In most cases when you apply pressure to the accelerator pedal, the engine slowly revs up and when in gear accelerates forward. The mechanical parts and systems that all work together to provide momentum are subject to wear and tear, which is why every vehicle on the road has a recommended service and maintenance program.

However, there are times when the fuel, electrical, and exhaust systems that provide smooth acceleration become dirty, damaged, or simply wear out and need to be replaced. When this occurs, it sometimes creates a hesitation in the application of the throttle or a "bucking" sensation. Knowing what component is causing this inconsistent behavior can reduce the expense involved in hiring a mechanic to service the vehicle or repair what is damaged.

With that being said, noted below are a few of the common ways to troubleshoot why your engine is stumbling, so that you can efficiently communicate with a mechanic and understand why it's necessary to make their recommended repairs.

Before we start to troubleshoot the potential issues, it's important to understand how all these systems noted below work together to provide smooth power and acceleration we depend on for efficient operation of our vehicles.

Part 1 of 3: Understanding what causes the engine to hesitate or cause bucking issues

When you press the gas pedal on your car, the fuel pump located either inside your fuel tank or just outside of the tank delivers your fuel (gasoline, diesel, or Natural Gas) through the fuel filter, up the fuel line, and to the fuel injection system. The fuel injection system combines liquid fuel with air to "atomize" (or convert into a fine mist) the fuel which is then delivered into the cylinders to begin the combustion process.

At this point the spark plugs inside the engine ignites the fuel/air mixture which combusts to propel the pistons and turn the engine crank shaft. Once the atomized fuel has ignited it creates fumes that need to be vented from the cylinder in order to complete this process again. This is done by opening an exhaust valve and allowing the piston to push the exhaust (or burnt fuel) through the exhaust manifold, through the exhaust pipes, cleaned by the catalytic converter, quieted by the muffler and expelled from the vehicle through the tail pipe.

Just to throw another element into the process of combustion, some of the unburnt fuel (called particulate matter or hydrocarbon molecules) are recirculated and burned again (using the EGR valve), and oxygen sensors monitor the exhaust process for the right levels of contaminants. The engine completes this process at an average rate of 1,200 revolutions per minute (RPM) during idle. As the throttle is applied the engine is asked to increase the rate of combustion, in most cases up to 4,000 RPMs during the acceleration process and before the transmission shifts into a higher gear.

Some of the more common reasons why an engine will stumble through this process may include the following individual components:

  • Dirty or worn out fuel filters
  • A damaged or worn out fuel pump
  • Faulty or damaged oxygen sensors
  • A dirty, clogged or damaged EGR valve
  • Worn out spark plugs
  • A dirty or clogged air filter

Since any of these six components can typically cause the engine hesitation, we'll focus on these items to show you how to troubleshoot problems with them.

Method 1 of 6: How to troubleshoot issues with the fuel filter

The fuel filter is responsible for cleaning liquid gasoline or diesel before it reaches the fuel injection system. As you can see by the image above, a clean filter (on the left) is transparent, while a dirty filter (on the right) is dark and filled with debris, dirt and other particles found in the fuel tank or the fuel that is put into your vehicle from a service station. The fuel filter (on some vehicle may be more than one), will be attached inside the engine compartment from the primary fuel line but before it reaches the fuel injection system. To troubleshoot whether the fuel filter is the issue, complete the following steps:

Step 1: Physically inspect the fuel filter. If the fuel filter is dirty like the image on the right, it's probably causing a restriction of fuel flow to the engine and should be replaced.

Step 2: Check for leaks coming from fuel filter connections. The fuel line is spliced between two fuel lines.

In some cases, the connections on the fuel filter may be loose, causing fuel to leak from the fuel lines or the fuel filter. This can cause engine stumbling issues. If you notice this problem, replace the fuel filter with a new one and retighten the fittings. Verify the leak has gone and inspect the vehicle for stumbling issues.

Method 2 of 5: Troubleshooting issues with the fuel pump

The fuel pump begins the combustion process by distributing fuel from the tank, through the fuel lines, to the filter and then to the fuel injector. This pump works by receiving an electrical signal that operates an electric motor inside the pump. The pump sucks in fuel from the tank and pushes it through the fuel lines at a predetermined pressure. As the throttle is applied and more fuel is requested, the fuel pump increases its pressure to keep up with the demands of the engine.

Due to these facts, problems with the fuel pump can cause the engine to not run or to stumble as the gas pedal is applied. Typically hesitation or stumbling issues are caused by an obstruction in the fuel pump or a pump that is starting to show signs of breaking or wearing out. To diagnose and troubleshoot issues with the fuel pump, you'll either need professional diagnostic equipment to complete a digital scan of any error codes stored in the ECM or remove the fuel pump and test it for operation.

Since the fuel pump can be very difficult to remove (as most of them are inside the fuel tank), it's recommended to replace the pump if you suspect the pump is damaged or wearing out in any ways.

Method 3 of 6: Troubleshooting problems with oxygen (O2) sensors

On most domestic and foreign four cylinders, six cylinder and eight cylinder engines, there are up to four oxygen or O2 sensors on a vehicle that all contribute to the smooth operation of the combustion engine. There are typically two different types of sensors:

  • Upstream O2 sensors: These sensors are located inside the exhaust manifold and are designed to measure the oxygen level in the vehicle exhaust as soon as it comes from the cylinders during the exhaust stroke. This allows the fuel system to introduce more outside air into the fuel mixture for a more efficient or clean burn.

  • Downstream O2 sensors: The other sensors are located at the back of the catalytic converter. These measure different molecules including NoX and CO2. They also relay information to the ECU to fine tune the mixture of air to fuel ratio for efficiency of the engine.

When either of these sensors are damaged or sending faulty information, they can cause the engine to make adjustments to the fuel trim that can cause the engine to stumble or cause the car to create that uncomfortable "bucking" sensation as you accelerate. In order to diagnose problems with the O2 sensors, you'll also need professional diagnostic tools like a digital scanner that will pull any stored error codes from the ECM.

Once the right codes have indicated a problem with the O2 sensors, a professional mechanic will be able to determine which ones are damaged and replace them or repair damage to electrical harnesses or connectors that are attached to the sensors.

Method 4 of 6: Troubleshooting issues with the EGR valve

Did you know that the typical combustion engine only burns about 80% of the fuel that enters the combustion chamber? The reason for this is that fuel molecules are comprised of multiple, large-chain carbon molecules that are very hard to burn. In fact, there are 13 different carbon molecules in unleaded gasoline (each with different sizes ranging from C-1 to C-13). The smaller ones, (C-1 through C-7) are very easy to burn. However, the larger molecules simply can't burn without increasing the rate of combustion (increasing timing) which may create more power, but also consumes more fuel.

The EGR system was invented and introduced in the late 1960s to recirculate unburnt fuel molecules (called particulate matter) from the exhaust and back through the intake manifold. This system over time will become clogged with excessive carbon build-up, which can create blockages and reduce engine operation efficiency. In order to diagnose or troubleshoot a problem with the EGR valve, you'll have to complete the following steps:

Step 1: Remove the EGR valve. Refer to your vehicles service manual for exact procedures on how to remove the EGR valve.

Step 2: Inspect the EGR chambers for excess carbon build-up and debris. Inside the EGR valve is a series of chambers that flow the particulate matter back to the intake manifold to be burned again during the combustion process. If the chambers are filled with excess debris, this can create engine stumbling problems.

Step 3: Check supporting ERG components for damage. The EGR system is comprised of multiple components beyond the EGR valve.

There are sensors, vacuum lines and vacuum canisters on many vehicles that all work together to ensure the EGR system works efficiently.

Any of these components may be damaged which can cause engine stumbling or "bucking" situations.

Method 5 of 6: Troubleshooting worn out spark plugs

Under normal operation, the spark plug receives and electrical signal from the distributor or electronic injection system and sends an electrical current to the electrode on the tip of the spark plug to ignite in a split second. Once this occurs, the electrical "spark" ignites the fuel/air vapor in the combustion chamber which creates power. However, as you can see by the image above, there are many ways that a spark plug can become damaged. When the spark plug is damaged, it will not properly ignite the fuel, which can lead to the engine stumbling when you apply the accelerator.

There are multiple methods for troubleshooting spark plugs, but it's best to let a professional mechanic who knows how to read a plug to determine why the damage is occurring in the first place to complete this process. For educational purposes, there are a few things that a novice mechanic can do to determine if the spark plugs may be causing your engine issues.

Step 1: Inspect the spark plug wires. Sometimes, choppy engine performance is caused by a spark plug wire that is damaged or loose as opposed to the spark plug itself. Check the spark plug wires on all cylinders to make sure they are properly attached, and not frayed or damaged.

Step 2: Remove spark plug and look for anything "odd". The spark plug is supposed to maintain its original shape, meaning that the end of the spark plug (the electrode) is supposed to look just like it did when it was new. If you remove the spark plug, refer to the image above to determine if something "just doesn't look right."

If you've noticed that the spark plug is damaged, coated with excess oil, looks burned or discolored, contact a professional mechanic to diagnose why the plug is in that condition. In order to fix the problem, that issue will need to be solved first.

Method 6 of 6: Troubleshooting a dirty air filter

Just as a fuel filter is designed to clean the fuel before it enters the engine, the air filter has the same job. Efficient running engines depend on clean fuel and air to mix together in order to burn efficiently and provide smooth acceleration when the gas pedal is pressed down.

If the air filter is dirty, the air will have a hard time pushing through the dirty filter. This can create a situation where the engine is trying to burn more liquid fuel as opposed to a consistent vapor (like it's supposed to).

To troubleshoot issues with the air filter, complete the following steps:

Step 1: Remove the air filter from the air filter housing. On most vehicles, there is an air filter housing that is attached to the throttle body of the fuel injector. There will also be a "lid" that holds the filter inside. Remove this lid and pull out the air filter.

Step 2: Inspect the air filter for damage, cleanliness. Generally, two things will cause an engine to stumble in regards to this part. First, the filter will be excessively dirty or second, the filter is damaged or torn. In either case, the filter should be thrown away and replaced.

If you're able to complete some of these troubleshooting steps on your own, it's very possible that you can find the source of your engine stumbling or developing that "bucking" motion when you try to accelerate. If, however, you're in need of professional assistance, contact one of our professional mechanics to complete an engine is hesitating and bucking inspection for you. This will allow them to pinpoint the precise source of your engine troubles and repair what is damaged.

This article originally appeared on YourMechanic.com as How to Troubleshoot a Hesitating or Bucking Car and was authored by Tim Charlet.


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