Diagnostics guide
Your car shakes at idle
You can feel it through the seat at a red light, and it smooths out once you are moving. The engine is already compensating for whatever is wrong — and it will show you what that compensation looks like.
Why is my car shaking at idle?
At idle the engine is doing the hardest version of its easiest job — producing just enough combustion, evenly, at the lowest speed it ever runs. There is no road speed and no gear to smooth anything over, so a cylinder contributing less than the others is felt directly, through the seat and the wheel. The cause is almost always one of three things the cylinders share: spark, fuel, or air. Which one it is, is usually a question the car can answer about itself, because the engine computer is already correcting for the problem and will show you what that correction looks like.
Before you read anything else: If your check engine light is flashing, that is an active misfire dumping raw fuel into the exhaust, and it can destroy the catalytic converter — stop driving as soon as you can pull over safely. A steady light means get it looked at soon. This page is about finding the cause; a flashing lamp is about not making it more expensive first.
Idle is where a weak cylinder has nowhere to hide
Every time a cylinder fires it gives the crankshaft a push. At speed those pushes come thick and fast, and the momentum of a spinning engine papers over one that lands softer than the rest. At idle there is no such cover. The engine is turning as slowly as it ever does, each push matters proportionally more, and an uneven one becomes a vibration you can feel.
That is why “shakes at idle but drives fine” is such a common description, and why it is not the contradiction it sounds like. It is the same fault, measured where the engine has the least margin.
Three things the cylinders share
When a rough idle is not confined to one cylinder, the cause is usually something every cylinder depends on: spark (worn plugs, a failing coil), fuel (low pressure, clogged injectors), or air (a vacuum leak letting in air the sensor never measured). Narrowing to one of those three is the whole job — and it is the part you can do yourself, before anyone quotes you for parts.
What to look at while it is happening
A rough idle is a live symptom, so it deserves a live reading. With a standard OBD-II adapter, Kaiju Mechanic shows what the engine is doing in the moment — and this read is free, with no subscription and no credits.
The values that earn their place on a rough idle
- Fuel trim — the correction the engine is applying to the mixture. The single most useful reading here, and the subject of the next section.
- Mass airflow — how much air, by weight, is actually entering the engine. A sensor reading low is a classic cause of rough running and poor economy.
- Engine load — the share of available output in use right now. It is context: airflow and fuelling readings make far more sense alongside it.
- Timing advance — how far before top-dead-centre the spark fires. The engine moves it constantly to balance power against knock protection.
What any given car exposes varies — this is a property of the vehicle, not of the app. Some engines report far more than others.
What is a normal fuel trim range?
This is the question everyone arrives with, and the honest answer is that the universal figure you have been offered elsewhere does not exist. It is worth explaining why, because knowing why is more useful than the number would have been.
Start with what the value is. The short-term correction is the small, moment-to-moment adjustment the engine makes to the mixture based on what the oxygen sensor reports; it swings around zero as the engine fine-tunes. The long-term value is the learned offset it keeps hold of. Both are the engine telling you how hard it is working to keep the mixture right.
Movement is not the fault
A correction that swings and settles is the system working exactly as designed. It is a consistently large pull in one direction that points to an air, fuel, or sensor problem — the engine adding fuel to make up for a lean mixture, or pulling it back for a rich one.
So the thing to read is the direction and the persistence, not a threshold.
And that is the reason no honest figure is available to quote. The limit that actually matters is the one your engine’s computer is built to — which is why P0171 is set when the correction the computer is applying runs past the limit it is allowed to compensate within, rather than when it crosses a round number you could look up. That limit is a manufacturer calibration. It is not the same on your car as on the car in the forum post.
Comparing the two sides of the engine is more informative than any single figure. On a V or boxer engine that reports both banks, a large difference between them points at something affecting one side rather than the whole engine. That is also why the lean codes come as a pair — P0171 for bank 1 and P0174 for bank 2 — on engines that report it.
What this page will not do: It will not give you a number to measure yourself against, because there is no source for one that would survive being checked. Kaiju Mechanic shows you the live values and what each one means; deciding what is normal for your specific engine is a judgement, and we would rather hand you the reasoning than a figure that is wrong on half of all cars.
What to do, in order
1.Check the check engine light first — steady or flashing
A flashing lamp is not the same warning as a steady one, and it changes what you do next rather than what you look at next. If it is flashing, stop driving as soon as you can pull over safely. If it is steady, or off entirely, carry on with the steps below.
2.Read the car while it is actually shaking
A shake at idle is intermittent for a lot of people — worse cold, worse in gear, gone on the motorway. The reading that tells you something is the one taken while the symptom is happening, with the engine warm and idling, not the one taken afterwards in the driveway.
3.Watch what the engine is correcting for
The engine constantly adjusts how much fuel it adds relative to the air it measures, and it reports that adjustment live. Watching it while the engine is rough tells you whether the mixture is being pulled one way, which separates an air or fuel problem from an ignition one before you buy a single part.
4.Compare the two banks, if your engine reports both
V-shaped and boxer engines have two cylinder banks and many of them report each side separately. A large difference between the sides points at something affecting one bank rather than the whole engine — a much narrower search than starting from scratch.
5.Read stored and pending codes, even if no light is on
A fault the car has seen once but not yet confirmed sits as a pending code with no lamp to announce it. On a rough idle that is common, and it is often the fastest route from "something is wrong" to a specific cylinder or a specific sensor.
Where this stops and a mechanic starts
Reading the car narrows the search. It does not finish it. Confirming a vacuum leak means finding the leak; confirming a weak coil means swapping it to another cylinder and seeing whether the fault follows. Those are hands-on jobs, and this page will not pretend otherwise.
What it does change is the conversation. Walking in with “it is rough at idle and the engine is consistently correcting the mixture in one direction on bank one” is a different appointment from “it shakes sometimes”. If a misfire code is what you are looking at, start with P0300; if there is no code at all, see why a scanner can report no codes with the light on.
Watch the engine while it is rough
Kaiju Mechanic shows live fuel trim, mass airflow, engine load and timing advance while the engine is running, so you can see what it is correcting for at the moment the shake is happening. Reading is free — no subscription, no credits. You need a standard OBD-II adapter; the one you already have will do.
Setup is install the app, add your car, connect your adapter, then watch it idle.
Ready?
Your car deserves better
than guessing.
Download Kaiju Mechanic free and start your first AI conversation in under a minute. No appointment. AI chat works without an adapter — live sensor reads use any standard ELM327 you bring. Just answers.