Why Is My Truck Idling High? (9 Reasons Why)

If you have a truck, you know that some trucks can feel rough to drive, especially if you have a manual transmission. However, some trucks may create odd symptoms, such as idling high.

If your truck is idling high, you will want to know what causes it. We did some research, and here is what we discovered about the matter!

Why Is My Truck Idling High?

Trucks idle high because of an issue with the ignition system, such as a bad spark plug, ignition wires, fuel injectors, and more. Plus, clogged air filters can keep clean air from entering the combustion chamber and make your truck idle high. Incorrect ignition and cam timing can lead to rough truck idling.

Do you want to find out other reasons why your truck idles high? We compiled a list of possible reasons below, so read on!

1. Bad Cam Timing

If you replaced your truck’s timing belt recently, the timing belt could be off or loose, causing your truck to idle high.

When the truck’s valves were not timed properly, the exhaust and intake valves will open when they should not be open.

That said, this will reduce compression, causing the combustion gasses to go where they should not be and make your truck idle high.

2. Bad Ignition Timing

Ignition timing gets controlled by your truck’s distributor and the ECU; if the timing is off, your truck can run through then stall.

Also, some trucks have a cam angle sensor or crank in place of a distributor, so you manually advance the ignition timing.

Read More:  Why Is The Inside Of The Car So Hot? (9 Reasons Why)

However, it is important to refer to your truck’s manual first, as each truck can have a different procedure for advancing the ignition timing.

3. Faulty Idle Air Control Valve

Your truck has an idle air control valve, which may be called the IACV or IAC, and if it is faulty, your truck could idle high.

Normally, the idle air control valve controls how much air goes through the throttle body when the throttle plate is not open.

This air regulates your truck’s engine idle speed and lets the idle stay on while you add more load to the engine, such as when you use your headlights or air conditioning.

Generally, the idle speed will drop if you use specific car accessories if your idle air control valve does not work properly, and it could make your truck stall.

4. Fuel Injectors

In most modern trucks, fuel injectors will be present as a way to bring fuel to the truck’s internal combustion chamber.

However, these fuel injectors can become clogged or dirty the more you use your truck. If you do not regularly use fuel injector cleaners or gas additives, you will have a dirty fuel injector.

Normally, dirty fuel injectors will make your truck idle badly, and you may also notice your gas mileage suddenly go down.

5. Dirty Air Filters

Dirty Air Filters

Most trucks will need a mixture of air and fuel for combustion to start the truck.

Generally, your engine will take air from outside the car and mix the air with fuel within the internal combustion chamber.

Read More:  Why Is My Coolant Orange? (7 Reasons Why)

Since your engine utilizes air from outside the car, there are air filters to keep debris and dirt from entering the combustion chamber.

However, there will be little to no clean air entering your engine if your air filters are dirty and clogged, making your engine idle rough.

6. Carburetor Problems

Before people started using fuel injectors for trucks, carburetors were used to make a spark in the engine with a mixture of air and fuel.

If you have an older truck, you likely have a carburetor in it that could be causing your truck to idle high.

Normally, you will know that you have an issue with your carburetor if you see black smoke in your exhaust pipe.

Usually, carburetor problems occur when there is an accumulation of engine deposits made from carbon, which also causes black smoke.

Typically, you will have to melt and clean out the carbon deposits from your carburetor to get rid of the rough idle.

7. Leak In The Vacuum

Most trucks have various hoses under the hood that make a vacuum for air and fuel. Your vacuum will draw fuel into the engine if you have an older truck with a carburetor.

However, newer trucks will have a throttle that regulates the airflow and engine speed through the truck’s system, making a vacuum in the intake manifold.

Also, a leak can appear in these hoses over time as these hoses get worn out, especially if you have not checked your hoses in a long time.

Your engine can misfire then idle high because a leaky vacuum can mix too much air with the fuel.

Read More:  Why Is My Coolant Not Circulating? (11 Reasons Why)

8. Spark Plugs

A very common reason your truck starts to idle high is that you have bad spark plugs.

With that, spark plugs work by sending an electrical current to ignite a combination of air and fuel in the combustion chamber.

However, a bad spark plug will not create a strong enough electrical current, causing your truck to idle high because the mixture of air and fuel will not burn consistently.

9. Bad Ignition Wires

When you have faulty ignition wires, you will experience similar symptoms to what you would have with a bad spark plug.

Ignition wires send a high voltage to your truck’s spark plug to help the electricity jump the gap, but the output will not be consistent, and your engine will misfire if your ignition wires are bad.

To know more, you can also read our posts on why your truck is burning oil, why your truck is blowing white smoke, and why your truck is blowing blue smoke


Normally, a truck will idle high because there is an issue within the combustion chamber, including the spark plugs, fuel injectors, carburetors, and more.

However, other aspects of your truck’s engine, including the cam timing and ignition timing, can lead to your truck idling high.

Leave a Comment