Re: fuel economy theory

[Oliver Seiler <oseiler@acm.org>]

On Thu, 17 Feb 2000, Chris DeLong wrote:

> Hey Oliver!
> 
> Have you checked the ignition timing? Incorrect ignition timing can directly 
> affect mixture settings and fuel mileage.

The timing is okay.

Did some tests today. I didn't have enough stuff with me to test
everything, but I hooked up the dwell meter and had a look at what the
frequency valve is doing. I had a hard start, idling really rough, so it
was rather interesting to see the duty cycle not at 50% (where it should
be during warm-up). I'm not sure what it was at, actually, because I'm not
sure if the scale on the dwell meter is correct. I'll do a more definitive
test tomorrow or Saturday when I have jumper wire. Based on closed-loop
operation, I think it was leaning out the mixture for some reason (at
start up on a cold engine; thermoswitch could be bad, or even the ECU,
or perhaps a short somewhere, but we'll see what happens when I do some
more rigorous tests).

Shutting the engine off and restarting had the engine starting smoother,
and the duty cycle was at 50%, where it should be while warming up. At the
transition from open-loop to close-loop operation, things got interesting.
Immediately the duty cycle increased (I think; the scale on my dwell meter
is reversed from what seems to be happening), almost to the maximum, and
then went back to 50%. Did this a few times and then settled into an
oscillation somewhere around 75 +/- 5%, which indicates that the ECU
thinks the mixture is lean and is trying to compensate. The result is that
my fuel use goes up. It does explain my erratic idle, since the
oscillations match the idle speed changes. Normally it should be around
50 +/- 5%.

Anyway, I need to run through all the various tests of the thermoswitch
and control unit. I'll probably try to replace the oxygen sensor before
doing this. I'm also worried about the catalytic convertor, not only
because I'm running rich now, but it's probably been running relatively
rich for a long time; for all I know the catalytic convertor might be the
problem! I was originally planning on replacing my suspension first, but I
might do the exhaust first; the money I save in gas could help pay a big
chunk of the suspension, I think! 

Thanks,
Oliver

Oliver Seiler -- Software Developer, Pivotal Software, http://www.pivotal.com
    Scirocco MkII Diary - http://erisian.com/~oseiler/scirocco/index.html


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