1999 Subaru Impreza Problem:
- Drastic reduction in mileage (about 50% of normal). It only happens on long trips after car has burned through about ¼ tank of gas. So for the first ¼ tank it gets 100% normal fuel economy, then we start to see a reduction in the next ¼ to about 75% or so, then for the last ½ tank it drops out completely to 50% of normal mileage.
- The problem can be slightly mitigated by stopping and letting things cool down a bit. Stopping for 30 minutes on a long trip gets me back up to 75% mileage for a little bit.
- It does not matter if we start the trip with a full tank or not, the problem seems to be more of a warm up issue
Things checked already:
- Not a fuel leak; the gauge does not go down when the car is sitting
- Just replaced the spark plugs and plug cables
- New fuel and air filters
- Does need an oil change but fluid is not too dirty and its barely overdue (just over 300 miles). I seriously doubt this could be it.
- Front pads and rotors were replaced and did not help the problem. It also coasts in neutral the same as ever so I don’t think it is a brake issue
- The car is slightly out of alignment, which needs to be fixed but I don’t think this would be the issue either. Alignment has been an issue for longer than the mileage problem.
- All fluids seem to be normal, including transmission fluid.
- Put in fuel injector cleaner and ran a full tank with that in but it didn’t help. (This doesn’t mean its not the injectors though.)
- No sign of clutch slippage, plus it was replaced only 2 years ago
Things not checked:
- There seems to be a leak in the exhaust somewhere, not sure when this happened. Maybe this could lead to one of the O2 sensors misreading and causing the fuel to be too rich? Haven't checked the exhaust to see if it burns black. It doesn’t seem this theory works with the whole warm up thing though. I would also think that this would throw a warning on the check engine light. I guess it depends on where the exhaust leak is; I haven’t looked into this yet.