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MPG drop after MSD coil upgrade


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I replaced the factory Diamond coil on my 96 legacy 2.2 with a MSD 8239 in hopes of gaining something. After all, I've read reports of "better throttle response and more MPGs". Well guess what! Mileage went from 260 per tank to 200 after the supposed "upgrade". This was done with a new NGK wire set at the same time. The plugs are the same set of NGK iridium's as before. (they were only 2 months old) The PCV, air filter, upstream O2 sensor have been replaced since the coil change to no avail. The car was running fine before, (no codes) and still does now. I was not expecting this MPG drop after the hype I read about the MSD coil. I did replace the tires and coil at the same time. Went from 185/70/14 with 4/32 tread to new 205/55/16. They aren't cheap tires either, Pilot Sport A/S+. That would not cause a 60 miles per tank drop though---no way. Are the plugs now improper? I'm at a total loss and am justifiably dissapointed.
RIP 96 Legacy 2.2 4EAT lost reverse @ 374,000 miles
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1) You bought new tires. Fuel economy always sucks with new tires.


2) You bought wider, stickier tires. That also hurts economy.


3) You bought tires significantly taller than your old tires. So not only are you at a mechanical disadvantage (it requires more power to turn your tires), but your odometer and speedometer are also reading off (which artificially deflates your fuel economy. You THINK you've gone 60 miles, but you've really gone 61.7 miles).


And all of that DOES add up.

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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  • 2 weeks later...

Those things add up, but 60 miles on a tank? Not sure about that.


When you have anything that increase the efficiency of the burn to any

significant level, the o2 sensor, will

sense a lean burn, will send an increased voltage signal and tell your computer

to pump more fuel sabotaging your mpg gains.


This is just an indisputable fact.


Is this what is happening in your case? I don't know but this IS what the

system will do with a burn that is too clean (not lean).


It is true that a lean burn will hurt your engine, but is this wholesale sabotage

of anything that makes the burn cleaner justified? No, so you can get around

it if this is what is happening in your case. The EFIE circuits will receive the o2

voltage and when it sees a signal that increases, it drops it (it is in series) and

sends a lower signal so your computer will not richen up the mixture sabotaging

your benefits.


The best resource on this EFIE circuit is George Wiseman and most EFIE's available

are simply knock offs and many people say they get the best results with his.


You can buy a how to book or buy one ready build. I don't make a penny for

recommending this. I built one before for a 91 Honda Civic and wound up getting

43mpg in the city from 28-30 in the city when I first bought it.


Anyway, NEVER run an EFIE without something that actually does increase

the efficiency of the fuel or it will lean it out and you don't even have anything

that gets more power out of a leaner burn.


I have a 1998 Legacy GT 2.5 liter wagon, what MSD or other CDI units fit this car?

I've had a hard time finding clear answers.

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