Terrible Gas Mileage. Why?

[5LugFoxFanatic]

After taking my 86 out of storage for the first time this year, I have noticed a decrease in mileage. I get about 120-140 miles per tank of gas. I used to get about 200-220 if I was being nice and easy. I still drive it easy, and I can almost watch the gauge go down as I drive. I do not have any leaks at all. After I did all of the swapping, I still got a decent amount, about 200. I changed the plugs, plug wires, cap, rotor and coil. I do have the fuel pressure jacked up a bit, I cannot remember the exact specs, but it is not too radical to max out the injectors. I ran an OBD II on my car and no trouble codes came up. I thought maybe the O2 sensors might be shot but according to this, they are fine. What else would cause my mileage to go way down?

Engine Modifications:

1986 Ford Mustang GT

-Ford E7 Cylinder Heads, Ported and polished with a 5 angle valve job, and Performance Vassco Dual Springs, Moley Keepers and Retainers
-Ported Lower Intake Manifold off of a 1986 Mustang 5.0
-Upper Intake off of a 1993 Ford Mustang 5.0
-Mass Air Conversion
-1/5/8 Jet-Hot Coated Pacesetter Headers
-Kirban Adjustable Fuel Pressure Regulator
-Pacesetter Off Road H-Pipe
-Flowmaster 3 Chamber Exhaust
-65mm Ford Racing Performance Throttle Body and EGR Spacer
-3.55 Rear End Gears
-19 Pound Fuel Injectors
-Ford Motorport 9mm Spark Plug Wires
-MSD Blaster Coil
-Modine 3 Row Radiator
-180 Degree Thermostat
-Pro 5.0 Power Tower Shifter
-140 Amp Powermaster Alternator
-K&N Conical Air Filter
-ARP Head Bolts

Suspension Modifications:

-Tru-Speed Strut Tower Brace
-Tru-Speed Shock Tower Brace
-KYB Gas-a-Just Shocks, Struts and Quad Shocks
-Subframe Connectors
-Steeda K-Member Brace
-Eibach Lowering Springs
-5 Lug Conversion Which Includes 11” Front Rotors with Dual Piston PBR Calipers and Rear Disc Brakes
-1999 Ford Mustang Cobra 17” Wheels with Yokohama AVS/ES100 245/45/17 Rubber
- Maximum Motorsports Caster/Camber Plates

 

[old_blue]

make sure you got a good air filter on there. I replaced mine and noticed I had gas longer.

 

[5LugFoxFanatic]

Yeah I just oiled my K&N last year. Maybe I will do it again since it sat all winter. But I doubt that would cause that much of a decrease. At least I hope not.

 

[Michael Yount]

Usually a noticeable drop in mileage like yours shows up with occasional check engine lights, and error codes for the O2 sensors. If they haven't been replaced recently, that's what I'd look at. But I'm surprised no error code is showing up.

 

[BlackGT89]

Pull your vacuum line off of your fuel pressure regulator and make sure it's dry and you're not sucking gas through the line. Check your tire pressures, etc.

 

[Michael Yount]

Good coaching BlackGT - I was gonna ask about the condition of the regulator. What's throwing me off is that if it were doing that, seems like he'd be getting rich HEGO error codes.

 

[Venom351R]

how long was it stored for?

 

[BlackGT89]

You're right Michael, does seem like a leaky FPR would throw a code. And his .sig says he has a Kirban and thay are supposed to be the best, so that's probably not it. Just seems to be a rash of FP regulator failures lately with high fuel consumption reported as a symptom so it might be worth a shot.

 

[mustangsquared]

I had a leaking AFPR (BBK) and it did NOT throw a code... I would check the O2 sensors and give it a tune up plus change the Fuel filter...she sat for the winter so who knows what has settled in the fuel tank..ie water from condensation in the tank. Do/check the basics. Bad timing/plugs/cap/rotor/wires will effect the MPG. Might want to check the TPS..set to high the car will eat extra fuel thinking the throttle is open more then it is.

 

[Michael Yount]

My bike just saw it's first duty this season after sitting up for about 5 months over the winter. It got the same 43-45 mpg on the "old" tank that it normally does. If the gas has been sitting long enough to go 'bad', drivability problems usually accompany the poor gas mileage.