Dose anyone know if the RX burns more fuel then it needs to in order to heat up the catalectic converter quickly. I am beginning to believe this is a reason the RX gets such bad mileage running short trips.

 

no- it runs rich (originally waaaayyy too rich but still rich now) to keep the cat from overheating. it injects extra air after initial start up to to help heat the cat quicker. it gets worse gas mileage during lots of short trips because it stays in open loop too much. assuming you mean short duration trips. if you mean short mileage trips that take a long time because of traffic thats a different equation.

 

I was reading an article that many cars are tuned these days to heat the cat as fast as possible in order to get it to operating temp, even at the expense of fuel economy. Seeing that the Rotary has a rap for poor emissions I can see Mazda doing this.

My original comment was focusing on many cold starts and short trips. My RX-8 seems to use the most gas in the 1st 5 minutes of my trip based on the movement of the gas gauge, which I understand can be a pretty poor way to measure. I am thinking of ways to better rate the actual gas used at this time.

Of course with the rotary running such a hot exhaust and leaning it out makes it hotter I maybe braking up the wrong tree here.

 

I would think open loop would help because it is always looking for the optimum air/fuel mixture?

 

"Closed" loop operation is where the sensor feedback is used to tune the fuel mixture.
Open loop, the sensor is ignored, the car runs off a fixed table of settings stored in the ECU i.e. the "feedback loop" is open.

S

 

Why would it be in open loop more for short trips. If the switching from Closed to open is a matter of rpms?

 

The actual setting used for startup are similar to the old "choke" settings - way too rich, to get the cat warmed up. Even if the wide-band was feeding back, it would be offscale.

Closed loop operation is used for a while after start up, based on settings recorded during the last drive cycle, kind of an "educated guess" at where the mixture needs to be, erring on the rich side for safety. Don't forget that the O2 sensors also need to get up to temp before they send good data. Only when the sensors and cat are all in operation can the system start to control mixture.

S

 

Both my parents 6 and my RX8 consume a tremendous amount of fuel during cold starts... while all cars go through a fuel rich cold start procedure, Mazda seems to be way richer then most for some reason

 

its not entirely rpm based. its stays open loop when its "cold" or not at a prescribed temp. then it switches to closed loop. Short (duration) trips may never let it get into closed loop. lots of these trips means the car is always in open loop mode which is less efficient than closed loop. One of the things that can cause bad mileage is a bad thermostat because if the PCM doesnt get correct info it could stay in open loop no matter the operating temp of the motor.

 

Yes, I remember you saying this before but if it was stuck open wouldn't the temp gauge stay below normal operating temp range. My gauge always gets to just below half way. This sounds like what everyone else’s gauges are at.