Have you ever experienced an engine misfire at low rpm, only to have it go away at high rpm?
A number of factors can cause misfiring, ranging from a malfunctioning coil pack, VVT solenoid, spark plugs, or fuel injectors.
This article will discuss the various causes of engine misfiring, the associated costs, and the steps you can take to get your engine running smoothly again.
Coil Pack
A coil pack is responsible for sending an electrical signal to the spark plugs to ignite the fuel-air mixture in the cylinders.
When a coil pack goes bad, it can cause the spark plugs to fire at the wrong time or not fire at all.
This misfire can cause a variety of issues, from a noticeable decrease in performance to a complete engine stall. The spark plugs ignite the fuel-air mixture in the cylinders, and if they are not doing that, the engine will not run properly.
At higher RPMs, the spark plugs fire more rapidly, and the engine shakes less, which is why the misfire may not be as noticeable. However, at lower RPMs, the spark plugs fire less frequently, and the engine shakes more, making the misfire more obvious.
This is why a faulty coil pack may cause an engine to misfire at lower RPMs but run fine at higher RPMs.
Often the ECU receives an error and produces code is P0351
Coil pack replacement costs from $150 to $350, including labor costs
Spark Plugs
The spark plug needs to work hard to ignite the fuel-air mixture, and it is much more likely to fail if it is dirty or worn.
As the RPM increases, the spark plug is subjected to much higher amperage from the alternator -this is good -and therefore is much less impactful. This is why an engine misfire at low RPM can often go away at higher RPM.
As spark plugs get dirtier or wear out, they become less capable of igniting the fuel-air mixture and start to misfire. This is why keeping your spark plugs clean and in good condition is important.
Oil from a leaking cover gasket can get into spark plugs and, even after cleaning, cause misfires.
Spark plugs are very cheap and range from a few dollars to $100 for a set of four – one for each cylinder. Labor costs are minimal, and best to use a car shop if you are unsure how tight to screw them in. Basically, hand tighten and then another half-turn with a wrench.
Fuel Injectors
A fuel injector is a device that is responsible for injecting fuel into the cylinder of an internal combustion engine. When these injectors become clogged or worn, they can cause misfiring.
When a fuel injector is clogged or worn, it will not be able to accurately deliver the correct amount of fuel to the combustion chamber.
This discrepancy in the fuel delivery will cause the engine to misfire. The misfire will be more noticeable at low speeds as the engine is not receiving the proper amount of fuel to create a consistent and steady combustion.
At higher RPMs, the engine works harder and needs more fuel to maintain consistent combustion. This increased fuel delivery means the misfire is less noticeable.
In order to fix the issue, the fuel injectors need to be replaced or cleaned.
Fuel injector cleaners work by breaking down the carbon deposits and other residues that can accumulate. The cleaners are typically sprayed directly into the fuel system and can be used on gas and diesel engines.
Some of the most popular fuel injector cleaners on the market include Wynn’s Injector Cleaner, Red Line Complete Fuel System Cleaner, and Liqui Moly Jectron Fuel Injector Cleaner. Expect to pay around $10 for a can.
Fuel injectors are an expensive replacement. Expect to pay around $300 for a four cylinder injector kit and more for a larger engine with six or eights cylinder cars.
VVT/VANOS Solenoid
When a VVT solenoid (or VANOS in BMW) goes bad, it can cause an engine to misfire at low RPMs. This is because the bad solenoid cannot accurately control the valves’ timing, leading to an irregular combustion cycle and reduced performance.
However, the misfire may go away at higher RPMs as the engine generates more power to compensate for the bad solenoid.
The reason for this is that the VVT solenoid is designed to work best at a particular RPM range. When the engine is running at a lower speed, the solenoid may not be able to adjust the valve timing quickly enough to keep up with the changing engine speed.
As a result, the valve timing can be out of sync with the engine, leading to an engine misfire.
However, when the engine runs at higher RPMs, the solenoid can adjust the valve timing more quickly, resulting in a smoother combustion cycle and improved performance. This is why the misfire may go away at higher RPMs.
You could try cleaning the solenoid.
Some people use throttle body cleaner or mass airflow cleaner to do this. Always check with the manufacturer, though, as you may invalidate any warranty doing this. It might be worth a try, as a new solenoid will cost from $200, including labor.
In conclusion
One of the parts mentioned above is the most likely cause for an engine misfiring at low RPM but not at high RPM. This could be a Coil pack, VVT solenoid, spark plugs, and/or fuel injectors.
These parts are relatively inexpensive to replace, and the spark plugs and fuel injectors can even be cleaned if necessary.
