Is this MX-5 running too cold, and could that explain poor emissions?

Question

Last year my MX-5 NC struggled to pass its emissions test.

5 years ago I replaced the front cat exhaust manifold with a 4-1 non-cat manifold, moving the oxygen sensors around the remaining mid pipe cat. The mechanic after some diagnostics felt the car was running too cold and recommended changing the thermostat. I did notice the temp gauge dropping when driving fast in high gear with low revs.

I’ve changed the thermostat, but the gauge looked low still, so I hooked up a ODB2 reader and the car is running at 85 deg C. Is this still too low? Can the temperature sensor give a low reading and thus still overfuel due to the ECU thinking that the engine is too cool? Any thoughts or guidance?

Answer 1

… the car is running at 85 deg C. Is this still too low?

Without getting into MX-5 specifics (for which the model year needs to be known), 85 °C isn't a crazy low value for engine coolant temperature (ECT). Many models are designed to operate at that temperature.

Can the temperature sensor give a low reading and thus still overfuel due to the ECU thinking that the engine is too cool?

The engine fuel management should not be adding extra fuel if ECT is low unless the temperature is in cold-start territory, which it isn't at 85 °C.

Any thoughts or guidance?

I don't think ECT explains what's happening with your emissions, especially considering that the exhaust system has been modified.

It's quite possible that the front cat delete and/or sensor relocation has had the effect of altering lambda readings, so unless the engine's fuel management was tuned to account for the hardware change, these discrepancies between reality and what is expected can lead to overfueling, which ends up clogging the cat and render it ineffective.