North American Turbocoupe Organization



Terrible fuel mileage
MN88TurboCoupe Offline
Member
#1
So I've been driving my T-Bird as a daily driver.  Got new tires, new computer and it's been running great but I get terrible fuel mileage (20 mpg).  When I first got the bird I was getting around 30 if I drove like a normal sane person and about 24 if I was into the throttle.  Totally different now and I suspect the wastegate.  
I suspect this for 2 reason.
The terrible fuel mileage and no turbo lag when I get into the throttle.

Is there a method for checking this?
Publicity, fame and accolades can make a theory popular. They can't make it true.
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anasazi4st Offline
Senior Member
#2
(04-30-2019, 01:44 AM)OKBMN88TurboCoupe Wrote: So I've been driving my T-Bird as a daily driver.  Got new tires, new computer and it's been running great but I get terrible fuel mileage (20 mpg).  When I first got the bird I was getting around 30 if I drove like a normal sane person and about 24 if I was into the throttle.  Totally different now and I suspect the wastegate.  
I suspect this for 2 reason.
The terrible fuel mileage and no turbo lag when I get into the throttle.

Is there a method for checking this?
Congratulations on that kind of MPG!

Now, the bad news: I average 18-24 mpg, depending on how much highway driving I do...and, I use ONLY 91 octane fuel.

I consider my car to be in a great running condition; I have solid boost up to 16-17 lbs, no problems with idle or hesitation or anything like that—and, except for the Gillis valve it’s presently all stock. Still, that’s what I get.

Our cars weren’t built for great MPG, but for performance.

It’s always been like that. I’m not sure if my experience is typical, others can confirm or refute it.

(04-30-2019, 01:44 AM)OKBMN88TurboCoupe Wrote: So I've been driving my T-Bird as a daily driver.  Got new tires, new computer and it's been running great but I get terrible fuel mileage (20 mpg).  When I first got the bird I was getting around 30 if I drove like a normal sane person and about 24 if I was into the throttle.  Totally different now and I suspect the wastegate.  
I suspect this for 2 reason.
The terrible fuel mileage and no turbo lag when I get into the throttle.

Is there a method for checking this?
Congratulations on that kind of MPG!

Now, the bad news: I average 18-24 mpg, depending on how much highway driving I do...and, I use ONLY 91 octane fuel.g

I consider my car to be in a great running condition; I have solid boost up to 16-17 lbs, no problems with idle or hesitation or anything like that—and, except for the Gillis valve, at present it’s all stock.

Our cars weren’t built for great MPG, but for performance.

It’s always been like that. I’m not sure if my experience is typical, others can confirm or refute it.
Another proud dues-paying member.

1987 Turbo Coupe w/T5OD, 8.8 axle, grey smoke; most options. Got it in 1991 with 41K miles: 3 turbos, 2 heater cores, 1 T5OD full rebuild, 5 clutches, 1 head gasket, 2 Teves II ABS units, etc. later....
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firebirdparts Offline
Member
#3
Tell us about the "new computer". Does your boost gauge work at all? The wastegate should be closed when you are driving like a sane person, and boost is limited by where the engine is operating, not by the wastegate. If it was leaking through then you do lose the potential for some efficiency there. I suppose you would benefit from being able to compare to another car. however, I don't get 30 mpg either.
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Jeff K Offline
Administrator
#4
Even on flat rural interstates with no headwind cruising at 65 MPH I will maybe hit 27 MPG with the cruise control on. Hilly terrain, a decent headwind, etc, cruising at 70 to 75 MPH with AC on, MPG will drop to the 22 to 25 MPG range. E10 gas will drop MPG a little bit also, as will cold weather, etc. I did hit 30 MPG once a number of years ago on a 200 mile one way trip with a 25 MPH tail wind on a 99% flat interstate with little to no traffic. Heading back home into the 25 MPH wind netted around 21 MPG.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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MN88TurboCoupe Offline
Member
#5
(04-30-2019, 01:42 PM)firebirdparts Wrote: Tell us about the "new computer".  Does your boost gauge work at all?  The wastegate should be closed when you are driving like a sane person, and boost is limited by where the engine is operating, not by the wastegate.  If it was leaking through then you do lose the potential for some efficiency there.  I suppose you would benefit from being able to compare to another car.  however, I don't get 30 mpg either.

The car idles great, starts up immediately.  It's been a great runner and no my boost gauge doesn't work.  I can go and blow into the line and the gauge will respond inside the car so I need to trace the vacuum lines and will probably find one with a hole in it or hooked up in the wrong place.  It's been in the 40's for the past few days so I haven't had a chance to look.

I do know I need to have the T-5 rebuilt.  I've got something there going on with a vibration so I think one of the shafts has an issue.  That's beside the point.  I know with the new computer and just thought about it with all the work I did is the timing may have changed from what it was.

Oh well.  Back to the drawing board.  I may run some fuel cleaner to see if cleaning up the fuel injectors might help.
Publicity, fame and accolades can make a theory popular. They can't make it true.
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Turbo8715 Offline
Junior Member
#6
(04-30-2019, 01:44 AM)MN88TurboCoupe Wrote: So I've been driving my T-Bird as a daily driver.  Got new tires, new computer and it's been running great but I get terrible fuel mileage (20 mpg).  When I first got the bird I was getting around 30 if I drove like a normal sane person and about 24 if I was into the throttle.  Totally different now and I suspect the wastegate.  
I suspect this for 2 reason.
The terrible fuel mileage and no turbo lag when I get into the throttle.

Is there a method for checking this?

I suggest that the fuel regulator on the fuel rail be changed if it is the original and the exhaust smells rich. That could be the issue with your poor fuel mileage.
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anasazi4st Offline
Senior Member
#7
(05-05-2019, 05:17 AM)The jTurbo8715 Wrote:
(04-30-2019, 01:44 AM)MN88TurboCoupe Wrote: So I've been driving my T-Bird as a daily driver.  Got new tires, new computer and it's been running great but I get terrible fuel mileage (20 mpg).  When I first got the bird I was getting around 30 if I drove like a normal sane person and about 24 if I was into the throttle.  Totally different now and I suspect the wastegate.  
I suspect this for 2 reason.
The terrible fuel mileage and no turbo lag when I get into the throttle.

Is there a method for checking this?

I suggest that the fuel regulator on the fuel rail be changed if it is the original and the exhaust smells rich. That could be the issue with your poor fuel mileage.
The problem is—his MPG as recently reported is around the same average that several of us have already said is normal, about 20 mpg on average in non-highway driving.

Which raises the bigger question: why was it so much before (he reported around 30 mpg)?
Another proud dues-paying member.

1987 Turbo Coupe w/T5OD, 8.8 axle, grey smoke; most options. Got it in 1991 with 41K miles: 3 turbos, 2 heater cores, 1 T5OD full rebuild, 5 clutches, 1 head gasket, 2 Teves II ABS units, etc. later....
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Pete D Offline
Administrator
#8
What was wrong with the old computer, they rairely fail.? Most of the other stuff being recommended to change can be tested first
Pete Dunham


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MN88TurboCoupe Offline
Member
#9
(05-05-2019, 10:22 AM)anasazi4st Wrote:
(05-05-2019, 05:17 AM)The jTurbo8715 Wrote:
(04-30-2019, 01:44 AM)MN88TurboCoupe Wrote: So I've been driving my T-Bird as a daily driver.  Got new tires, new computer and it's been running great but I get terrible fuel mileage (20 mpg).  When I first got the bird I was getting around 30 if I drove like a normal sane person and about 24 if I was into the throttle.  Totally different now and I suspect the wastegate.  
I suspect this for 2 reason.
The terrible fuel mileage and no turbo lag when I get into the throttle.

Is there a method for checking this?

I suggest that the fuel regulator on the fuel rail be changed if it is the original and the exhaust smells rich. That could be the issue with your poor fuel mileage.
The problem is—his MPG as recently reported is around the same average that several of us have already said is normal, about 20 mpg on average in non-highway driving.

Which raises the bigger question: why was it so much before (he reported around 30 mpg)?

(05-05-2019, 04:18 PM)Pete D Wrote: What was wrong with the old computer, they rarely fail.? Most of the other stuff being recommended to change can be tested first

The car wouldn't start.  Came down to computer was the culprit.  Why it failed I am not sure.

When I first got the car I didn't change it much and drove it for that summer while I worked on things (interior, audio, etc).
Publicity, fame and accolades can make a theory popular. They can't make it true.
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