🩺 Rough Idle / Stalls at Stops
An engine that idles rough — shaking, hunting up and down, or dying at stoplights — is almost always struggling with one of three things: air it didn't meter, fuel it didn't expect, or spark it didn't deliver. The causes sort from a $20 hose to real engine work, and the order below is the order that finds it cheapest-first.
The causes, in the order a tech checks them
A cracked vacuum hose, a leaking intake gasket, or a stuck-open PCV valve lets in air the computer didn't meter. Idle suffers most because that's when unmetered air is the biggest share of the total. High or hunting idle plus rough running is the classic sign.
How to confirm: Listen for hissing; a tech smoke-tests the intake in 15 minutes and finds it definitively. Fuel trims on a scan tool (high positive at idle, normal at cruise) point straight at it.
Carbon and oil vapor coat the throttle plate and idle passages until the computer can't meter idle air anymore. Especially common on higher-mileage engines that stall right when you come to a stop.
How to confirm: Pull the intake tube and look — a visibly black, crusty throttle plate is your answer. Cleaning is a 20-minute job.
A single misfiring cylinder makes an idle shake you feel in the wheel — often with a flashing or steady check engine light and a P030x code that literally names the cylinder.
How to confirm: Read codes (free at most parts stores). P0301 = cylinder 1 misfire, and so on. Old plugs are the first suspect; a coil that swaps the misfire to another cylinder when moved is the second.
A contaminated mass-airflow sensor lies about incoming air; a weak fuel pump or clogged filter starves the engine at low speed. Both produce rough idle plus hesitation on takeoff.
How to confirm: MAF cleaning is a $10 can of sensor cleaner and ten minutes. Fuel pressure is a gauge test any shop can run.
A collapsed mount transmits totally normal engine vibration straight into the cabin at idle in gear. The engine isn't running rough — you're just finally feeling it.
How to confirm: Idles rough in Drive but smooths in Neutral/Park? Suspect mounts. Watch the engine while a helper power-brakes it gently — excessive rocking is the tell.
Describe your exact situation — vehicle, when it happens, what changed — and get a free diagnosis ranked for your specific car. Then, if you want professional certainty, a vetted master technician reviews your case for $25, answered in 24 hours or it's free.
Common questions
Why does it idle rough only when cold?
Cold engines run rich on purpose, which masks some problems and exposes others. Cold-only roughness that fully smooths out warm often points at a marginal vacuum leak (gaps close as metal expands) or early carbon buildup on intake valves — common on direct-injection engines. Annoying, worth watching, rarely urgent.
Rough idle but no check engine light — possible?
Absolutely. Vacuum leaks, dirty throttle bodies, and worn mounts frequently never set a code, because nothing is electrically out of range. No light doesn't mean no problem; it means the computer can't see this particular kind of problem.
Will fuel injector cleaner fix a rough idle?
If the cause is mildly dirty injectors, a quality cleaner can genuinely help — it's a reasonable $15 first experiment. But it can't fix a vacuum leak, a bad coil, a dirty MAF, or a broken mount, which together cause far more rough idles than injectors do. One bottle, one tank; if nothing changes, stop buying bottles and start diagnosing.