Posted this originally on subaruforester.org, but didn't get any bites so am asking over here. Any ideas gratefully received.
I've been measuring a big drop in fuel economy over the past two months - 19mpg on average down to (as I filled up this evening) 15.1mpg. I'm at a loss to explain what might be causing it. Before I get into the suspicion that I have, however, here's what has remained constant:
- 1999 Forester S, 4EAT, N/A. 151,000 miles on the body; 15,000 miles on fully-rebuilt SOHC EJ25 (heads and block). Compression and timing are good.
- Drop in economy is noticeable in daily commuting or on longer trips at constant speed. Total loss in fuel economy is approximately 20% across the board.
- Oil is changed every 3,000 miles; level is good and oil is clean. No leaks or burning.
- Tyres are brand-new (less than 800 miles on them). Economy drop was measured before changing tyres.
- Brand of fuel and station it is purchased at have varied, so it's not a case of one station having poor fuel storage leading to contamination and poor economy.
- Front oxygen sensor was replaced after the rebuilt engine was swapped in. Rear sensor has not been touched.
- Air filter is clean, spark plugs have normal brownish-grey residue, and the fuel filter was also replaced after the engine swap. I believe that the original injectors were reused with the current engine, though I'd have to check some fairly buried paperwork to confirm.
- Engine gets up to temperature normally, and there are no signs (whistles, surging idle, etc.) of possible vacuum leaks.
- There are no smells of fuel leaks, flooding, etc.
- A/C is not a factor as it hasn't been turned on since somewhere in the October timeframe.
There are two of us using the car on a regular basis, we're both seeing the same drop in terms of percentage, and driving styles haven't changed. However, for reasons that are completely inexplicable at this point, we've both seen the fuel economy go through the floor over the past eight to ten weeks. Frankly, the Jeep Cherokee that this Forester replaced managed 17-18mpg in mixed driving, and that was with a 4.0; in the nearly two years I've owned this vehicle, it's never been anywhere near this bad (with the exception of when the previous engine exploded).
One thing I will mention: the catalytic converter was not replaced after the engine replacement, and the old engine (which had failed rings and burned valves) was definitely dumping raw fuel and oil down the exhaust. I would not be surprised if the cat was burned out internally, but there are no rattles from it or signs of external damage. Ditto no exhaust leaks, either from the pipes or gaskets. I'm not entirely certain that I can blame the cat at this stage, though given that it's probably the original one it's likely time to replace it anyway.
That said, does anyone have any ideas on what to check? The vehicle is otherwise behaving perfectly normally apart from the huge drop in fuel economy, and I'm at a loss as to where to start probing since all the usual culprits seem to be in working order. Seeing a drop this sharp and this sudden with no obvious indicators of what may be causing it has me completely stumped.