Rough Idle

[Salfd]

Hi all .. I have a 2005 Baja Turbo in good condition.. but my issue is rough idle when warmed up.

I have replaced both Up and Downstream O2 Sensors
I replaced the MAP sensor
I cleaned the throttle body off the vehicle.
Replaced Air Filter and cleaned MAF
I replaced plugs and coil pack on cyl 1 and 3 ...didnt get to 2 and 4 yet but I get NO Codes on ODB2 Scanner and no Check Engine lights.
I did the idle relearn procedure..correctly.

When the car warms up, it will start ok but idle poorly and even stall if I don't hit gas pedal.

I checked for vacuum leaks but did not find any
I do not know where else to look. Help !!

 

[bmart]

What diagnostic work/results had you replace those parts?

 

[Salfd]

About 8 mos ago ..got p303 misfire on 3 ..changed plug and coilpack ..did relearn and it was fine.
Then 4 mos ago got misfire on cyl 1 code.. changed plug and coilpack...did relearn again abd ok again
Then car was hit in front.. nit much damage ..replaced radiator/A/C core and bumper.. Car looks new but started to get idle issue.
Scan showed Zero volt to O2 s

So... we changed up and downstream O2s ..no change

Cleaned MAF and replaced Air filter.....clean throttle body... replaced MAP because original one was broke at the tube that enters the manifold.
I get NO codes and no check engine !

Car starts fine cold and runs ok... but after warm up.. will idle rough on startup and will sometimes stall unless I hit gas pedal. Once in gear, seems to run ok ..but rough idle at traffic lights etc.

Scanner shows no codes but I don't know what to look for when I look at LiveData ! Cheap home type scanner.

I have new fuel pump/filter coming.. but unsure if that is issue.

I have not changed plugs and coil packs on cyl 2 and 4 yet since it is a big PAIN to do that besides, I am not getting error codes and NO check engine.

Car is very well maintained

 

[bmart]

Maybe a vacuum line? A quick AI search gives me:

A rough idle on a 2005 Subaru Baja Turbo without any error codes can stem from several common issues, including vacuum leaks, spark plug problems, throttle body or IAC valve concerns, or issues with the turbocharger's boost control system. Since there are no codes, focusing on mechanical and airflow-related problems is a good starting point.

Here's a breakdown of potential causes and how to address them:
1. Vacuum Leaks:

• Explanation: Air leaks into the engine's intake system can disrupt the air-fuel mixture, causing a rough idle.
  
  
• Diagnosis: Inspect all rubber hoses, vacuum lines, and connections for cracks, splits, or looseness. A smoke test can help pinpoint leaks.
  
  
• Solution: Replace any damaged hoses or connections.
  

This video demonstrates how to diagnose vacuum leaks in a Subaru Baja:




1m



Budget Mechanic
YouTube · Aug 2, 2025
2. Spark Plugs:

• Explanation: Worn or fouled spark plugs can lead to misfires, causing a rough idle.
  
  
• Diagnosis: Check the spark plugs for wear, damage, or fouling. Inspect the spark plug wires for damage or excessive resistance.
  
  
• Solution: Replace the spark plugs and wires if necessary.
  

3. Throttle Body and IAC Valve:

• Explanation: A dirty or malfunctioning throttle body or Idle Air Control (IAC) valve can restrict airflow, leading to a rough idle.
• Diagnosis: Clean the throttle body and check the IAC valve for proper function and cleanliness.
• Solution: Clean or replace the throttle body and/or IAC valve if needed.
  

4. Turbocharger and Boost Control:

• Explanation: Issues with the turbocharger's wastegate or other boost control components can affect engine performance and cause a rough idle.
• Diagnosis: Inspect the wastegate and other boost control components for proper operation and leaks.
• Solution: Repair or replace faulty components.
  

5. Fuel System:

• Explanation: While less likely without a code, fuel delivery issues can also cause a rough idle.
  
  
• Diagnosis: Check the fuel pressure, fuel filter, and fuel injectors.
  
  
• Solution: Replace the fuel filter and consider cleaning or replacing the fuel injectors.
  

6. Exhaust System:

• Explanation: Blockages or leaks in the exhaust system can also affect engine performance and lead to a rough idle.
• Diagnosis: Inspect the exhaust system for any blockages, leaks, or damage.
• Solution: Repair or replace any damaged components.
  

7. Idle Relearn Procedure:

• Explanation:
  After performing maintenance, it's sometimes necessary to reset the engine's idle settings through an idle relearn procedure.
• Procedure:
  Disconnect the battery, clean the throttle body, reconnect the battery, turn the key to the on position, and then start the engine. Let it idle for a specified time (usually 10 minutes) with no electrical loads.
• Solution:
  Perform the idle relearn procedure as described in the owner's manual or online resources.
  

By systematically addressing these potential causes, you can effectively diagnose and resolve the rough idle issue in your Subaru Baja Turbo

 

[Salfd]

Maybe a vacuum line? A quick AI search gives me:

A rough idle on a 2005 Subaru Baja Turbo without any error codes can stem from several common issues, including vacuum leaks, spark plug problems, throttle body or IAC valve concerns, or issues with the turbocharger's boost control system. Since there are no codes, focusing on mechanical and airflow-related problems is a good starting point.

Here's a breakdown of potential causes and how to address them:
1. Vacuum Leaks:

• Explanation: Air leaks into the engine's intake system can disrupt the air-fuel mixture, causing a rough idle.
  
  
• Diagnosis: Inspect all rubber hoses, vacuum lines, and connections for cracks, splits, or looseness. A smoke test can help pinpoint leaks.
  
  
• Solution: Replace any damaged hoses or connections.

This video demonstrates how to diagnose vacuum leaks in a Subaru Baja:
View attachment 3241
1m
View attachment 3242
View attachment 3243
Budget Mechanic
YouTube · Aug 2, 2025
2. Spark Plugs:

• Explanation: Worn or fouled spark plugs can lead to misfires, causing a rough idle.
  
  
• Diagnosis: Check the spark plugs for wear, damage, or fouling. Inspect the spark plug wires for damage or excessive resistance.
  
  
• Solution: Replace the spark plugs and wires if necessary.

3. Throttle Body and IAC Valve:

• Explanation: A dirty or malfunctioning throttle body or Idle Air Control (IAC) valve can restrict airflow, leading to a rough idle.
• Diagnosis: Clean the throttle body and check the IAC valve for proper function and cleanliness.
• Solution: Clean or replace the throttle body and/or IAC valve if needed.

4. Turbocharger and Boost Control:

• Explanation: Issues with the turbocharger's wastegate or other boost control components can affect engine performance and cause a rough idle.
• Diagnosis: Inspect the wastegate and other boost control components for proper operation and leaks.
• Solution: Repair or replace faulty components.

5. Fuel System:

• Explanation: While less likely without a code, fuel delivery issues can also cause a rough idle.
  
  
• Diagnosis: Check the fuel pressure, fuel filter, and fuel injectors.
  
  
• Solution: Replace the fuel filter and consider cleaning or replacing the fuel injectors.

6. Exhaust System:

• Explanation: Blockages or leaks in the exhaust system can also affect engine performance and lead to a rough idle.
• Diagnosis: Inspect the exhaust system for any blockages, leaks, or damage.
• Solution: Repair or replace any damaged components.

7. Idle Relearn Procedure:

• Explanation:
  After performing maintenance, it's sometimes necessary to reset the engine's idle settings through an idle relearn procedure.
• Procedure:
  Disconnect the battery, clean the throttle body, reconnect the battery, turn the key to the on position, and then start the engine. Let it idle for a specified time (usually 10 minutes) with no electrical loads.
• Solution:
  Perform the idle relearn procedure as described in the owner's manual or online resources.

By systematically addressing these potential causes, you can effectively diagnose and resolve the rough idle issue in your Subaru Baja Turbo

Yep.. thank you.. I did see all that... only thing I did not do yet was change plugs and coilpacks on cyl 2 and 4 which are on drivers side...which looks very hard to do in such a tightvspace.. When I change #3 plug etc.. it left the coilpack boot on the plug..took me 2 hours to remove it and another almost hour to replace plug and coil... Cyl 2 and 4 look worse. Also I see a lot of talk about IAC valve .. but I do not see that on my engine !! I looked at many videos but none show me where that is ...if it even exists... on a 2005 Baja Turbo.

I do hope it is not the turbo.

Maybe a vacuum line? A quick AI search gives me:

A rough idle on a 2005 Subaru Baja Turbo without any error codes can stem from several common issues, including vacuum leaks, spark plug problems, throttle body or IAC valve concerns, or issues with the turbocharger's boost control system. Since there are no codes, focusing on mechanical and airflow-related problems is a good starting point.

Here's a breakdown of potential causes and how to address them:
1. Vacuum Leaks:

• Explanation: Air leaks into the engine's intake system can disrupt the air-fuel mixture, causing a rough idle.
  
  
• Diagnosis: Inspect all rubber hoses, vacuum lines, and connections for cracks, splits, or looseness. A smoke test can help pinpoint leaks.
  
  
• Solution: Replace any damaged hoses or connections.

This video demonstrates how to diagnose vacuum leaks in a Subaru Baja:
View attachment 3241
1m
View attachment 3242
View attachment 3243
Budget Mechanic
YouTube · Aug 2, 2025
2. Spark Plugs:

• Explanation: Worn or fouled spark plugs can lead to misfires, causing a rough idle.
  
  
• Diagnosis: Check the spark plugs for wear, damage, or fouling. Inspect the spark plug wires for damage or excessive resistance.
  
  
• Solution: Replace the spark plugs and wires if necessary.

3. Throttle Body and IAC Valve:

• Explanation: A dirty or malfunctioning throttle body or Idle Air Control (IAC) valve can restrict airflow, leading to a rough idle.
• Diagnosis: Clean the throttle body and check the IAC valve for proper function and cleanliness.
• Solution: Clean or replace the throttle body and/or IAC valve if needed.

4. Turbocharger and Boost Control:

• Explanation: Issues with the turbocharger's wastegate or other boost control components can affect engine performance and cause a rough idle.
• Diagnosis: Inspect the wastegate and other boost control components for proper operation and leaks.
• Solution: Repair or replace faulty components.

5. Fuel System:

• Explanation: While less likely without a code, fuel delivery issues can also cause a rough idle.
  
  
• Diagnosis: Check the fuel pressure, fuel filter, and fuel injectors.
  
  
• Solution: Replace the fuel filter and consider cleaning or replacing the fuel injectors.

6. Exhaust System:

• Explanation: Blockages or leaks in the exhaust system can also affect engine performance and lead to a rough idle.
• Diagnosis: Inspect the exhaust system for any blockages, leaks, or damage.
• Solution: Repair or replace any damaged components.

7. Idle Relearn Procedure:

• Explanation:
  After performing maintenance, it's sometimes necessary to reset the engine's idle settings through an idle relearn procedure.
• Procedure:
  Disconnect the battery, clean the throttle body, reconnect the battery, turn the key to the on position, and then start the engine. Let it idle for a specified time (usually 10 minutes) with no electrical loads.
• Solution:
  Perform the idle relearn procedure as described in the owner's manual or online resources.

By systematically addressing these potential causes, you can effectively diagnose and resolve the rough idle issue in your Subaru Baja Turbo

 

[DragonWagon]

Yep.. thank you.. I did see all that... only thing I did not do yet was change plugs and coilpacks on cyl 2 and 4 which are on drivers side...which looks very hard to do in such a tightvspace.. When I change #3 plug etc.. it left the coilpack boot on the plug..took me 2 hours to remove it and another almost hour to replace plug and coil... Cyl 2 and 4 look worse. Also I see a lot of talk about IAC valve .. but I do not see that on my engine !! I looked at many videos but none show me where that is ...if it even exists... on a 2005 Baja Turbo.

I do hope it is not the turbo.

i dont remember if bajas got the ej255s like the fxt's, but if you take the battery and windshield washer reservoir out they actually have more clearance to the body than the passenger side! recently did my plugs and cylinders 1+3 were way harder

 

[Salfd]

Hi .. thank you for reply.. My Baja is the turbo by the way..I did take the Bat and Washer out as I did attempt to do plugs and coilpacks.. I chickened out partly because it was too hot outside and I am afraid that if I pull the coikpack, the boot will stay on the plug.. not good.. so I stopped.. but I think I have to change plugs on cyl 2 and 4. Then I can eval better why the rough idle.
What bugs me is that the car ran just fine before It was hit square on the front.. There was no major damage.. Radiator, A/C condensor, and headlights took the hit... no frame damage...The shop said they detected a small exhaust leak near the downstream sensor so they did a weld repair.. but when I got car back, that is when the idle issue started.. The shop did a scan abd said no voltage detected from downstream senor so I replaced it.. did not cure issue..

Only thing left is to change cyl 2 and 4 plugs/coils and take it from there.