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Benino · Posted: Tue Feb 06, 2007 6:23 am Post subject: air fuel ratio problem

I installed an air fuel ratio gauge in my '80 931. The computer seems to be responding normally to O2 sensor output when I start up the car. The air fuel ratio goes from lean to rich and bounces back and forth. Driving around town this works normally, but when I get on the freeway, after a few miles just cruising (off boost, although this problems seems independent of boost) the gauge will slowly stop bouncing back and forth and start showing almost completely maxed out on the rich side. It stays pretty steady there whether I floor it or let off the gas etc. If I flick the ignition off and back on it instantly starts responding to o2 sensor output like normal again and will work normally for a few miles. Any ideas? I'm thinking maybe my computer (o2/frequency valve control unit) is going bad. Has anyone had this problem before?
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morghen · Joined: 21 Jan 2005 Posts: 9102 Location: Romania

probably your computer has a bug or some sort of missunderstanding with the O2 sensor.
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flosho · Posted: Tue Feb 06, 2007 7:30 am Post subject:

The bouncing around from lean to rich will always be there especially when driving around town.

If you are on the highway and put it down, hold it for a few seconds and then let off, does it bounce back to lean for a second on decel?

I can't remember what mine looks like but I think your a/f gauge is doing the same thing mine does after it warms up.

The main problem is that its a narrow band gauge. I would suggest going to a wide band. The difference is that (IIRC) the narrow band gauge works on voltage between .5 and 1 volt. The wide band sensor/gauges read from .5 to 5 volts.. a much more precise measurement.

Now, you didn't waste your money on that narrow band gauge, all you have to do is search ebay for the LC-1 wideband gauge by Innovative.. It will be about $190-$200 and you can actually convert your narrow band gauge to wideband. The innovative LC-1 kit comes with just the o2 sensor, bung, software, and the wiring.. no actual gauge. You can connect it to the serial port on a laptop and actually log the a/f's.

The other way you can go is AEM UEGO, which is an all in one kit for about $250. But its essentially stand alone in that it wont work with megasquirt or other EMU other than the AEM ones. But this kit does include everything.
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Benino · Posted: Tue Feb 06, 2007 8:08 am Post subject:

Paul · Posted: Tue Feb 06, 2007 9:12 am Post subject:

Just make sure you are not diluting your oil with all that fuel...
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Smoothie · Posted: Tue Feb 06, 2007 4:33 pm Post subject:

Just to allay your fears some, I've noticed the same thing - the gauge reading high for seemingly extended periods. -And I don't have a fuel-in-the-oil problem. You should see a steady reading during warm-up and approximately the first 5 min., then at idle and steady cruise it should bounce up-down in a regular pattern indicating closed-loop mode. Then while cruising, if you step on the gas (doesn't have to be full throttle), it comes out of closed-loop and gives a higher, steady reading. Then when you go back to a steady cruise at part-throttle it should go back to closed-loop. Mine sometimes doesn't go back to closed-loop as soon as I'd think it should, but eventually it does. Anyway, I don't think you should worry yourself too much about it going so rich that its' contaminating the fuel. At least in my case it isn't.
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