2005 Subaru Impreza Slight Underperformance

Tiny
TONOKLEIN
  • MEMBER
  • 2005 SUBARU IMPREZA
  • 4 CYL
  • AWD
  • AUTOMATIC
  • 69,500 MILES
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
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Thursday, January 21st, 2010 AT 8:23 PM

5 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
I have two converns. At a specific speed, the RPM's shouldn't have changed regardless of how the engine runs. The gear ratio hasn't changed. Is there any chance the trans is slipping?

Next concern is if there was damage done to the catylatic converters or O2 sensors. Has the check engine light ever come on?

As far as the MAP / TPS, I really think there would be other trouble you would experience such as engine dieing, rough idle.

Let me know about the items I mentioned.
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Friday, January 22nd, 2010 AT 10:12 PM
Tiny
TONOKLEIN
  • MEMBER
No change in the transmission. It doesn't slip.

The check engine light never came on.
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Tuesday, January 26th, 2010 AT 9:14 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
I read both articles. Basically you have replaced everything they mentioned. I'm sure you have gone through several tanks of fuel by now. Has there been any change as a result of that? I still question if there has been damage to the O2 sensors or the converter, but you mentioned it is running much better.
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Thursday, January 28th, 2010 AT 10:56 PM
Tiny
TONOKLEIN
  • MEMBER
Yes, it runs better since the battery and fuel filter replacements. But like I said the engine is working a bit harder to move the car. I've burned through dozens of tanks of gas with no improvement. Do you think that flood of ethanol in my fuel system could have done some structural damage which is making the engine work harder?
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Saturday, January 30th, 2010 AT 2:57 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
I can't think of anything other than the O2 sensors. If they are not working properly, it will affect the performance and fuel mileage.
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Saturday, January 30th, 2010 AT 11:41 PM

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