John - 5x Racing

Got a question for the ecu experts out there: What is the ecu changing when it is in open/closed loop and how does this affect mpg? From my scangauge II I noticed when I am traveling above 78 mph the ecu goes to open loop mode. It seemed to hurt the mpg, I just was wondering what the different modes do.

Thanks,

Brettus

iTrader: (2)

think you are seeing the secondary injectors kick in there rather than it going to open loop .
It is very hard to stay in closed loop at anything close to that speed unless you are going down hill

John - 5x Racing

What is the difference between the two modes though? The scangauge II manual describes it as:

Open: when the oxygen sensors are not being used to control the mixture.
Closed: when the oxygen sensors are being used to control the mixture.

This is a pretty generic explanation, so I wanted to know how this related to the rx-8.

How many injectors does the car have?

Brettus

iTrader: (2)

that is actually a pretty good explanation - inside closed loop the ecu is contantly adjusting to get the optimum settings . Outside it does nothing but follow prexisting tales that are modified by a factor that was determined in closed loop operation .....

6 injectors
3x2 pairs

MazdaManiac

iTrader: (3)

Secondaries never come on at cruising loads, regardless of RPM.

Brettus

iTrader: (2)

/\ what makes it suddenly go richer at that rpm ?

John - 5x Racing

So the mixture goes rich in open loop states, this is obviously not improving mpg. The reason that this intrigues me is the talk of in gear coasting saving gas, which I found odd as the engine turns at a higher rpm than it would while idle coasting in neutral. The theory was that the injectors cut off when the engine was being turned by the tranny, and idling was using more gas. I was researching this theory using my scangauge and found that the ecu goes into open loop while coasting in gear and stays in closed loop while idling. So does open loop in off power situations still richen the mixture? Has anyone tested this theory of in gear coasting saving gas?

cjkim

just check your AFRs while coasting

zoom44

if you decelerate in gear you will see your AFRs shoot so high it will probably peg your meter. as for testing it- yeah everyone who has ever logged this car has tested it. thats many many people.

Georgia8er

Going into open loop when coasting in gear makes perfect sense. While it does do the deceleration fuel cut-off, the O2 sensor readings would cause it to go into open loop regardless.

zoom44

what reading from the o2 sensor would cause it to go into open loop regardless?

Georgia8er

If the ECU saw a reading beyond the range of the O2 sensor, that should default to an open-loop state, similar to not having a sensor hooked up or a bad sensor. I think the ECU would recognize it is decelerating and that wouldn't be a concern, but I don't really know how that is programmed.

John - 5x Racing

I don't have the ability to monitor my ratios unless I hook up my innovate meter. I am sure someone here already knows what happens to the ratios during in gear decel conditions, does it get maxed out rich or lean as one of the other posts said? So my question remains, does being in an open loop state use more gas? I recently took a weekend trip with tons of interstate driving and was getting good mpg readings at 70 mph, but when above 78 mph the ecu goes into open loop and I was getting crazy high readings for mpg averages as the scangauge shows 9999 for mpg when in open loop mode. After most of the trip in open loop I saw a bad mpg average for the tank. All of this info might seem senseless except for the fact that I am doubting this in gear coasting saving gas theory, as I saw lower mpg in open loop mode.

zoom44

it will default to a limp home mode store a code and light your CEL if it some how detected an out of range reading. but thats not what happens when decelerating. when decelerating it moves to its hysteresis maps and actually cuts injectors off to achieve very lean burns. it would be considered open loop since it just goes by the tables without feedback loop

John your answer is - it depends. normal open loop acceleration or crusing will be less fuel efficient than closed loop. however deceleration is another matter. it goes as lean as possible with actually shutting off on deceleration

your scanguage is giving you false mpg readings it must have either a fault or a compatibility issue.

John - 5x Racing

Zoom44, the scangauge is working correctly as it does exactly what the manual says, it shows 9999 when in open loop mode. I guess it cannot calculate accurate mpgs when in open loop. I still find it strange that the car is more efficient when decelerating in gear rather than coasting in neutral, you would think the engine would require the same amount of fuel for the relative rpm it was turning, at least just as much as it would need at idle as the throttle position is the same.