Fuel Mileage Problem - Diagnosis?

[BobN]

In the past 6 weeks my 98 OBW has suffered an increase in fuel consumption
from its normal 25MPG to 21-23 MPG. I'm trying to figure out what's causing
it. So far I've come up with these ideas:

1. Worn sparkplugs: Yes, they were completely worn. I replaced them 2 weeks
ago but while it starts better and doesn't stumble when accelerating from
rest any more, mileage has not improved. Plug wires are all good, no kinks,
no arcing. Changing the plugs was a royal pain, I believe I don't have the
right kind of ratchet extension.

2. Oil: Oil is changed every 3K miles (that's monthly for me), needs a 1QT
topup after 1500. I check and top up once per week. Probably not the
problem.

3. PCV valve: replaced about 12K miles ago. Shouldn't be the problem.

4. Air filter: also replaced about 12K miles ago.

5. O2 sensor: I don't know if these have ever been replaced in the 227K mi.
life of the car. Possible culprit? I have not had any CELs. There is a
hole or a worn-out flange gasket in the mid-pipe that I'll check this
weekend, but that shouldn't affect mileage.

6. Transmission? Fluid is full, but looks a little brown and probably
should be flushed. Shifts perfectly smoothly.

7. Differentials? I plan to check and change the diff fluid this weekend.

8. Wheels? No unusual noises, bearings seem fine without play. Two tires
are worn nearly to limit, two others are newer (one old and one new each
front and rear). Pressures always checked at least weekly, running 35PSI.

9. Air conditioning: Mileage is noticeably worse with the AC on. I wonder
if the compressor bearing or clutch is getting worn and creating excess
load. How would I test this?

10. Fuel formulation: Who knows what they put in regular gas these days for
EPA or state EPA reasons? Maybe the Massachusetts gas has lower energy
content these days.

11. Compression: I suppose I should test the compression. I was hoping to
get another 200K out of it before a rebuild, but who knows? If changing the
sparkplugs is any indication, checking compression is going to be a real
PITA.

All serious comments appreciated. Thanks, guys.

 

[Dave Morrison]

In the past 6 weeks my 98 OBW has suffered an increase in fuel
consumption from its normal 25MPG to 21-23 MPG. I'm trying to figure
out what's causing it. So far I've come up with these ideas:

[big chop]

All serious comments appreciated. Thanks, guys.

12. Dragging brakes. Have them inspected.

 

[mike]

My Pick would be No 5
A lazy O2 sensor was a problem in my old car. It would still respond so
wouldn't through up an error code but would be very slow to respond to
exhaust gas changes. the result was poor gas mileage.
If, after a long drive, the exhaust gas smells rich like you have just
started a cold car this would be the most likely culprit.
I also had a problem once when the o2 sensor behind the catalytic converter
become unplugged this resulted in the same symptoms but again didn't cause
an error code.
hope this helps
Mike

 

[Bruce Hoult]

In the past 6 weeks my 98 OBW has suffered an increase in fuel
consumption from its normal 25MPG to 21-23 MPG. I'm trying to figure
out what's causing it. So far I've come up with these ideas:

[big chop]

All serious comments appreciated. Thanks, guys.

12. Dragging brakes. Have them inspected.

If that's a problem then you should be able to tell yourself by feeling
the temperature of the wheel hubs after you've driven a few miles.

 

[l.lichtman]

I have a question about your tires. Do I understand that you have one
worn tire on the front and one on the rear? Is that so that the center
viscous clutch will not get unequal driveshaft speeds when you are
going straight? I was discussing this with another Subie owner, and we
couldn't agree whether this would be a good approach.

 

[Edward Hayes]

I'd also suspect a lazy front O2 sensor. Subaru had a recall on front
O2 sensors on my 2000 Forester but, I don't know what years were
included. I think O2 sensors are part of the 80,000 miles emissions
warranty? Worth a check with www.subaru.com

 

[Paul Harding]

All good suggestions, you've just got to look
at everything. I had a similar problem and found it was caused by a tire
change. The new tires handled a lot better but they were softer with more
stick and caused a 3mpg drop in my WRX. I previously owned a '00 Outback and
was getting between 19 and 21 on the OEM tires with a 5MT, so you are right
there!

 

[hippo]

You could try the Subaru Upper Engine Cleaner - sorry , can't remember the
part # ATM! Cheers

 

[BobN]

I have a question about your tires. Do I understand that you have one
worn tire on the front and one on the rear? Is that so that the center
viscous clutch will not get unequal driveshaft speeds when you are
going straight? I was discussing this with another Subie owner, and we
couldn't agree whether this would be a good approach.

I tried putting the two good ones on the front, and the vibration was
terrible. I went back to the f/r stagger after 50 miles and the vibration
was much less.