2005 Subaru Impreza Slight Underperformance

I got a good one for you guys. My Impreza is taking just a couple hundred extra RPMs to go the same speed as it did before a massive problem I had recently. For example, it used to take 2k RPM to go 55-57. Now it goes about 47 at the same RPM. Yes, I’m carefully tuned to my car.

Back in July, after getting a problematic tank of gas at a gas station that sells ethanol-free fuel, it gradually lost a lot of power over the next 1-3 tanks of (ethanol) gas. I had to rev it up way higher than normal to get it up to speed and keep it going – we’re talking like 3k RPM to get it to go 65. When I would try to punch it, it would hardly go. At start up, at times it would almost shut right back off, but once it was idling it was fine. It never quit on me.

I talked to the owner of the gas station, basically accusing him of watering his gas down. But his government tests for water came back normal. He thinks the problem was all the years of using ethanol gas. He's had at least one other customer have this happen to them. He said there was likely 3 layers of fluid in the tank at all times – the regular gas at the bottom, ethanol on top of that, and water on top of that. I suspect the switch to his fuel flushed the ethanol and/or water into my fuel system.

The dealer gave the car a tune up (fuel system clean and new spark plugs), but that did nothing for the problem. A friend told me the problem was in the fuel filter. So I had them change that out (as well as the fuel pump because it’s the same assembly). After this fuel filter/pump repair, it was much better, but there was still a noticeable problem. Another dealer found the battery operating at only 97 amps, so that was replaced. After the battery repair, more power came back to the car, but there is still this small loss of power. I used about 4 or 5 bottles of dry gas, with no results. I replaced the air filter too, just in case. No change.

Any idea why the engine would take a couple hundred extra RPMs to go the same speed as it did before? It still takes a PTO off the pedal, no hesitations, sluggishness or anything.

I was beginning to suspect a bad fuel pressure regulator, until I came across the info at
http://www.buzzle.com/articles/symptoms-of-a-bad-fuel-pressure-regulator.html. The symptoms they list all look pretty severe. I also suspected bad MAP (manifold absolute pressure) sensor or bad TPS (throttle position) sensor. But I'm not a mechanic so those are just shots in the dark.

Great site btw