Open vs. Closed Loop
 

I'm trying to clarify something. From what I've read open loop is when the engine is using a default a/f mix, and closed loop is when it is using an a/f mix based on the reading of the O2 sensor and such.

O know the engine runs in open loop until it is warmed up.

Once warmed up should it stay in closed loop or should it be cycling between open and closed depending on throttle, gear, etc?

I ask because I was noticing that Box cycles back and forth, but I have no idea if that is normal or not.

I was always under the understand that once in closed loop (when warmed up) that it stayed there except for WOT. Probably about 90-100% of throttle it ignores the O2 and just starts dumping in extra fuel.

I've never had a meter to actually verify this on any of my cars though. My father has a Chevy scanner and I know our trucks stay closed once they close as does his Caprice.

I'm not sure about your car since it's a lot newer, I'm sure most new cars just read the TPS, but on my old Audi there was actually a switch at the end of the physical throttle movement, if this was adjusted too far in you could be hitting at 50-60% throttle maybe? Or actually just check your TPS, measure resistance at 0-50-100% and see if it lines up with what others report.

Are you sure that's what that switch is for? My old 1980 Bonneville had a switch at the end of the throttle travel as well, when the switch was activated the transmission kicked down to a lower gear. That switch seems kinda redundant on a newer car as the computer already knows how much the pedal is being pushed from the TPS.

-Jay

Right that's why I suggested to check the TPS. Well my Audi was manual and I know for a fact when the switch was pressed that it goes to WOT, but the Audi isn't a good example of anything because it didn't have a TPS and measure fuel requirements with a flapper :-).

My 1996 Civic goes to Closed Loop after about 10 seconds from a cold start. Warm start and it's almost immediate. It switches to Open Loop above about 95% load / throttle.

...according to Scangauge.

Would a bad TPS possibly cause this? Also the idle is uneven when in neutral and stopped, it drops til the engine nearly stalls then jumps back up to over 1000rpms, then drops steadily again.

This is how the 02 sensor is supposed to work. Sometimes it will even go to closed loop before the engine is totally warmed up to normal running temp, as long as it does not need the extra rich fuel mixture.

Could be something wrong with the 02 sensor if it is continually running open loop and wasting fuel.

Almost sounds like a vacuum leak somewhere. My grandfather's Grand Marquis did that and the dealer found a cracked vacuum tee under the hood.

-Jay

bulletin - EG012-04

INTERMITTENT ROUGH OR LOW
ENGINE IDLE AT A STOP

A small number of 2004 model year Scion xA and Scion xB vehicles equipped with manual transmissions may experience an intermittent rough or low engine idle at a stop due to a possible misadjusted throttle cable.

That sounds exactly like my car. I dug up a copy of that TSB online and will have to look into it.
Sadly it also sounds like I've got another issue what with the going in and out of closed loop.

I played around with the Loop gauge today too and it seems mine is closed loop when TPS is greater than 0 and open loop when TPS is 0 (closed throttle). Does this seem normal?

Doesn't sound normal to me. What kind of vehicle do you have again?

-Jay

I've got a 03 Mitsubishi Lancer ES. I mean, would it matter if its open loop when its closed throttle? it goes right back to closed loop right when i apply the accelerator...

It just doesn't sound right... There's got to be something else going on there.

-Jay

I don't know about modern cars....but my '83 Cadillac goes into closed loop at 81 degrees centigrade coolant temp. The only thing that WOT does AFAIK is cut the A/C compressor.

You might check your wires going to the temp sender, O2 sender...hell...just check them all if you have a chance.

On my '83 Cad, I have had issues with wires that were old, heat and oil cracking the insulation, and rodents chewing on the insulation. Had the car 4 years, and I'm still finding bad wires!

I realize your car is pretty new compared to my Cad, but things still can happen...maybe a dirty connection, something snagged a wire at some time and broke some of the conductors...a little corrosion somewhere...

That's pretty cool...a car named after my favorite character from "Logan's Run"...

I had a chance this weekend to experiment more with this. By paying close attention I noticed Box goes into Open loop a half second before the instant mpg goes to 9999mpg. So apparently what I was seeing is the engine turning off as I coast in gear, and the scanguage was reading that as going into open loop. As soon as I touch the gas or put in the clutch and the engine turns back on it goes right back into closed loop.

I've heard the same, that SG will report open loop in DFCO, but my truck doesn't do that. I guess that's why my SG does not report DFCO as 9999 MPG. To confirm DFCO I either have to watch live O2 sensor data (DFCO O2=0) or ignition timing advance (DFCO = -10).

-Jay

The only EFI cars that don't go into open loop at(or near) WOT are usually those that don't have a fuel enrichment mode or those that come factory equipped with a wide-band O2 sensor.

Since manufacturers try to get you good HP numbers and good fuel economy at the same time, almost all vehicles have a fuel enrichment scheme of some sort for heavy load.