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 Post subject: Want more MPG's
PostPosted: Sun Jan 25, 2015 10:21 am 
Third Gear
Joined: Wed Oct 26, 2011 7:02 am
Posts: 881
Location: Anchorage,Alaska
My 90 5speed wagon has always had marginal gas mileage. I've been trying to get the a/f numbers above 13.0 but not being very successful. I've changed the coolant sensor and reset the ecu and replaced the o2 sensor. None of these has changed anything. I average 20mpg and I know it could be better because my wife's is getting 26mpg's.

This came to my attention going over my tax records when my gas was 7500.00 this year just for my car. So I need to sort this out before I drop in a 2.5.

My question is could it be the 90 ecu? I've read 92-94 ecu's have better fuel mapping, thinking thats why my wifes get better mpg's. Also I think I need to change anyway before the 2.5 swap.

What's your guys thoughts.

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Charles

90' White/Gray Outback DD workhorse (670,xxx) miles
92' Onyx Metallic (430,xxx) Wife's DD
68' Barracuda formula S 340 4spd coupe (ongoing project)
66' Mustang 2+2 4spd wife's (ongoing project)


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 Post subject: Re: Want more MPG's
PostPosted: Sun Jan 25, 2015 11:53 pm 
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Yeah, the 90-91 MT cars had the Hitachi ECU which wasn't as fast as the JECS ecu's found in the 90-91 AT & 92-94 MT/AT non-turbo Legacies. Also, the 90-91 MT had a different IAC valve, reciprocating vs. rotary. I don't think that necessarily is causing any worse mileage, but just one of those things that is different.

If you're planning to swap in a 2.5 what ECU are you planning on running?

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Want more MPG's
PostPosted: Mon Jan 26, 2015 5:23 am 
Third Gear
Joined: Wed Oct 26, 2011 7:02 am
Posts: 881
Location: Anchorage,Alaska
I figured since the 2.5 runs the red injectors I would run 92-94 ecu. I have the IAC and the MAF in my pile of parts for the swap. I'm not sure how my ecu would react to the red tops.

Things I've noticed, if I let it warm up for 30 min idling its like a switch is flipped and lean out to 14-15 and when I drive it it will stay in the high 14's. I thought heat to the engine was the key so I completely blocked off the radiator to see if it would keep my a/f high. After some driving it seems it didn't change anything. One other thing I observed when my a/f ratios are normal (high 14's) if I were to increase my speed to 75mph it will lean out again and hit 17. I'm not sure if I'm maxing out the injectors and it leans it out.

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Charles

90' White/Gray Outback DD workhorse (670,xxx) miles
92' Onyx Metallic (430,xxx) Wife's DD
68' Barracuda formula S 340 4spd coupe (ongoing project)
66' Mustang 2+2 4spd wife's (ongoing project)


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 Post subject: Re: Want more MPG's
PostPosted: Mon Jan 26, 2015 6:10 am 
Fourth Gear
Joined: Thu Aug 11, 2005 4:34 pm
Posts: 1980
Location: Lake Elsinore, CA
Check the constant hot wires to the ECU. Maybe it's having to relearn the fuel curve every startup. The idea that it gets better after an extended warmup leads me to think along these lines.

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 Post subject: Re: Want more MPG's
PostPosted: Mon Jan 26, 2015 8:15 am 
Third Gear
Joined: Wed Oct 26, 2011 7:02 am
Posts: 881
Location: Anchorage,Alaska
I will check the back up power wire, I believe that would be the one that keeps the memory. But if I was having a power issue would it still store codes? I have a vss code that pop up only in the summer but never in the winter.

Also it will never lean out if I drive it before it warms up. Outside temp doesn't seam to effect it either. Last week was 30's-40's and today was around 0 and I was running high 14's. When I don't let it warm up it stays at a steady 13.0 even climbing hills.

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Charles

90' White/Gray Outback DD workhorse (670,xxx) miles
92' Onyx Metallic (430,xxx) Wife's DD
68' Barracuda formula S 340 4spd coupe (ongoing project)
66' Mustang 2+2 4spd wife's (ongoing project)


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 Post subject: Re: Want more MPG's
PostPosted: Mon Jan 26, 2015 6:56 pm 
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Charles,

I've got a couple comments for you.

I ran the red top 92-94 injectors with my 90 ECU for a little while and found that the 90 ECU ran a little rich and had slight decrease in "peppiness" with the red top injectors. When I put the 92 ECU in the mileage increased slightly and the throttle response was a bit more crisp.

I believe the 90-91 injectors flow the same as the 92-94. I've heard mixed feedback that they flow a little less, but don't have any flow data to confirm or deny that. If they do flow the same then the difference is likely in the injector design and latency. If the injector latency is different then the ECU's fueling will be off slightly....which could explain what I experienced.

When you do the ECU swap, I found that the MT/AT identifier pin was wired backwards to the 90-91 cars and was not wired how the ECU I/O depicted. I don't know if that was only on the 92 ECU I had or if all the 92-94 ECU's are like that. Just something to keep in mind during the swap. It'll either mean you need to snip the wire going to that pin or add a wire, depending on what the car originally had setup.


On your current AFR's, are you using a wide band O2 sensor to get those values? Are you seeing the 17's in a certain type of situation? The ECU has an overrun fuel cut table which will shut off fueling when the throttle is closed and the engine is decelerating above a certain RPM. So this is typically why you see very lean AFR's under deceleration.

Assuming you're using a wide band O2 sensor to get those AFR's it really looks like you're seeing variability. The things that can primarily cause variability in the fueling is input from the MAF sensor, coolant temp sensor, O2 sensor, and ambient pressure sensor.

Honestly, I bet when you change the ECU & MAF sensor your problem will probably go away. You are replacing two of the above sensors (MAF & ambient pressure sensor) with the ECU swap.

The other thing to look into I guess is whether the wiring is all good and you don't have an intermittent wiring issue which is causing the inputs from various sensors to change under different circumstances.

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Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

If you need to get a hold of me please email me rather then pm


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 Post subject: Re: Want more MPG's
PostPosted: Thu Feb 26, 2015 6:15 am 
First Gear
Joined: Sun Jan 22, 2012 8:16 am
Posts: 119
Location: Yakima WA
Is the ecu different between years on the turbo models? The donor car I picked up for my 5 speed swap got way better mileage than my 91 the donor was a 93 and my 91 was a 10/90 so it was one of the first ones made. My car also runs better than the donor and is so much faster.

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 Post subject: Re: Want more MPG's
PostPosted: Thu Feb 26, 2015 9:20 am 
Third Gear
Joined: Wed Oct 26, 2011 7:02 am
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Location: Anchorage,Alaska
From what I read I know the n/a ECU from a 92-94 have slightly different fuel mapping, I'm sure it's because of the newer injectors. But not sure about the turbo ECU's.

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Charles

90' White/Gray Outback DD workhorse (670,xxx) miles
92' Onyx Metallic (430,xxx) Wife's DD
68' Barracuda formula S 340 4spd coupe (ongoing project)
66' Mustang 2+2 4spd wife's (ongoing project)


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 Post subject: Re: Want more MPG's
PostPosted: Thu Feb 26, 2015 6:35 pm 
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Location: Houston, Tx
The turbo ECU's are all interchangeable. There are different part #'s between the earlier years I believe, but all are interchangeable.

The non-turbo cars are a little more complicated because you have Hitachi ECU's for the 90-91 MT cars and JECS ECU's for the 90-91 AT cars. The MAF sensors, injectors, and a few other things are different. The 92-94 non-turbo cars all had JECS ECU's.

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Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

If you need to get a hold of me please email me rather then pm


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 Post subject: Re: Want more MPG's
PostPosted: Fri Feb 27, 2015 12:14 am 
First Gear
Joined: Wed Dec 14, 2011 11:43 pm
Posts: 125
Location: Bakersfield CA
Interesting so is it possible to plug in a Hitachi ECU into a JECS car? Its not something i necessarily want to do but i might have done something along those lines when i did a 5speed swap i just changed the outer case and i feel as if the car does behave a bit different i dont entirely recall but i think i might have gotten the ecu from a preface lift model

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 Post subject: Re: Want more MPG's
PostPosted: Fri Feb 27, 2015 4:16 am 
First Gear
Joined: Sun Jan 22, 2012 8:16 am
Posts: 119
Location: Yakima WA
Would having a 3 inch downpipe made a big difference in mpgs because that's what my donor had was a full 3 inch no cats or resonators I plan on putting on my car when I find a quieter muffler because I don't want to be pulled over all the time

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 Post subject: Re: Want more MPG's
PostPosted: Mon Mar 09, 2015 6:11 pm 
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Chrisg03 wrote:
Interesting so is it possible to plug in a Hitachi ECU into a JECS car? Its not something i necessarily want to do but i might have done something along those lines when i did a 5speed swap i just changed the outer case and i feel as if the car does behave a bit different i dont entirely recall but i think i might have gotten the ecu from a preface lift model


If you swapped the appropriate sensors and injectors it would probably work.

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Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

If you need to get a hold of me please email me rather then pm


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 Post subject: Re: Want more MPG's
PostPosted: Mon Mar 09, 2015 6:12 pm 
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zeke wrote:
Would having a 3 inch downpipe made a big difference in mpgs because that's what my donor had was a full 3 inch no cats or resonators I plan on putting on my car when I find a quieter muffler because I don't want to be pulled over all the time


It might help the engine breathe a little better, but I don't think it would change anything significantly.

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Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

If you need to get a hold of me please email me rather then pm


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 Post subject: Re: Want more MPG's
PostPosted: Mon Mar 09, 2015 10:31 pm 
Fourth Gear
Joined: Sun Jun 10, 2007 4:35 am
Posts: 1732
Location: All over the place
I saw no noticeable mileage difference between stock, open downpipe, and full 3" turbo-back on my Touring Wagon. Although I noticed I was driving more aggressively more frequently with the open downpipe, which dropped mileage on certain fillups :)

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