuel injection that varies in strength depending on the load on the engine.

The varied vacuum strength allows a mechanism to rotate with the help of a diaphragm, adjusting the ignition timing, usually by rotating the plate inside or near your distributor, depending on the design.

If you remove the vacuum hose from it while the engine is idling, you will notice a drop in RPM. If you didn’t notice any change, you know something is up with the vacuum advance.

Causes of a faulty vacuum advance can be a sticking mechanism, a broken diaphragm, or a leaky hose that drops the vacuum strength and, in turn, lowers the amount of timing advancement.

There are also mechanical advance units that can sometimes become a little sticky or require spring replacements.

Service kits from your local car parts store are available for these advance units.

11. A Faulty Mass Air Flow Sensor

A mass airflow sensor.

A mass airflow sensor detects the volume of air that flows through the air intake inlet. This helps the engine control module gauge the amount of fuel to be regulated, affecting the overall mixture.

If this sensor malfunctions, you will have a bad idle and all-round poor running engine. The fuel economy will also be affected.

This component should be fairly easy to replace yourself if you do enough research about your car’s specific sensor and its location etc.

12. A Faulty Temperature Sensor

A couple of examples of engine temperature sensors.

A temperature sensor is necessary to help the onboard computer know when to gradually reduce the richness of the fuel mixture via the fuel injection system until normal operation.

If the temperature sensor is faulty, the computer will assume that the car is cold and tell the fuel injection unit to keep running a richer mixture to help smooth the running of the cold engine.

13. Alternator not running at peak efficiency

A car's alternator.

One of the ways you can possibly tell if your alternator isn’t producing enough charge is by turning on the lights at idle and seeing if the idle drops a lot.

You may also notice that your car isn’t starting as well as it should.

For this problem, most battery centers will quickly test your alternator to establish whether it requires replacing.

14. Ignition timing is misadjusted

If the ignition timing is retarded too much, it will affect the idle speed. It can cause quite a drop in idle speed, in fact.

To correct it properly, you will need a timing light to see exactly where it needs to be set.

The timing light will flash at the correct time so you can follow a notch on the harmonic balancer that must be aligned to a specific marker somewhere on the engine housing or a tab.

It is best to take it into a shop to do this for you, seeing as it will be unlikely that you will have a timing light. Unless you want to take care of this for yourself in the future and wish to invest in one.

Here is a video demonstrating how to do this correctly.

Conclusion

As you can tell, many things cause a low idle speed. These are just some of the most common causes, and I hope you get to the bottom of the cause for your car using this article.

Another cause could also be a faulty ECM unit, which can also be costly. After you’ve been through all of these potential causes in this article, it may be best to take your vehicle to a repair shop to be checked out.

However, take your time and check all these causes carefully to ensure you don’t overlook the potential cause.

And if things are easy enough, you may be able to watch a YouTube video on how to replace the offending part on your model vehicle and save yourself some hard-earned cash!