Rough Idle

[jesse malhotra]

Hey guys,

i am originally from DC and i moved last week to Miami for college. when I got here I was idling In a parking lot and it was pretty hot and all of a sudden it did this studder where the RPMs dropped and the car shook a lot for a few seconds and then it would go back to normal idle for a couple of mins and then that cycle would repeat. Since then it hasn't been as hot outside, but now when I stop at lights I get a pretty rough idle and the rpms move around but are lower than usual.

I tried unplugging what I thought was the MAF or MAP while the car was rough idling and it completely fixed the idle and once it plugged back in it went back to rough instantly. I now realize that since this is an Ecoboost it was the IAT so I'm not sure what that means or what I should do to fix it.

I'd really appreciate any advice on how to fix this, but also if there's anyone from Miami and knows a good mustang shop I could go to for some other issues aswell that would be super helpful.

Thanks everyone!

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[CAELUM]

Hey guys,

i am originally from DC and i moved last week to Miami for college. when I got here I was idling In a parking lot and it was pretty hot and all of a sudden it did this studder where the RPMs dropped and the car shook a lot for a few seconds and then it would go back to normal idle for a couple of mins and then that cycle would repeat. Since then it hasn't been as hot outside, but now when I stop at lights I get a pretty rough idle and the rpms move around but are lower than usual.

I tried unplugging what I thought was the MAF or MAP while the car was rough idling and it completely fixed the idle and once it plugged back in it went back to rough instantly. I now realize that since this is an Ecoboost it was the IAT so I'm not sure what that means or what I should do to fix it.

I'd really appreciate any advice on how to fix this, but also if there's anyone from Miami and knows a good mustang shop I could go to for some other issues aswell that would be super helpful.

Thanks everyone!

I am chasing a EcoBoost rough idle when in D and stopped. It appears not to be an issue when in N. It also appears to be related to ascending a hill but not as apparent descending a hill. I have change plugs and coils but the issue remains.

 

[jesse malhotra]

I am chasing a EcoBoost rough idle when in D and stopped. It appears not to be an issue when in N. It also appears to be related to ascending a hill but not as apparent descending a hill. I have change plugs and coils but the issue remains.

Yup same thing. So weird right! Have you tried replacing your motor mounts yet?

 

[CAELUM]

No, but I am at step 3A for ChatGPT to check them, see below.


Misfire / Rough Idle Diagnostic Decision Tree (2017 Mustang EcoBoost)
Step 1: Confirm Basics

• Spark plugs = new, correct type & gapped (~0.028–0.031").
• Coils = new, seated properly.
• No active CEL, but pending misfire feel at idle.
  Since you already did this, we move deeper.

Step 2: Check When the Rough Idle Happens

• Only in Drive (with brake on) → go to Step 3A (Load / Mount / Torque Converter).
• Also in Neutral / Park → go to Step 3B (Engine/Fuel/Air-related).

Step 3A: If Only in Drive at Idle

1. Inspect Engine Mounts
   • Put in Drive, foot on brake, pop hood.
   • If engine rocks/shakes excessively → bad mount.
   • EcoBoosts often kill passenger-side mounts first.
2. Torque Converter Check
   • Lightly feather throttle at idle in Drive.
   • If the “shudder” smooths out immediately → could be torque converter clutch dragging.
   • Transmission fluid condition matters here (burnt fluid = higher suspicion).

If both mounts and torque converter check out → continue to Step 3B, since load may just be making a minor engine stumble more noticeable.


Step 3B: If Rough Idle Happens in Park/Neutral Too

1. Fuel Trim Check (Scan Tool)
   • At idle: are STFT/LTFT lean (+10% or more)? → likely vacuum leak.
   • At 2000 RPM steady: if trims normalize → confirms vacuum leak.
2. Vacuum Leak Test
   • Inspect PCV hoses, brake booster line, intake manifold gaskets.
   • Spray carb cleaner around suspected spots → idle will stumble if leak present.
3. Injector Check
   • Use scan tool “cylinder balance test” (if available) OR manually disable injectors.
   • If Cylinder 4 barely changes idle when disabled → suspect bad/clogged injector.
4. Carbon Buildup Check
   • If all else looks normal but idle is still rough → intake valves may have carbon.
   • Symptoms: stumble at idle, fine under load.
   • Only fix = walnut blasting or chemical cleaning.

Step 4: Mechanical Health Check (If Still Unresolved)

• Compression test or leak-down test → to rule out bent valves, head gasket, or piston ring issue.
• This is less likely since your car drives smoothly at speed.

Likely Priorities for Your Mustang

Based on your description:

1. Injector #4 or minor vacuum leak near #4 → most probable.
2. Engine mount → since it’s worse in Drive + brake.
3. Carbon buildup → if mileage is higher (>70k) and it’s never had intake cleaning.

If you want, I can also give you a cheap “DIY order” checklist (what to check first in your driveway with minimal tools, vs what needs shop equipment). Would you like that version too?

 

[robvas]

Don't use ChatGPT to fix cars...

 

[CAELUM]

Don't use ChatGPT to fix cars...

Yep, shows how desperate I am.

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