Having your car misfire or run roughly after filling up the gas tank is a common problem many drivers face. This issue is frustrating and can leave you stranded if the engine stalls altogether. But don’t worry in most cases, it’s an easy fix.
This article will explore the most likely causes of misfires after refueling and provide actionable tips to diagnose and resolve the problem yourself or with a mechanic’s help. This will help you to gain a deeper understanding of what’s happening when your car misfires after gassing up.
Table of Contents
- Main Causes of Misfires After Fueling Up
- Diagnosing the Cause of Post-Refueling Misfires
- Fixing a Malfunctioning Evaporative Emissions System
- Cleaning Out Bad Gasoline That Causes Misfires
- When to Seek Professional Help with Fuel-Related Misfires
- Preventing Future Gas-Related Misfires in Your Vehicle
Main Causes of Misfires After Fueling Up
There are two primary causes of misfires and rough running immediately after putting fuel in the gas tank:
1. Evaporative Emissions System Malfunction
All modern vehicles have an evaporative emissions system that traps fuel vapors from the gas tank and routing those to the engine to be burned off. A malfunction in this system is the most common reason for misfires after filling up.
Problems like a loose gas cap, cracked evaporative hoses, or a bad charcoal canister that stores the vapors can cause fuel vapors to escape into the air instead of getting routed to the engine. This results in a lean fuel mixture and misfires.
2. Bad Gasoline
Getting a tank of substandard or “bad” gas with contaminants like water, dirt, or the wrong octane rating for your car can also cause misfires after filling up. The bad fuel doesn’t combust properly, causing the engine to run roughly.
So issues with your evaporative emissions system or bad gas are the two most likely causes.
Diagnosing the Cause of Post-Refueling Misfires
Determining if the issue is a malfunctioning evaporative system versus bad gasoline requires methodically testing and ruling out possible causes:
Step 1 – Check for Loose or Missing Gas Cap
The first step is the easiest when you filled up, did you tighten the gas cap until it clicked? A loose or missing gas cap allows fuel vapors to escape, causing a vacuum leak and lean fuel mixture. Tighten the cap properly and see if the problem persists before moving to the next steps.
Step 2 – Scan for Engine Codes
Use an OBD2 scanner tool to pull any diagnostic trouble codes (DTCs) stored in the engine computer. Post-fueling misfires will often set EVAP system and fuel trim codes that point to the source.
For example, a P0442 small EVAP leak code would indicate a malfunctioning evaporative system. Lambda sensor codes suggest an overly lean or rich fuel mixture. Research any codes to understand what system is affected.
Step 3 – Perform a Fuel Pressure Test
Connect a fuel pressure tester and measure fuel rail pressure right after the misfires occur. If pressure is low, it points to a faulty fuel pump or clogged filter not delivering enough fuel.
Step 4 – Test Drive with Fuel Cap Loose
Carefully drive with the fuel cap loosened to allow vapors to escape. If the misfires and roughness go away, it zeroes in on an evaporative emissions problem. Be sure to tighten the cap after testing.
Step 5 – Fill Up at a Different Gas Station
Trying a different fueling station with a fresh tank of gas can determine if bad fuel is to blame. If the problem disappears, contaminated gas is likely the cause.
By following these diagnostic steps, you can methodically pinpoint whether the issue is bad gas or a problem with the EVAP system. The solution will depend on which one is causing the post-refueling misfires.
Fixing a Malfunctioning Evaporative Emissions System
If you’ve determined the misfires stem from a problem with the EVAP system, follow these tips to restore proper operation:
- Replace a leaking gas cap with a new factory cap to maintain a tight seal. Aftermarket caps can cause issues.
- Inspect all evaporative hoses for cracks or damage. Replace any degraded vapor lines. Ensure connections are tight.
- Replace the charcoal canister if it’s clogged with fuel or has internal cracks. A mechanic can diagnose canister issues.
- Clear any EVAP codes and then perform an evap system monitor test. This verifies leaks are fixed and all components work properly.
- Consider replacing the purge valve if it’s sticking open. Only open when the engine is running to prevent leaks.
With repairs, the EVAP system should once again maintain tight control of fuel vapors, eliminating post-refueling misfires.
Cleaning Out Bad Gasoline That Causes Misfires
If adding fresh gas leads to rough running and misfires, here is how to clean out the contaminated fuel:
- Use a fuel injector cleaner like Techron regularly for 3-4 tankfuls. It helps remove water, deposits, and stabilizes fuel.
- Replace the fuel filter to ensure any particulates or contaminants are trapped and don’t reach the engine.
- In extreme cases, draining the fuel tank, and lines may be required to eliminate all traces of bad gas. This can be done by a shop.
- Discuss fuel additives with your mechanic if the issue persists despite draining the tank. They help bind water molecules and clean injectors.
- Consider higher-octane premium gas for 1-2 tanks once the fuel system is clean to prevent knocks.
With contaminated gas, full removal is key to stop misfires and restore smooth running. Avoid the station with bad fuel in the future.
When to Seek Professional Help with Fuel-Related Misfires
While many causes of post-refueling misfires are simple DIY fixes, it’s best to have a professional mechanic inspect the vehicle if:
- You’re unsure of the root cause after diagnosing. A pro can test components and pressures.
- The check engine light remains on signaling an unresolved issue.
- Misfires or roughness continues despite repairs and cleaning the fuel system.
- The engine is misfiring or running erratically even when the tank is not full.
- You don’t have the tools, equipment or knowledge to safely make fuel system repairs.
Searching for and fixing evaporative leaks requires specialized tools and knowledge beyond an average car owner’s capabilities. Seek professional assistance to ensure repairs are done completely and correctly when necessary.
Preventing Future Gas-Related Misfires in Your Vehicle
To help prevent evaporative leaks and issues with bad gas leading to misfires after refueling, keep these tips in mind:
- Always visually inspect gas before filling up your tank. Avoid stations with dirty/contaminated pumps.
- Tighten the gas cap until it clicks every time you refuel to maintain a tight seal.
- Replace the gas cap if it’s damaged or over 6-10 years old. The seal degrades over time.
- Be consistent about where you buy gas from. Try sticking to major name brands.
- Listen for whistling noises near the fuel tank indicating potential air leaks.
- Keep an eye on your fuel trims with a scan tool. Steady negative values can mean leaks.
Staying diligent gives you the best chance to avoid both evaporative issues and bad gasoline that commonly create those frustrating misfires after gassing up.
Misfires and rough running immediately after filling up the gas tank is a nuisance but fairly simple to remedy in most cases. Confirm the cause then fix evaporative leaks or clean out contaminated fuel to stop the drivability issues. Or seek professional assistance if needed.
Following the diagnostic steps outlined and being proactive with maintenance greatly reduces the chances of experiencing future refueling-related misfires. But should they occur again, you now have the knowledge to tackle them efficiently and get back to smooth driving.