Ignition timing determines how much of the charge has burned before the exhaust valve opens. Retarded (late) timing means incomplete combustion at exhaust opening — you pay for it in the tailpipe.
The late-timing signature
- HC elevated — unburnt fuel reaches the exhaust.
- CO elevated — partial combustion.
- CO₂ depressed — combustion didn't complete.
- NOₓ suppressed — peak cylinder temperatures never reached.
- Exhaust temperature high — late burn continues into the port and downpipe.
Advanced timing — the mirror pattern
- HC low
- CO low
- NOₓ high (peak temps exceed normal)
- Knock risk under load
Causes of late timing
- Knock sensor active — ECU pulling timing to escape detonation.
- Crank / cam position sensor drift.
- Timing chain stretch, tensioner failure.
- Faulty coolant temp sensor preventing timing advance maps.
- Low-quality fuel triggering knock retard.
Confirmation
Read timing on OBD (advance angle PID) and compare to manufacturer spec at warm idle. Use a timing light where mechanical timing marks are accessible.