Ignition timing significantly impacts exhaust emissions. Advanced timing increases power and NOx but risks detonation. Retarded (late) timing reduces power and creates a characteristic gas signature: high CO and HC, low NOx.
The Effect of Late Timing
When the spark fires too late, combustion occurs while the piston is already moving down. Peak cylinder pressure happens later, wasting energy as heat in the exhaust rather than as torque on the crankshaft. The incomplete burn leads to:
- High CO — fuel doesn't fully oxidize
- High HC — unburned fuel exits the chamber
- Low NOx — combustion temperatures are lower because the burn extends into the expansion stroke
Diagnosing with 5-Gas Data
Look for:
- CO > 2%
- HC elevated (500-2000 ppm)
- NOx low or zero
- Lambda may be normal or slightly low
This pattern is distinct from rich faults (which also have high CO) because NOx is low, whereas a rich mixture typically produces some NOx (though less than stoichiometric).
Verifying Timing
If available, check timing advance data from OBD (Mode 01 PID 0x0E). Compare to specifications at idle and load. Alternatively, use a timing light directly on the engine to verify base timing.
Some vehicles have variable valve timing that can mimic timing effects. Consider VVT position as well.
Common Causes
- Faulty crankshaft position sensor (timing signal delayed)
- Mechanical distributor wear (if equipped)
- Incorrect base timing after component replacement
- ECU logic issues (software, Learn reset)
Repair
Once identified, correct the timing to manufacturer specifications. After repair, CO and HC should drop significantly, and NOx may increase slightly as combustion efficiency improves.