I have a 2015 Ford Fusion with a 2.5L engine. When the car is idling, it runs rough. The rpm drops after about 4 seconds, the car rumbles, and then it recovers. This keeps happening as long as the car is not moving. Sometimes it even stalls when the rpm drops too low. The shaking is worse when the engine is loaded, like when I shift from park or neutral into drive or reverse. I also see this happen when the purge valve closes completely. When I’m driving, the car runs great, so this issue only shows up when it’s idling.
-----Initial Diagnosis-----
I noticed the engine surging when the purge valve was closed. I figured the Charcoal Vent Canister Vent Solenoid might be bad. To be safe, I replaced the Purge Valve, Charcoal Canister, and the Vent Solenoid. All that’s left in that system is the hoses, but this didn’t change anything. I’ve checked the Purge Valve and used my thumb to leak test it. The replacement parts look good and don’t leak air or fumes when closed.
----Fuel Trim Diagnostic Results-----
After the first try didn’t solve the problem, I did more testing. The Short Term fuel trim at idle is around -0.8 to -1.6%. When I rev the engine, it drops to 0% and then to -10%. This indicates that there might be too much fuel or not enough air as it runs rough at idle. The Long Term Fuel Trim sits around 8% at idle, then drops to 0% when I rev the engine, but goes back up to about 8%.
----Next Steps Taken----
Given those results, I thought about cleaning the Mass Airflow Sensor and changing the spark plugs since it’s cheap and needed. But this didn’t help. The issue keeps happening.
----Repairs Attempted So Far-----
Evap Purge Valve
Charcoal Canister
Charcoal Canister Evap Vent Solenoid
New Spark Plugs
Cleaned Mass Airflow Sensor
What should I do next? I feel stuck and wonder if it could be a vacuum leak or something related to the fuel pressure. I’m not a mechanic, just someone who likes to learn and tackle challenges myself. Any advice would be really helpful.
You should check the fuel pressure. It could be the fuel filter. These are cheap and easy tests. Do you have a diagnostic tester? Make sure to check for any codes.
Wynn said:
You should check the fuel pressure. It could be the fuel filter. These are cheap and easy tests. Do you have a diagnostic tester? Make sure to check for any codes.
I do have a diagnostic tester and found no codes, but I can diagnose further. Let me know how I should proceed and what I need to test. I have access to a basic OBD2 scanner and a more advanced Autel Maxisys. Can I check fuel pressure with a reader? I’m still learning to use the advanced scanner, so I am not sure where to start.
Check the throttle body for carbon buildup. How many miles are on it?
Make sure to disconnect the battery first before you do anything with the throttle body.
Open the throttle plate gently with a wooden dowel and look inside for any carbon. I had a similar issue with my 2007 model because the throttle body was ‘sticky’ from buildup. After I cleaned it with some throttle body cleaner and a rag, it ran much better. Just don’t flood it with cleaner—use just a little, let it soak, and wipe it with a rag. Don’t use paper towels; you don’t want bits to go down the intake.
After cleaning, reassemble and reconnect the battery, then run an idle learn cycle. You can find instructions online. Not sure if the 2015 is the same as the 2007.
@Nico
The car has 235K miles. It’s pretty high mileage. I will remove the throttle body for cleaning next. I’ve also been checking the MAF readings and noticed some rough running while holding a constant rpm (1500 or 2500 for testing) in park. It shakes as if I’m driving on a highway. Since I recently changed the spark plugs, I hope it’s not a misfire from my install.
@Jamie
Good thought! The EGR valve can also affect idle and cause off-idle response problems. If you press the gas hard, you may not notice, but if you accelerate slowly from idle, you might feel a stutter that will show up in the fuel trim as the trim has to adjust for air bypassing the sensor.
A dirty throttle body causes idle issues, but won’t affect fuel trim. The throttle plate can stick mostly closed, so when the motor struggles to open it, that can throw off idle control. Cleaning it usually resolves the issue.
@Nico
Thank you! That is really helpful. I will check out the EGR valve next. It looks a bit tough to reach since it’s next to the throttle body. I have access to Pro Demand, so that should help when I get to it tomorrow.