From: neal@cmptrc.lonestar.org (Neal Howard)
Subject: Re: Sudden and Drastic Drop in Gas Mileage
Date: Wed, 11 Nov 1992 05:48:40 GMT
Organization: CompuTrac Inc., Richardson TX

In article <1992Nov10.134924.11873@engage.pko.dec.com> sontakke@helix.enet.dec.com (Vikas Sontakke) writes:
>
>I am experiencing a drastic and very sudden drop in the gas mileage (i.e. from
>30's into 20's).  I have not done anything to the car recently.  The oil was
>changed 1.5K miles ago, spark plugs were changed around 4K miles.  The
>driveability and the performance of the car has not changed a bit.  There is
>NO smoke, NO smell.  Choke opens as it should when the car warms up.
>
>The weather has become very cold in this part of the country.  But few extra
>minutes of warm-up does not explain such a drop in the mileage.
>
>All tune-up components (such as plugs, cap, rotor, wires etc) are new or nearly
>new.  The mileage of the car has always dropped in the winter months (from 30 to
>27) but since last tankful, every time I start the car, I swear the gas needle
>loses a tick or two.
>
>Since the car had been running pretty decently, I do not want to mess with it
>much.  Besides it is freezing out there to take a wrench.
>
>I will be switching gas stations to rule out bad gas.  However, I can't see how
>the car can start drinking the gasoline by gallons (last 300 miles) and not
>change its performance or driveability by an iota.  Gas is not leaking and it is
>not being siphoned off.  Nothing else can explain this drop.
>
>For crying out loud, my other car, Taurus SHO got 27.5 miles on its last 1400
>miles trip!  
>
>(car is 83 Honda Prelude, 115K miles, carburated, KNOCK_ON_THE_WOOD-> runs as
> nice as it did when it was new <-KNOCK_ON_THE_WOOD)

Since it is carburated, the automatic choke has some sort of heating device
attached, probably electric (or maybe a heat riser tube from the exhaust
manifold like older American cars used to have). It sounds as though the choke
may not be opening fully when the engine is warmed up to operating temperature
which could cause the gas mileage to drop and maybe actually very slightly
increase performance a bit since Hondas normally run a little lean as part of
their design. If you could stand to freeze just a little, try looking into the
carb after the engine has run just enough to barely warm up to normal from dead
cold. If the choke is not completely 100% open then either the choke heater is
not working properly of the thermostatic spring in the choke may be worn out
since it is ~10 years old. If the choke is 100% open and you're still getting
reduced mileage, gasoline quality may be the culprit. During the winter months,
a lot of states require gasoline to have a lot of ethanol or MTBE in it for
pollution control laws and this will reduce gas mileage and power output
somewhat. I have a '92 GMC pickup with the Vortek V-6 engine and I just noticed
a huge drop in gas mileage when using regular unleaded gas here in Dallas a
few weeks ago (from ~25 highway to ~15 highway). Switching to unleaded
premium brought the mileage back up to where it belongs. The unleaded regular
I had been using (Texaco) also smells different than it did during the summer
months and has the tendancy to make my motor detonate a lot easier than it did
earlier in the year. I suspect that all the major gasoline companies are    
diluting  gas with oxygenating additives for distribution everywhere instead
of only in gas destined for areas which require it by law so they don't have to
bother with the additional storage and transportation hassles of more blends
than they already had to deal with before the laws requiring emissions-reducing
fuels. I just hope the quality of the unleaded premium I've been getting lately
holds out for the rest of the winter since the regular fuel has been giving the
detonation sensor/dynamic ignition timing portions of the electronic engine
control system in my truck a heavy duty workout ( I can actually feel the timing
retard under mild acceleration as soon as the engine begins to clatter with the
crappy gas, at least I know the system is doing it's job and keeping the motor
from running in constant detonation). I thought MTBE was supposed to increase
the motor octane rating of fuels too, but at the expense of energy content.
Maybe whatever they're putting in the cheaper regular grade isn't MTBE, 
maybe they're just blending with more of the straight chain shorter hydrocarbons
like hexane and heptane, I dunno.....

Ramblin on and on,
Neal Howard

-- 
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Neal Howard   '91 XLH-1200      DoD #686      CompuTrac, Inc (Richardson, TX)
         "Let us learn to dream, gentlemen, and then perhaps
          we shall learn the truth." -- August Kekule' (1890)
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