I had a couple questions re: open vs. closed loop plus the o2 sensor adaptation function.

My car runs almost perfect in open loop just running off the map and maf voltage. I still have the WB o2 sensor in the driver's bung, with the NB o2 sensor in the passenger side.

I've copied/pasted the open loop map into the closed loop map.

When I switch to the closed loop map, the car runs less well and errs on the side of being a little lean (as measured by the WB)

When I enable o2 adaptation, it runs even worse.

Is there any other explanation other than the NB and WB are getting different mixtures, thus when switching to closed loop, the NB is giving a signal that no longer squares with the map? But why would adaptation then make it even worse?

My hypothesis is that once I get the tuning dialed in as well as can be expected, and then replace the NB where the WB currently resides, I should expect identical driving characteristics. I would also expect adaptation to smooth things out.

Currently my NB gauge reads just a wee bit rich, suggesting the driver's side runs a bit leaner than the passenger side. So when I switch to closed loop, the driver's side leans out as the passenger side attempts to find stoich.

Are any of these assumptions wrong or misguided?

I don't really mind running open loop all the time, it just seems like closed loop should be "even better," but that's currently not the case. Before I just switch out o2 sensors from side to side again, I want to be sure that there's not something else I may be overlooking.

 

If you have cats, you don't want to run in open-loop mode all the time. The life of the cats will be greatly shortened. The life expectancy of cats running in open-loop mode is around 5 years, as opposed to an O2 closed-loop system where the cats last sometimes upwards of 20 years or even more. In closed-loop mode, the computer doesn't just try to set the mixture at 14.7:1, it actually cycles it from rich to lean and back again many times per second to facilitate the function of the cats.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Brian,

I'll try to answer your questions but hopefully John Speake will weigh in here. I think your assumptions are all correct, and either there is still some left-right difference, or the NBO2 sensor is lying.

When closed-loop is selected (NBO2 sensor connected, and ST's "obey coding plug" mode is selected) then the LH monitors the NBO2 signal: If it says "lean" then the LH increases fuel, if it says "rich" then it decreases fuel. This happens quickly, and you will see the AFR "hunting" between 14.2-15.2 (or so) on a WBO2 gauge, that is characteristic of closed-loop operation. And as Dan said, the cats need this, to alternately have a bit of excess fuel and then a bit of excess oxygen.

So if your map is set up for an AFR of 14.7 then it should make absolutely no difference whether it is closed-loop or open, except for this "hunting" (which you can't see without a WBO2).

Exactly correct-- except adaptation should theoretically do nothing. In reality there are always some small fueling differences which adaptation will adjust out, but these should never be noticeable.

"Adaptation" is nothing more than an average of the "O2-adjust" corrections, over a longer period of time and (I believe) also saved for each map cell. The purpose is to get an adjustment made more quickly, and also during periods when the NBO2 signal is unavailable (i.e. during warmup)-- but this only has an effect if a large adjustment is needed. If your map is correct then it shouldn't be doing anything.

I am suspicious of the NBO2 gauge. As I mentioned the NBO2 sensor is basically a binary device, either rich or lean compared to 14.7. So an NBO2 meter can't tell you much about the actual AFR, to the extent that it reads anything more than "rich" or "lean" they are basically reading tea leaves.

If you want a AFR meter, then get a gauge that be connected to the WBO2 controller.

But taking the NBO2 gauge-readings out of the picture, there is still the issue that the car runs better in open-loop mode, which means that the NBO2 signal getting to the LH is not correct-- either a sensor issue, or a left/right difference.

Running closed-loop won't be any better, and in fact closed-loop might be smoother because the LH is not running around fiddling with things. The problem is that things age, and as MAF output changes a little over time the fueling drifts away from where you set it.

928's for the rest of the world don't have cats or NBO2 sensors and run open-loop all the time. However, they also have an extra pot for adjusting mixture at idle (and low RPM's), to account for production tolerances and aging. This is the "CO" pot (located near the LH/EZK) and is set using a gas analyzer for a tailpipe CO of 1% IIRC. If a WBO2 sensor (with gauge) is fitted then that can be used instead, to set AFR to stoich (14.7). But there's little point, with an O2 sensor (NB or WB) just let the LH run closed-loop and it will do that automatically.

So three things to try: with the LH in open-loop, swap the WBO2 to the other side and see if you get the same AFR under the same conditions.

Try disconnecting your NBO2 gauge, and just connect the NBO2 sensor directly to the LH in the factory configuration. The gauge might be messing up the NBO2 signal-- e.g. picking up some interference.

And then try a new NBO2. A generic 3-wire sensor works fine and is not expensive. I chopped the cable on off the old one and used that as a pigtail with insulated spade-disconnects. (That also provides a convenient point to disconnect the NBO2 signal for testing and tuning, just put a "test" switch in series with the NBO2 signal wire).

 

Are these tests you are doing, WOT, or just cruise? you know that, and it has been mentioned already, that the cruise map, is the closed loop mode where you get wild mixture hunting. lean to rich, but average in the 15:1 range. Probalby a little leaner in some cases. in closed loop mode , its all about the MAF and mixture maps, with no feedback from the 02 sensors. this happens generally around 75% of throttle position and greater.
What exactly are you trying to do?

as far as learnng goes, my car, if the power has been pulled overnight, will run lousy for about 10min of hard driving, plus 5min of warm up. after that, it revs and performs normally. But, before that time, it will have a flat spot and some misses near the max rpm range (5000-6000rpm range and generally down 50hp). after, everything is normal.

mk

 

What you describe in your car once you disconnect the power is the O2 adaptation happening. Your base cruise map is off and the LH forgets the O2 adaptation matrix it has saved. When you start up with no adaptation matrix, the car runs like crap until the LH relearns how to adapt the base map to actual conditions.

This is what Brian is trying to avoid by tuning the base cruise map. I'm about to do the same in my car. With the maps set perfectly to the engine, there should be very little difference whether the car has run with power for 6 months, or if the power was disconnected 5 minutes ago because with a properly set map there doesn't need to be much O2 adaptation going on.

Dan
'91 928GT S/C 475hp/460lb.ft

 

I don't have much to add to Jim's excellent response, except to reiterate that any left/right imbalance will fool the stock NBO2 sensor and also the WBo2 of course. That issue needs to be addressed you can tune correctly.

When you go closed loop, monitor the "O2 adaptation %" parameter on the LH System Screen. Zero out the adaptation, and monitor. It should be reasonably centered in it's range with the engine running. Retune to better centre it, but only once you get equal NB and WB readings whatever the position of the sensors, side to side.

Did you Autotune to 14.7:1 when you were on the open loop map ?