04 Mustang GT Gas Mileage


New Member
Aug 11, 2019
Hey guys, new mustang owner here. I’ve always wanted a New Edge mustang and I’ve finally got my hands on one. I picked up a 2004 Mustang GT Automatic with 138,000 miles. She needed a little elbow grease but I’ve got her where I want her cosmetically and now it’s time for some maintenance. Since purchased the car has had an off-road h pipe (no cats) down to SLP resonators. The rear O2 sensors were clipped and I think that’s why the car is throwing P0171 and P0174 (engine running lean in both banks). I replaced the front O2 sensors thinking it would help my gas mileage but honestly I haven’t seen a difference. I was reading other threads and that seemed to fix everyone’s problems but not mine. A couple things to throw in about this car, when going over 70mph (right over 70 only) the car seems to engines brake if you will? You hear a sound in the rear and it starts slowing down, I read somewhere it could be something to do with the rear end? Another thing to add in would be the car does seem a little slow in first, but I’m starting to think the car is maybe tuned but the last owner never did anything and he said it’s always been like that. When you step on it, the car MOVES. It pulls you when it shifts and literally feels like a race car. I’m thinking it’s tuned but don’t know how to truly find out. It also has an Edelbrook cold air intake on it. I fixed the tire pressure on all the tires as well. OH and the MPG is about 13-15. I get 16 with the AC turned off the whole tank. Even when I drive super super good on a tank it just seems poor. I don’t know. Looking for some advice? Thanks guys.

(Yes the car does smell of gas because of the off-road H-pipe (with no cats), not too worried about that tho)
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SN Certified Technician
Aug 14, 2009
Houston Texas
The rear O2 sensors were clipped and I think that’s why the car is throwing P0171 and P0174 (engine running lean in both banks)
A clipped rear O2 sensor wire will NOT cause a P0171/P0174. Why? First the PCM knows the difference between an invalid O2 sensor and ZERO O2 sensor signal. A P0171 DTC codes requires a working O2 sensor that has a means there's an operational "problem" inside the motor.

The odds favor there's a vacuum leak. But you should do the work to find out. Also IMO it should be expected to have driveability issues with outstanding DTC.

P0171 - System to Lean (Bank 1)
P0174 - System to Lean (Bank 2)

Air Measurement System
  • MAF sensor (contaminated, damaged, malfunctioning, etc.)

Fuel System
  • Fuel pressure regulator (leaking, malfunctioning, etc.).
  • Fuel filter plugged, dirty.
  • Fuel pump (weak, check valve leaking, etc.).
  • Leaking/contaminated fuel injectors.
  • Low fuel pressure or running out of fuel.
  • EVAP canister purge valve leaking (when canister is "clean").
  • Fuel supply line restricted.
  • Fuel rail pressure sensor (incorrect reading).

Air Induction System
  • Air leaks after the MAF.
  • Vacuum Leaks.
  • PCV system (leak, valve stuck open, etc.)
  • Improperly seated engine oil dipstick.

Exhaust System
  • Exhaust leaks before or near the HO2Ss (exhaust manifold gasket, mating gaskets, etc.).

EGR System​
  • EGR valve tube/gasket leak
  • EVR solenoid vacuum leak

Secondary Air Injection
  • Damaged/malfunctioning secondary air injection system (mechanically stuck valve).