Poor mileage/plugs indicate lean running/13-16MPG (avg 14mpg)

Phil and Judy Rose pjrose at frontiernet.net
Sat Nov 10 10:05:50 PST 2007

Dwanca, IMHO you're using some (potentially) very inaccurate 
measurement tools--the gas gauge and the computer's "mpg" readout. 
I'd forget about the gas gauge altogether in assessing the mpg--as 
the gauge will never be accurate enough for anything other than a 
very crude assessment. The dashboard computer can be quite accurate 
-- but often it's been improperly calibrated/adjusted.  And then 
there's the matter of the kind of test-driving you should do in order 
to assess your car's fuel economy. So, step (1) IMHO would be to 
determine the accuracy of your car's computer and  (2) would be to 
use well-controlled driving conditions that permit comparison of your 
car's mpg with "typical" performance levels.

[Apologies if the following seems obvious or you've already done it]

(1) To determine whether or not your computer is presently giving 
accurate info about the mpg, you first need to do some fairly long 
distance driving (perhaps 350-400 miles) to use most of a tankful of 
gas (I'd recommend to use at least 15-20 gallons and don't worry 
about the driving conditions--fast or slow, it doesn't matter for 
this purpose--except avoid excessive idling of course). Make sure to 
reset the computer mpg at the start of this test period. Also make 
sure your tires are properly inflated and that the tire size doesn't 
produce a significant error (anything over 1 or 2 %) in odometer 
readout. Do not reset the computer until the completion of the test 
period--all the while keeping careful track of total miles driven on 
the odometer. Then (before running out of gas ;-) ) fill up the tank 
and "do the math" so you can compare your actual mpg to what your 
computer is claiming. The percentage difference, if any, is caused by 
the error in the computer's "fuel consumption factor" and this error 
can be minimized by making the appropriate +/- adjustments with those 
little instrument-cluster switches. Alternatively, you can avoid the 
ordeal of removing the IC to make the adjustments and simply 
calculate (and record) the percentage error in the computer value and 
use it manually to correct all future mpg measurements. Don't be 
surprised if you find the computer was "off" by as much as 15-30%.

(2) Next, for assessing whether if your car's MPG performance is OK 
(or not), I think you need to avoid doing tests that involve driving 
up a mountain and then coasting back down. That might be a number you 
want to know, but it's a pretty useless measurement when trying to 
ascertain if your car is performing "normally". Surely there's a lot 
of fairly level highway not too far from Denver. I'd advise finding 
such a stretch of highway  and go for a drive maintaining about 65-70 
mph  in 5th gear with a really feather-light touch on the go-pedal 
(that probably means: no passing!)  Naturally you need to reset the 
computer  (the mpg average) at the start of your test. Drive at least 
30 miles in one direction and almost immediately return along the 
very same route. If the (overall) highway gradient is pretty 
flat/level and there is no major amount of change in head- or 
tailwind to confuse the issue, I'd say you ought to see an overall 
average "reading" of at least 25 mpg (true value). My chipped 
(Lehmann stage 1) '91 200q will typically have a true mpg of 26-29 
mpg under similar conditions. IMHO, unless you see much below a 
(true) 25 mpg average during the "65 mph/level-road" test, you 
probably don't have much cause to worry about sub-par fuel economy.

Of course you can always forget about any of that computer 
"calibration" stuff and just do your test the old-fashioned way: 
top-off the gas tank carefully--then drive at least 300-400 miles on 
a level highway at 65-70 mph (minimal passing allowed)--then 
carefully refill the tank and "do the math".


At 7:12 AM +0000 11/10/07, dwanca at att.net wrote:
>Took out the car for a ride in the mountains.  Instead of watching 
>the Turbo Boost I changed the trip computer to MPG.  Going up I-70 
>out of Denver using the Boost I would expect lower mileage but not 
>less than 17 MPG.  It is about a 30 mile drive one way going up. 
>Car runs great boosts well 1.8-9 BAR in O.D. passing everything. 
>Coming back is 30 mi of down hill 6 to 7% grades all coasting.  Best 
>I could get was 16MPG at the end of the ride.  It came up from 14.5. 
>Ran between 2500 and 3200 RPM and nursed the gas for mileage on the 
>way home.  In 60 miles I burned 1/4 tank.
>Last look at the plugs indicated that the motor was running lean :?
>Checked the carbon canister frequency valve (N80)and I know that it 
>is bad as I was supposed to get a 0 ohm reading across the connector 
>posts per Bentley's and I got 289 ohms.  But I can't believe that 
>this would make mileage go this bad as it is supposed to have a 
>manual override if the solenoid fails.
>So where is all the gas going?
>It had a new 02 sensor installed by the PO but the fuse was not 
>hooked up.  It passed emissions with flying colors.
>I hooked up the fuse and then the cats turned to powder inside :?  I 
>gutted them since I have 2 years before another test and no extra 
>cash to replace cats now.
>Does anyone have any ideas about what might be wrong?
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*  Phil & Judy Rose           Rochester, NY  *      
*        mailto:pjrose at frontiernet.net       *

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