ScanGauge and Open Loop

[George7941]

On my truck the ScanGauge displays Loop Status as OPEN during extended deceleration on the highway. Today I hooked up my Tech2 and it only displayed OPEN during the first five minutes after a cold start. Tech2 displays Closed during extended deceleration and extended WOT. So the ScanGauge and Tech2 do not agree with each other. Which one is right?

While the Tech2 is the GM factory tool and one is therefore inclined to trust it, some facts are puzzling. During extended deceleration the injector pulse width is 0.2 ms as per Tech2( at idle, it is 3.4 ms). With that small a quantity of fuel injected, how could it be in closed loop?

Also, at WOT, don't most engines run OPEN loop?

 

[supton]

I recall in OBDI days, yes they ran open loop. Or they could. Technically the technology keeps changing, and every OEM is free to do as they wish (as long as they hit emission targets). I'd think with wideband sensors they could not only stay closed loop but would want to: if they need to run pig rich to avoid engine issues, then they know exactly what a/f mixture they need. Likewise, if they only need to be just so rich, then they are going to still want to make sure they aren't too lean (due to some otherwise not yet detected issue, like a plugged fuel filter).

To me, as an engineer, closed loop just means the ECU knows what the O2 reading is, and can adjust accordingly. It could be 10:1 or 16:1. The ECU has the feedback needed to alter ratio without resorting to only the knock sensor. The really small injected fuel is probably "open loop" in the sense that the O2 sensor can't get a good reading--but that is a small region of operation (engine over-run). Likely injecting just enough fuel to keep the cat warm, and otherwise considers the injectors "off" (thus the O2 should be leaned to the max, and thus... reading just how it expects it to).

 

[The_Jeff]

Originally Posted By: supton


To me, as an engineer, closed loop just means the ECU knows what the O2 reading is, and can adjust accordingly. It could be 10:1 or 16:1. The ECU has the feedback needed to alter ratio without resorting to only the knock sensor.


Agree. There are other "closed loop" systems in life besides automotive fuel control. Corporations can have them for example where an accountant checks on the CEO and vice versa.

IOW, "closed loop" does not have to mean "perfect stoichometric ratio", merely, "I, the car, have elected to listen to the o2 sensor to make fueling decisions."

While it's weird two scanners have two results, I find it entirely plausible that you're in a fuel decel cut-off mode, and that it would be considered non-closed loop.

 

[JHZR2]

The generic throwing of open vs closed loop terminology may not help.

Here's some good reading:

http://m.controleng.com/index.php?id=9113&tx_ttnews%5Btt_news%5D=112995&cHash=f8d8023a15738d0fcfe78d6a2d71dd60

From there you might speculate reasons that they are saying one thing or another...

 

[SHOZ]

Newer cars with FI will cut the fuel back to idle levels when decelerating. This shows up as 0v on the O2s. Also open loop.

 

[gathermewool]

Originally Posted By: SHOZ
Newer cars with FI will cut the fuel back to idle levels when decelerating. This shows up as 0v on the O2s. Also open loop.


Idle or ZERO injector duty cycle?

 

[The_Jeff]

I've personally witnessed zero FI duty cycle on decel. Forget about open/closed loop though.

Have coasted down through 35 mph/ 1200 RPM and seen the FI kick back in. Felt it, too.

 

[gathermewool]

Originally Posted By: eljefino
I've personally witnessed zero FI duty cycle on decel. Forget about open/closed loop though.

Have coasted down through 35 mph/ 1200 RPM and seen the FI kick back in. Felt it, too.


At least in the recent Subarus I've owned, this is true, too. There's a gear/RPM limit at which the zero IDC will no longer be active, and the engine will effectively idle. I FEEL the same thing in my Civic, but I have no way to know, other than OL/CL logic, as displayed on the Scangauge. As it is, the feeling and OL --> CL display seem to coincide. On occasion, my Civic will not decel as aggressively as usual, and sure enough the scangauge is not displaying OL, and I assume that the IDC is not zero.