Poor mileage problem

[BobN]

My 98 OBW (240K miles) gets ok highway mileage of around 26mpg at an average
speed of 70 MPH. But if most of my driving is around town that falls to
19-20mpg. I have noticed that the smell of raw gas is quite strong on cold
starts, and the mileage seems worst when I have a lot of short trips and
start the car more often per miles driven. Could a sensor or something be
haywire that causes the engine to run too rich?

 

[mulder]

My 98 OBW (240K miles) gets ok highway mileage of around 26mpg at an average
speed of 70 MPH. But if most of my driving is around town that falls to
19-20mpg. I have noticed that the smell of raw gas is quite strong on cold
starts, and the mileage seems worst when I have a lot of short trips and
start the car more often per miles driven. Could a sensor or something be
haywire that causes the engine to run too rich?

It's perfectly normal to get less mileage with city driving vs.
highway, although you may be getting less than the expected city
mileage if something is wrong. Is that smell coming from the engine
area or the exhaust? If it's from the exhaust it could be excessively
rich, if it's from the engine itself there is a leak someplace.

 

[Ragnar]

My 98 OBW (240K miles) gets ok highway mileage of around 26mpg at an average
speed of 70 MPH. But if most of my driving is around town that falls to
19-20mpg. I have noticed that the smell of raw gas is quite strong on cold
starts, and the mileage seems worst when I have a lot of short trips and
start the car more often per miles driven. Could a sensor or something be
haywire that causes the engine to run too rich?

Engines often run rich when cold. And starting the car does tend to use
more gas than idling. Doesn't sound like a sensor; sounds like normal
usage.

 

[BobN]

Engines often run rich when cold. And starting the car does tend to use
more gas than idling. Doesn't sound like a sensor; sounds like normal
usage.

Thanks for the comments so far. Normally I'd agree that it might be normal,
and I sure have a lot of history with this car (original owner). But the
mileage deficit has become markedly worse over the past 10,000 miles -
that's about 4 months driving for me.

For example, on the first quarter-tank after a fillup with highway driving I
get 110-115 miles. I know that figure includes excess in the filler hose,
I'm very consistent in my fillup routine. The most recent mixed-driving
first quarter tank only gave me 79 miles. This has happened several times
since November.

I changed the oil this afternoon so I took a good look at the engine. Other
than a small oil leak from the front seal, the engine bay doesn't indicate
any leaks. There is a small exhaust leak about 1 foot ahead of the
mid-pipe/tail-pipe joint (right before the heat shield ends, so it will be a
major pain to fix). Spark plugs are fresh - new platinum plugs installed
14K miles ago.

The smell seems to be from the exhaust. I typically smell it most when
backing out of the driveway.

 

[Carl 1 Lucky Texan]

My 98 OBW (240K miles) gets ok highway mileage of around 26mpg at an average
speed of 70 MPH. But if most of my driving is around town that falls to
19-20mpg. I have noticed that the smell of raw gas is quite strong on cold
starts, and the mileage seems worst when I have a lot of short trips and
start the car more often per miles driven. Could a sensor or something be
haywire that causes the engine to run too rich?

If it has just begun this behavior - I'd suspect the Engine Temp Sensor
(often near - but disticnt from the temp gauge sender). Sometimes it
'fools' the computer into pumping in more fuel after warmup, reporting a
still cold engine temp.

Could possibly be a crack in some vapor recovery part I suppose.

Carl

 

[hippo]

Thanks for the comments so far. Normally I'd agree that it might be
normal,
and I sure have a lot of history with this car (original owner). But the
mileage deficit has become markedly worse over the past 10,000 miles -
that's about 4 months driving for me.

For example, on the first quarter-tank after a fillup with highway driving
I
get 110-115 miles. I know that figure includes excess in the filler hose,
I'm very consistent in my fillup routine. The most recent mixed-driving
first quarter tank only gave me 79 miles. This has happened several
times
since November.

I changed the oil this afternoon so I took a good look at the engine.
Other
than a small oil leak from the front seal, the engine bay doesn't indicate
any leaks. There is a small exhaust leak about 1 foot ahead of the
mid-pipe/tail-pipe joint (right before the heat shield ends, so it will be
a
major pain to fix). Spark plugs are fresh - new platinum plugs installed
14K miles ago.

The smell seems to be from the exhaust. I typically smell it most when
backing out of the driveway.

New plugs 14K back & problem around 10k back...
I know it's a pita job, but maybe worth pulling the plugs and checking the
gaps?

BTW, granted the symptoms are recent, but what was your hwy and town
mileage previously just out of interest? Also - always same fuel from same
place? Cheers

 

[Rick Courtright]

If it has just begun this behavior - I'd suspect the Engine Temp > Sensor (often near - but disticnt from the temp gauge sender). > Sometimes it 'fools' the computer into pumping in more fuel after > warmup, reporting a still cold engine temp.

Hi,

That sensor defaults to a "full rich" condition on failure in the older
models--my highway mileage also dropped about 10% when mine went out.
Not sure what the failure mode is on the OP's model, but it's a good
place to start looking.

Also, depending on the area, "winter" fuel is reported to return far
worse mileage than "summer" fuel. Since the problem first surfaced about
the time the fuels would have been switched (at least in my area), I'm
wondering if there's a new "recipe" this year that's affecting mileage?

Rick

 

[BobN]

Sensor (often near - but disticnt from the temp gauge sender). > Sometimes
it 'fools' the computer into pumping in more fuel after > warmup, reporting
a still cold engine temp.

Hi,

That sensor defaults to a "full rich" condition on failure in the older
models--my highway mileage also dropped about 10% when mine went out.
Not sure what the failure mode is on the OP's model, but it's a good
place to start looking.

Also, depending on the area, "winter" fuel is reported to return far
worse mileage than "summer" fuel. Since the problem first surfaced about
the time the fuels would have been switched (at least in my area), I'm
wondering if there's a new "recipe" this year that's affecting mileage?

Thanks for those ideas, guys. Would the engine temp sensor throw a code if
it failed? I haven't had any codes in the past six months.

I did notice that a lot of pumps have labels saying the gas contains 10%
ethanol, I think as a winter formula. I believe that hurts mileage as well.

The funny thing is, I did a long highway trip yesterday and got my usual
25mpg at a virtually steady 3000RPM for 300 miles.

 

[Rick Courtright]

Thanks for those ideas, guys. Would the engine temp sensor throw a > code if it failed? I haven't had any codes in the past six months. ....
I did notice that a lot of pumps have labels saying the gas contains
10% ethanol, I think as a winter formula. I believe that hurts
mileage as well.

Hi,

When my sensor went out, it triggered a CEL and threw a code. But mine's
a '90--if yours is OBDII (the current computer diagnostic version in
case I screwed the letters up!), it may behave differently from mine.

On the ethanol issue: since ethanol's got less BTU content per unit than
gasoline, I think the mileage issue's gonna show up more markedly at
times when you need more power, like accelerating around town. Also, I
believe ethanol doesn't "light off" as well at lower temps as gasoline,
so a lot of cold starts could have a negative effect on fuel
consumption.

OTOH, your road trip sounds "ideal" for mileage, since the engine stayed
warmed up and you held a steady speed, which takes far less power than
getting the car going, so I could see how an ethanol blend COULD return
similar mileage under that particular condition.

Rick