1. Home
  2. Subaru Cars Forum
  3. Fuel Mileage Problem - Diagnosis?

Fuel Mileage Problem - Diagnosis?

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.

  1. 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.

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

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

  2. 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.

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

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

  1. 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.

  2. 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?

  1. 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.

  2. 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.

Reply to
BobN
Loading thread data ...

On Fri, 02 Sep 2005 23:09:55 GMT, "BobN" wrote in news:eZ4Se.9251$8q.6389@lakeread01:

[big chop]

  1. Dragging brakes. Have them inspected.
Reply to
Dave Morrison

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

Reply to
mike

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.

Reply to
Bruce Hoult

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.

Reply to
l.lichtman

Reply to
Edward Hayes

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!

Reply to
Paul Harding

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

Reply to
hippo

wrote

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.

Reply to
BobN

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.