Always wondered, so I'll ask... Our '94 has had this problem for as long as I can remember, but I can't remember back before I changed from a pre-cat to a downpipe. The smell goes away in about two minutes, and after that there's no problem. Cat converter is a relatively new stock unit, voltage at the coolant temperature sensor is in spec for a cold engine, so the A/F ratio is probably whatever Mazda wanted for that condition. Is it normal for a near-stock FD's exhaust to smell overpoweringly rich on initial startup if the pre-cat has been removed?

 

I imagine that would be the case, also with the original engine, the engine won't be making as much compression when cold, so on a cold startup you'll have more unburnt fuel going out the exhaust.

Dale

 

My FD is tear eye rich during warmup. Smells normal after warmup. Thats with downpipe and possibly bad cat converter.

 

There is a relatively easy way to change the A/F ratio when cold without affecting it (much) when up to temperature. By putting a parallel resistor across the coolant temperature sensor's input to the PCME, the mixture at cold could be leaned out. Has anyone has ever done that, and if so what was the result?

 

I've heard of that being done. I'd probably get a variable resistor that you can turn and adjust and play around with the setting.

Dale