Engine & warning lights
Rough Idle: 6 Causes and the Cheapest Order to Check Them
Why your car shakes at a red light and how to test each cause from cheapest to most expensive.
The short version
Throttle body cleaning $27–65, air filter $13–40, plugs $55–160, engine mount $80–240 each.
- 4 symptoms you can check yourself
- 4 diagnosis steps in order
- 2 FAQs answered directly
- Last editorial review: 2026-08-01 · 5 min read
Overview
Idle shake happens when the engine can't hold a steady speed (usually 650–850 rpm). Checking in the right order saves hundreds.
Key takeaways
- Steering wheel and seats vibrate at a standstill
- Worn engine mounts
- Open the hood and watch engine movement when the A/C kicks in — excess motion means mounts
- Not directly, but it signals a fault that wastes fuel and wears mounts and brackets.
Symptoms
- Steering wheel and seats vibrate at a standstill
- Tachometer needle hunting up and down
- Shake disappears when you shift to Neutral
- Shake worsens with the A/C on
Likely causes
- Worn engine mounts
- Dirty air filter or spent spark plugs
- Carbon-clogged throttle body or idle air control valve
- Vacuum hose leak
Diagnosis & repair steps
- Open the hood and watch engine movement when the A/C kicks in — excess motion means mounts
- Clean the throttle body and change the air filter first (cheapest, often enough)
- Inspect vacuum hoses for cracks or disconnections
- Scan for OBD2 codes before replacing any part
Expected cost
Throttle body cleaning $27–65, air filter $13–40, plugs $55–160, engine mount $80–240 each.
Figures are a guidance range calculated with a published method — read the methodology.
How to prevent it
Clean the throttle body every 25,000 miles and change the air filter every 10–12,000 miles in dusty regions.
FAQ
Is a rough idle harmful?
Not directly, but it signals a fault that wastes fuel and wears mounts and brackets.
Why is it worse with A/C on?
The compressor loads the engine, exposing weakness that already exists.
DriveMet review
Throttle body cleaning $27–65, air filter $13–40, plugs $55–160, engine mount $80–240 each. Clean the throttle body every 25,000 miles and change the air filter every 10–12,000 miles in dusty regions.
What we verified
- Cross-checked 4 warning signs against 4 likely causes before publishing.
- Reviewed 4 repair steps and confirmed the order a shop would follow.
- Compared the cost range with our published methodology and refreshed it on 2026-08-01.
- Answered 2 questions people actually search for on this topic.
Reviewed by the DriveMet editorial team on 2026-08-01 under our editorial policy. This is an editorial review of the page, not visitor ratings.
Related tools
Apply what you just read to your own car with your own numbers.
Reference sources
- NHTSA vPIC — vehicle specifications and manufacturer recalls
- EPA fueleconomy.gov — reference fuel-economy figures
- SAE J1979 standard for generic (Pxxxx) fault-code definitions
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