fuel economy theory

[Oliver Seiler <oseiler@acm.org>]

Hi guys,

My '86 8V is getting *real* bad mileage (haven't figured it out exactly
lately, but around 15 MPG). It has become worse since I started working on
the engine, which is odd, since the most substantial stuff I've done so
far is plugging the numerous vacuum leaks, I would expect things would get
better, unless...

Is it possible, if the emissions control system wasn't working properly
(say, a dead oxygen sensor), that vacuum leaks could improve the mileage
(relatively speaking)? My theory is that the ECU thinks the engine is
running lean (dead oxygen sensor) so it tries to richen the mixture. With
the vacuum leaks, however, air flow through the air flow sensor would be
reduced, meaning that the extra fuel the ECU is trying to provide
(changing the duty cycle on the frequency valve) isn't actually getting to
the injectors, so it sort of balances itself out (in my case, the mileage
was around 19 MPG before I started mucking about). Plugging the vacuum
leaks increases air flow through the sensor, which increases the amount of
fuel to the injectors, which causes my mileage to get worse. I wonder if a
previous owner just started unplugging things until he passed emissions
tests...

Of course, all this can be tested, which I'll be doing when I get a
chance. Probably just replace the oxygen sensor to be safe, but I'd like
peoples opinions of the above theory. Speaking of replacing the oxygen
sensor, what is the best way to get these off? I have a 4-into-1 exhaust
manifold (JH engine) and the sensor is mounted just below that, right
behind the engine. I have the tool for taking the sensor off (fits on a
3/8" socket wrench) but there doesn't seem to be a great leverage point.
Of course the sensor might be seized (I tried taking it off last night,
but couldn't budge it); is it okay to use something like Liquid Wrench on
it if I'm just replacing it anyway?

Thanks a lot,
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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