Drivable — worth looking into
Idle speed hunts
The tachometer needle wanders up and down with the car standing still: 1200, then 600. Sometimes the engine nearly stalls and picks up again.
There is no danger, but the symptom will not go away by itself: it is the early stage of something that gets pricier.
What to look at in the live data
- Fuel trims at idle. A steady positive value means an air leak, a steady negative one means it is running rich.
- Air flow: at idle on a warm engine it is roughly the engine size in litres times 2–3.
- Throttle position: it should sit still, not jitter.
- Coolant temperature — on a cold engine hunting idle can be normal.
Common causes, cheapest first
- A dirty throttle body — half an hour to clean.
- An air leak downstream of the air meter.
- The idle air control valve, on cars that have one.
- A tired mass air flow sensor.
- Lost adaptations after the battery was disconnected — sometimes half an hour of driving is enough.
Codes that show up with this
P0171System too lean (bank 1) — air leak or not enough fuelP0101Mass air flow circuit range/performanceP0505Idle air control system malfunction
These parameters in the reference
Engine RPMShort term fuel trim B1Long term fuel trim B1Mass air flow (MAF)Throttle positionCoolant temperatureShort term fuel trim B2Long term fuel trim B2Relative throttle positionAbsolute manifold pressureCommanded EGREGR error
Lamps that may be on
Do it step by step
Four readings on one screen show what exactly is swinging.Unstable idle — how to find the cause
A symptom is not a diagnosis. The same sign comes from a dozen different causes — the readings are what tell them apart, not the feeling.
How the app helps
CarScan shows exactly these readings live, remembers the highest and the lowest value of the trip, and puts them into a report you can hand to a mechanic.