An engine misfire at low RPM is alarming and may lead to reduced performance and fuel efficiency. As it can happen for a couple of reasons, you should not lose yourself in what to do and what not.
So, what causes the engine to misfire at low rpm? It can happen due to cylinders failing to ignite properly, lean fuel mixture, and loss of compression. You have to perform a couple of tests, inspections, and replacements to resolve this problem. For example, spark plugs, ignition coil, vacuum lines, and MAF.
If you are still confused, keep on reading this article. This will be a guideline featuring all the potential causes and fixes for this problem.
3 Key Reasons Why Engine Misfires at Low RPM (Including Symptoms And Potential Fixes)
There are a couple of reasons that cause the engine to misfire. Have a go through all the causes and potential fixes of these.
1. Ignition System Failure (No Spark)
When your engine experiences misfire at low RPM, it is often due to an ignition system failure. It eventually leads to a lack of spark within the cylinders. This absence of a spark causes a misfire by preventing the timely ignition of the air-fuel mixture.
How to Identify?
To check if your car ignition system is acting up, watch out for these signs:
- The engine runs rough at a stop.
- The car has trouble starting.
- Gas mileage gets worse.
- Car stalls or hesitates.
What To Do?
You have to check and fix the issues as follows.
Step 1: Spark Plug Inspection
- Using a spark plug socket, carefully remove each spark plug.
- Examine the spark of the plugs for signs of wear, such as eroded electrodes or fouling ( carbon deposits).
- Measure the spark plug gap using a gapping tool, ensuring it falls within the manufacturer’s specifications.
- Replace any spark plugs that show wear, damage, or improper gap settings.
Step 2: Ignition Coil Evaluation
- Test each ignition coil by using an ohmmeter or a specialized ignition coil tester.
- Check for proper resistance within the coil. The appropriate resistance varied, actually. It should be 0.7-1.7 ohms, while the secondary coil winding resistance should be 5-20 kΩ. If the resistance is out of spec, replace the faulty ignition coil.
- Inspect the ignition coil’s physical condition for any visible damage or cracks.
- Ensure that the coil’s electrical connectors and wiring are securely connected and free of damage.
Step 3: Wiring and Connection Verification
- Visually inspect the wiring harness and connectors related to the ignition system.
- Check for loose, frayed, or corroded wires.
- Utilize a multimeter to measure the electrical continuity of the wires and connections. If the electrical read seems to be more than 2 ( 0 to 2 indicates good electrical flow), you should contact a professional. The wiring and connection should be changed according to the damage.
2. Lean Fuel
When your engine runs on a lean fuel mixture, it means there is an excess of air and insufficient fuel in the combustion process. This condition can lead to engine misfires, especially at low RPMs.
Because the air-fuel mixture burns inefficiently, causing fluctuation in power delivery. And guess what? an engine misfire occurs as a result.
What to Do?
All you have to do now is do a technical inspection and fix it. Here are two steps you should follow to make things alright.
Step 1: Inspect Vacuum Lines
Thoroughly inspect the vacuum lines and intake manifold for any cracks, loose connections, or signs of damage.
One effective method is to use a can of carburetor cleaner or a propane torch (unlit) to spray around the suspected areas gently. If the engine idle changes when you spray near a particular spot, you have likely found a vacuum leak.
Once identified, you can fix the leaks by replacing the gaskets of the damaged hoses or sealing connections properly.
If you still don’t get it, don’t worry. Watch this video to get a visual idea.
Step 2: Examining the MAF Unit
Carefully remove the MAF sensor and clean it using a specialized MAF sensor cleaner. Follow the manufacturer’s instructions, as it is a delicate component.
If the sensor is beyond cleaning or shows signs of damage, it may be necessary to replace it with a new, properly calibrated unit.
One special tip: If you want your car not to show these problems, higher octane fuel might be helpful. If you ask me for my recommendation, I’m gonna name 93-octane automotive fuels.
This video might come in handy if you think you should replace the MAF unit:
3. Loss of Compression
A loss of compression is a risky thing, especially if it’s on more than one cylinder. This means that the air-fuel mixture is not being squeezed as it should be, causing issues with ignition.
The symptoms of compression loss are as follows:
- Reduced Power
- Hard Starting
- Rough Idling
- Poor Fuel Efficiency
- Excessive Exhaust Smoke
What to Do
To diagnose compression loss, you need to perform a compression test. Here’s how:
Step 1: Get a compression tester, usually available at auto parts stores.
Step 2: Start your engine and let it run until it reaches normal operating temperature.
Step 3: Switch off the engine and disconnect the ignition system to prevent sparks.
Step 4: Carefully take out the spark plugs from each cylinder.
Step 5: Screw the compression tester into the spark plug hole of the first cylinder.
Step 6: Turn the engine over by either using the starter or manually cranking it with a wrench. The compression tester will gauge the pressure.
Step 7: Perform this test for each cylinder, recording the readings.
Step 8: Compare the compression readings. If one or more cylinders show significantly lower pressure than the others, you likely have a compression problem.
Along with the cylinder problems, a damaged or blown cylinder head gasket can also be a major cause here. Sometimes, it’s the valves themselves that are the problem. This is a more complex procedure. So, we recommend seeking professional help in such circumstances.
Bottom Line
Engine misfire at low rpm is disturbing. I hope this article gave you a clear picture of why it happens and how to recognize and resolve this problem. In some cases, the repair might be costly; for an MAF fix, it will cost $384, or a vacuum line repair can cost up to $1000.
But you can prevent all these problems, or at least minimize costs by looking after your car properly. You should maintain the car as per the company’s maintenance criteria. Regularly inspecting the spark plugs and ignition modules will also be helpful.
However, do not drive recklessly if you do not want your vehicle to fall into trouble like this.