2004 Mercury Grand Marquis 109K miles.

Last transmission service at 100K by Ford as well all new spark plugs, all fluids, all filters, belts, hoses. Belt tensioner, idler pulley and belt replaced two weeks ago due to belt tensioner bouncing about an inch. Full rear axle rebuild four months ago due to failed pinion bearing and cover plate leak; all bearings, all seals, gears and two new axles.

The engine feels like something is heavily dragging it down. It's using an 1/8 of a tank of gas more over a known distance. You take your foot off the gas to exit a freeway and it feels like engine braking. The drag is a consistent amount across all speed ranges. Some times, not always, from stop you need to give the engine 50% throttle before the rpms climb above idle then suddenly rpms jump and you surge forward (this car has a mechanical throttle cable, not drive by wire). Slight surge/buck/drag occurring at all speeds even at 70mph with cruise set. 

We ran ForScan on it at cold idle, hot idle, freeway cruise and variable speed city street driving.

- Zero OBDII diagnostic trouble codes.

- Zero misfires detected. Zero misfire history.

- Fuel rail pressure at or above 38 PSI which is normal. Fuel tank ambient pressure at 0 psi. No fuel pump faults reported. Fuel filter is 5K miles old. 

- Battery at 100%. Alternator dead on perfect for output voltage at idle and under load.

- Transmission fluid temperature stable at 189 degrees in Phoenix afternoon heat. 

- Transmission fluid temperature perfectly clean with no abnormal smells with 9K miles on it. 

- No differential fluid leaks, Plug removed and diff fluid is at normal level. 

- No dragging brake pads.

- Engine turning a normal and stable 1750 rpms at 70mph. Engine load driving is 25 to 35% with the higher load climbing small hills on the freeway. 

- Drag occurs with a/c on or off. Disconnected the belt and a/c, water pump and alternator pulley spin freely by hand smoothly with no drag or noise. Power steering pump is a little stiff by hand but smooth with no bearing noise and the fluid appears brand new still after 9K miles on it. No fluid losses.

- No apparent coolant leaks anywhere, coolant looks normal with no odd smells. No coolant loss. Engine temperature normal. 

- Ran the full engine and transmission diagnostics in ForScan and everything was normal with zero faults/failures detected. Beyond what I mentioned above, I also included the torque convertor clutch solenoids, torque convertor slip, transmission range switches 1 through 4, turbine speed sensor, fuel injectors, mass air flow (about 7 g/s at idle), shift solenoids 1 and 2, torque converter clutch solenoid, engine load, fuel rail pressure transducer status, fuel tank pressure transducer status, fuel pump monitors, fuel system closed loop, idle air control, throttle position sensor. And everything showed no fault and normal reading.

I was hoping to see at least one fault code or one odd reading to give me a starting direction, but there isn't any.

Any ideas on what is causing this massive drag and what direction to take other than dropping it off at Ford? Thanks.