Date: Mon, 13 Oct 1997 12:47:38 -0700
Reply-To: Malcolm Holser <mholser@ADOBE.COM>
Sender: Vanagon mailing list <Vanagon@Gerry.SDSC.EDU>
From: Malcolm Holser <mholser@ADOBE.COM>
Subject: Re: Using O2 sensor for fuel mix adjustment?
> I've seen references to using the O2 sensor present in some Wasserboxer
> vans to adjust the fuel mix, or CO level, or whatever. As my '86 Digijet
> Caravelle doesn't have one of those sensors, I'm thinking of using an
> external one.
>
It cannot be really "external" -- it must be placed into the exhaust pipe
before any catalytic converter or muffler.
> 1:st question: Which kind should I use? I can easily get one-wire,
> three-wire and four-wire sensors. I suppose the ones with three or four
> wires have some kind of heating element in them.
>
Single wire ones are fine, but they must be placed more carefully. The
"other" wires are for heating elements for installations where the exhaust
gases are not hot enough (about 300'C) to get good readings. If you use
a three-wire, get the one with two white and one black wire. The black
wire is the sensor, the two whites are for a 12v internal heater.
> 2:nd question: Where should I put it?
>
Intall a fitting into the exhaust pipe (18mm threads) as close as you can
to the engine, but where the exhaust all converges. If you have dual carbs,
you may have to use two ports. In the US, you can buy these fittings at
any auto-parts store as "spark plug anti-fouling extenders", and easily modify
them. You have to drill a hole into the exhaust pipe and weld in the fitting.
*ALL* exhaust leaks must be fixed.
> 3:rd question: How should I connect it, and how to interpret measurements?
>
Simply read the voltage between the black wire (or only wire) and the
metal housing of the sensor. About .4v is right, but it is almost impossible
to get it stable. A rich setting will give more voltage, up to about .9,
while a lean setting will show little if any voltage. Typically, for a
carburetted car, you want to test this fully warmed-up, while driving under
a normal load. You will be testing the main jets only, basically. Best
power is a bit rich, best gas mileage is a bit lean. Best emissions are
right between, but CO and HC go up very fast as you get rich, and NOx
emissions go up very fast as you get lean. If you are concerned mainly with
HC and CO, a lean setting is best. NOx emissions are not tested in California
for instance.
The sensor almost acts like a switch, going from 0 to .9v very quickly right
around the ideal mixture point. FI systems acutally place a .45v reference
on the wire, and if they ever read this perfect value, they know the sensor
is probably failed. FI systems constantly go rich, then lean, watching the
voltage go up and down.
malcolm
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