Rough Idle While Stopped?
Idle is the engine running on its own with nobody touching the gas, whether that’s at a stoplight, in the driveway, or waiting in a drive-through line. A rough idle means it’s not holding steady there: it shakes, surges, hunts for the right speed, or drops a cylinder instead of running smooth. What’s wrong depends on how it’s acting, not just that it’s acting up. Four patterns cover almost every case, and each one points at a different repair.
A steady hard miss with the check engine light on is almost always an ignition coil or spark plug, sometimes an injector, running $150 to $500 to fix. An idle that surges up and down with no code stored usually traces back to a dirty throttle body or idle air control valve, a $150 to $300 cleaning job. A shake that smooths out once the engine climbs past 1,500 RPM points at a motor mount, $300 to $600 per mount. A miss that only shows up when the AC compressor kicks on is usually a vacuum leak or a fuel system struggling under the added load, $100 to $400 depending on what’s leaking.
As of 2026, idle behavior alone sorts most of these cases before a scan tool even gets plugged in. The code confirms it. The behavior finds it.
A Steady Miss With the Check Engine Light On
A hard, steady miss usually means one cylinder isn’t firing evenly, and the check engine light is almost always on when it’s happening. Pull the code and it typically names a specific cylinder. P0301 is cylinder 1, P0302 is cylinder 2, and so on down the line. P0300 means the misfire is bouncing between cylinders instead of sitting on one.
The most common cause is a worn ignition coil, especially on coil-on-plug setups like GM’s V6 and V8 engines or Ford’s EcoBoost lineup, where each cylinder runs its own coil and one going bad only kills that one cylinder. A single coil runs $150 to $400 installed. Spark plugs wear out on a similar timeline and often get swapped at the same visit, running $200 to $500 for a full set depending on how buried they are.
Less often, the culprit is a fuel injector not delivering fuel to that cylinder. Injectors cost more. Figure $150 to $300 per injector plus labor, more on engines where the intake manifold has to come off to reach them.
Surging Up and Down With No Code
An idle that climbs and dips on its own, without ever setting a code, points somewhere different. That’s usually the throttle body or the idle air control valve, both of which manage how much air the engine gets at idle when nobody’s touching the gas.
Carbon builds up on the throttle plate and inside the IAC passage over time, especially on vehicles that spend a lot of time idling in traffic. That buildup makes the computer’s air correction hunt for the right spot, and the idle surges while it hunts. A throttle body and IAC cleaning runs $150 to $300 and solves it outright more often than not.
If cleaning doesn’t fix it, the IAC valve itself has likely failed and needs replacing rather than another cleaning.
A Shake That Smooths Out Above 1,500 RPM
If the whole car shakes at a stoplight but the shake disappears once RPM climbs past 1,500 or so, that’s usually not an ignition or fuel problem at all. It’s a motor mount.
Mounts hold the engine in place and damp normal vibration before it reaches the cabin. A mount that’s cracked or collapsed lets more of that vibration through at idle, when the engine has the least momentum to smooth itself out. Above idle, the added momentum covers for it. Replacing a mount runs $300 to $600 depending on the vehicle and how many mounts are sharing the load.
A Miss That Only Shows Up When the AC Kicks On
Some rough idles only show up the instant the AC compressor engages, since that’s extra load the engine wasn’t carrying a second earlier. That points at either a vacuum leak or a fuel system that’s already borderline.
A vacuum leak, often a cracked hose or a failing intake gasket, gets exposed the moment idle load increases and the engine has less spare air to cover for it. Fixing a leak runs $100 to $400 depending on where it’s hiding. A weak fuel pump or a failing fuel pressure regulator can cause the same symptom under load, though that shows up less often than a plain leak.
Reading Your Own Misfire Code
For anyone who already pulled a code before calling the shop, the number does most of the sorting. P0301 through P0308 point at a specific cylinder misfiring, which lines up with the coil, plug, or injector list above. P0300 means the misfire moves around, and that more often points at something feeding every cylinder at once, like a vacuum leak or low fuel pressure, rather than one bad coil.
That distinction saves a diagnostic step before the vehicle even gets on the lift. A cylinder-specific code narrows the search fast. A random misfire code means the problem is systemic, not local to one spot.
Rough idle rarely fixes itself. Waiting usually means driving on a misfire long enough that the catalytic converter starts absorbing unburned fuel it was never built to handle, and that’s a repair with a lot more zeros on it than a coil or a cleaning. None of these four patterns get better with time, and most get worse gradually enough that it’s easy to talk yourself into ignoring them for another month. Pay attention to how the idle acts, not just that it’s acting up, and the fix is usually cheaper than expected.
FAQ
Why does my car idle rough only at stoplights? Most of the time it’s one of four things: a coil or spark plug misfire, a dirty throttle body or IAC valve, a worn motor mount, or a vacuum leak that only shows up under load. How it acts, steady versus surging versus shaking, usually points at which one.
What does a P0301 code mean? It means cylinder 1 is misfiring. P0302 is cylinder 2, and so on. A cylinder-specific code usually points at a coil, plug, or injector on that cylinder rather than a problem affecting the whole engine.
Can a bad motor mount cause a rough idle? Yes. A cracked or collapsed mount lets more engine vibration reach the cabin at idle, and the shake typically fades once RPM climbs above 1,500 or so.
Is it safe to keep driving with a rough idle? For a short while, yes, but an ongoing misfire sends unburned fuel into the catalytic converter, which can turn a $200 coil job into a $1,200 converter replacement if it’s ignored too long.
Why does my idle only get rough when the AC is on? That usually means a vacuum leak or a fuel system that’s borderline, since the AC compressor is extra load the engine wasn’t carrying a moment before. It’s worth getting looked at before the leak or fuel issue gets worse.