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Lousy fuel economy on '91 200TQW; advice, please.

I've probably run this across the list before, but let me try again.

I've got a car that runs splendidly and it's very reliable, with 105k.
Since  buying it about 20 months ago, I've carefully put money into it, and
all the regular maintenance items are taken care of.

But here's what's puzzling me.  The car gets absolutely miserable gas
mileage around town.  Mind you, I'm not expecting a lot, but it's probably
getting about 13mpg if I stick to city driving.  On the freeway, of course,
it's much better, and I've gotten in the mid-20s on the freeway.  Around
town, I drive it pretty reasonably, and I'm not putting my foot into it.

Here's some further info:

1. It's got an IA Stage II chip
2. I had some mild  bucking at high boost several months ago, and  replaced
a couple of vacuum lines, and that problem is gone.
3. I replaced the O2 sensor this past spring, with the generic Bosch unit
from a Ford Probe through the Parts Connection.  Thinking there might be a
problem with the new O2, I checked the voltage, and the range is .3 to .9
volts (or 3-9, can't remember which).  A friend with a shop in the Bay Area
tested it and his gauged showed the average voltage at the O2 to be .59 (or
is it, 5.9, I forget).  He said it was a bit high, but within range.  The
car idles fine.  Could this be a problem?

4. I've checked pretty much all the sensors per the Bentley, and they read

5. Scott Mockrey said a thermostat that causes engine temp to run cool
could be a contributing factor.  I  bought a new one and I suppose I should
try installing it and see what happens.

6. I generally use 92 octane, and occasionally run Techron through it.

7. Three months ago or so, I put it on an emissions tester, and it was very
clean.  I even ran the probe into the pipe off the headpipe before the
converters, and it would've passed (I think).

8. Some time back, I checked the plugs and they looked okay.  I replaced
them a year ago with OEM.  The resistances on the plug wirers all checked
out.  And I'm not getting any codes.

9. Could there be a fuel pressure regulator problem and still pass smog? It
has a new vacuum hose to it.

10. I am checking fuel economy according to how much the car actually uses,
not relying on the computer (though it's fairly accurate).  With the last
tank, including close to 100 miles of freeway driving, I averaged exactly
16 mpg.  This doesn't seem right.

Any BTDT?  Advice?


- Jim