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Bad fuel efficiency/emissions

Tiny
LBEAN
  • MEMBER
  • 1991 SUBARU LEGACY
  • 4 CYL
  • AWD
  • MANUAL
  • 200,000 MILES
I have a 1991 Subaru legacy that is getting around 14/15 mpg and won't pass an emissions test. The idle is ok after a cold start until it warms up a bit, then it starts surging more and more until it drops too low and dies. That also happens while moving. I was told by the previous owner that it has a bad ECU and it would cost 1200 for a mechanic to fix it all up, but ecus seem pretty easy to replace which makes me think there's something else involved. This issue does not seem related to temperature. Can you recommend some other tests or smaller repairs to do that would help me narrow down the problem?
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Monday, December 27th, 2010 AT 8:39 PM

6 Replies

Tiny
ERNEST CLARK
  • MECHANIC
  • 1,730 POSTS
If your car fails emissions, the testing facility should give you a reason why. This will be your first indicator pointing you in the right direction.

Also, when was your last tune up? This in itself could fix your problems. If you've already done a tune up, then a more thorough diagnosis is in order. Have your tech check the EGR valve, Cat efficiency, O2 sensors.

Also, keep in mind that your engine has 200K miles on it. No engine last forever, and when they begin to wear significantly, they produce more emissions, and will eventually need replacing/overhaul/rebuild.
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Monday, December 27th, 2010 AT 9:39 PM
Tiny
LBEAN
  • MEMBER
  • 9 POSTS
The engine itself seems to be in good shape. I'm a diy guy and can't afford mechanics right now. I got the car for $50, so I'm trying to see if there is any tinkering I can do to get it going better. My area doesn't require emissions that's why I got the car, I was just told that it wouldn't pass last time they tried it. I'll get going on a tune up, but the person I got te car from did say something about the ecu.
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Monday, December 27th, 2010 AT 10:18 PM
Tiny
ERNEST CLARK
  • MECHANIC
  • 1,730 POSTS
If the ecu as failing, you'd have drivability problems. If you haven't yet performed a tune up, start at the plugs and read them. You can search the net for pictures of all the various conditions of plugs and what they relate to. Also, I'd suggest changing the 02 sensor as well, since you don't know how old this is.

To elaborate further on your ecu, when these begin to fail, the will usually fail to send a signal to the coils and or fuel injectors telling them when and how to fire. Of course the computer can fail in other ways, but this would be the most common and obvious sign. Don't replace the computer unless you're positive it's gone bad.
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Monday, December 27th, 2010 AT 10:28 PM
Tiny
MHPAUTOS
  • MECHANIC
  • 31,937 POSTS
I will add my Bob's worth here, have you done a scan for any unresolved fault codes, check the engine temp sensor as well, check all the usual tune items including comps, also it will be a good idea to have the injectors check for spray patten and flow rates.

Mark (mhpautos)
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Monday, December 27th, 2010 AT 11:19 PM
Tiny
LBEAN
  • MEMBER
  • 9 POSTS
Thanks, that gives me something to start with. Another thing is, the check engine light is not on, does that change anything?
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Tuesday, December 28th, 2010 AT 12:53 AM
Tiny
MHPAUTOS
  • MECHANIC
  • 31,937 POSTS
Not all codes will set a MIL, so do a scan anyway.

Mark (mhpautos)
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Tuesday, December 28th, 2010 AT 2:17 AM

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