Extremely poor fuel economy and performance when engine is warm

[Fenceingmadman]

2007 ford mustang GT, changed the alternator and heater hoses for a friend, new gaskets on intake and coolant crossover + thermostat, as well as throttle body and injector seals. I also fixed a vacuum leak on the passenger side PCV. I also cleaned the throttle body.

I'm not sure if this was an issue before I did this work but now when the car is warm, (I'm assuming the change is caused by the car entering open loop) the car runs poorly, it will light the tires when cool for the first several miles of driving. but will then run poorly and barely accelerate. I also somehow managed to burn through 16 gallons of 87 octane in 124 miles which would mean an average MPG of 7.5 Even with how hard my friend and I was driving it that should be impossible especially considering it feels like its running lean and I'm not smelling any fuel. Any idea what would cause this? 2 bad O2 sensors that aren't throwing a code? something else?

 

[Dino Dino Bambino]

When the engine warms up, the ECU goes into closed loop mode whereupon it processes the voltage signals from the O2 sensors to deliver a precise ~14.7:1 air:fuel ratio when running pure gasoline.
Fuel consumption of 7.5mpg points to a rich condition. It seems too much of a coincidence for both O2 sensors to go out at the same time. If scanning the ECU doesn't yield any helpful codes, I'd recommend that you try to collect and record some live data (called datalogging) from various sensors. This will tell you what's wrong with the car so you can avoid the common mistake of randomly throwing parts at the problem in the hope that it goes away. Mechanics love to do that and charge you for the privilege!
To record live data, I use a Total Diagnostics ELM 327 Bluetooth dongle that I plug into the OBD port under the dash, and have the Car Scanner app on my smartphone paired up to it. The App includes a data recording feature that I've found extremely useful.

 

[WJBertrand]

If all the data logging seems normal consider checking for any driveline issues, i.e. dragging brakes, tire inflation, etc.


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[86GT351]

Did you change the temperature rating of the thermostat?

 

[Dino Dino Bambino]

Did you change the temperature rating of the thermostat?

Even a 160* thermostat would only have minimal if any effect on gas mileage because the upstream O2 sensors (assuming stock location) should already be warmed up about a minute after a cold start and send the ECU into closed loop. By that time you'd be lucky if the coolant temp. had reached 120*F.
I'm leaning towards the problem being something very simple like a vacuum leak either from an improperly seated intake manifold runner gasket or TB flange gasket.
Time to blow some smoke!

 

[Derf08]

Are you smelling any fuel? How's the performance of the car otherwise? Down on power or seems to be fine?

Having replaced the intake manifold gaskets, I would look at the Fuel Trim data to see if it's now burning really lean (leak at the intake) and the Fuel Trims are all maxed out on '+' trying to make up for it. That would cause poor mileage and wouldn't necessarily throw a code if it's a small enough leak and the PCM can compensate for it.