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:
- 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.
- 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.
- PCV valve: replaced about 12K miles ago. Shouldn't be the problem.
- Air filter: also replaced about 12K miles ago.
- 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.
- Transmission? Fluid is full, but looks a little brown and probably should be flushed. Shifts perfectly smoothly.
- Differentials? I plan to check and change the diff fluid this weekend.
- 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.
- 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?
- 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.
- 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.