Dear OwenF,

Hope you're well. The engine coolant temperature sensor supplies a signal to the engine control unit (ECU).

A separate sensor sends an engine temperature signal to the instrument cluster fuel gauge.

Thus, the ECU can "think" the engine is still cold, when the engine's temperature - as shown on the instrument cluster temp gauge - is normal.

If the ECU "thinks" the engine is cold, the air/fuel mixture will be "rich", i.e., extra, unneeded fuel will be supplied.

Thus, I'd check the engine coolant temperature sensor. If memory serves, that sensor is mounted under the third intake manifold runner, i.e., towards the back of the engine (closest to the firewall).

I'd not try to clean the inside of the down-pipe, the pipe from the exhaust manifold to the catalytic converter. Any loose soot will be blown towards the cat and there burned-off. Soot that stays in the down-pipe doesn't hurt anything.

Hope this helps.

Yours faithfully,

Spook