My 1991 lexus Ls 400 seems to be forming a vacuum in the gas tank. I replaced the gas cap but still having a massive amount of air escaping when i go to fuel up. Having trouble with power especially on the freeway it wont maintain a speed above 60mph. Transmission shifts fine. However you can smell raw fuel coming from the engine.

SOURCE: Lexus won't stay running

Having same issue here. Try to change mass air flow sensor (bad news for you, it costs about $400)

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With that Burning Smell it is the Transmission Burning up the Clutches on the Inside. Gonna have to get it Rebuilt.

SOURCE: 03 honda Odyssey gas smell and check engine light

I've got a '99 Oddy with the same symptom of gas odor inside and outside of the vehicle. Gas cap was secure and the level of fuel made no difference, so by process of elimination, I checked and followed the fuel lines and made sure there were no cracks that I physically could put an eye and hand on and eventually ended up at the fuel injectors and fuel rail. From there, all you need to do is remove the plastic cover plate from the intake plenum and the injectors will be exposed as well as the fuel rail. Turn the car on and wrap some paper towel over your hand like a mitten and run it over the suspected areas. You'll either find your leak there or the fuel filter located under the rear sliding doors.

SOURCE: escape will not go above 60mph and will not downshift w/gas pedal

It could very well be a restriction in the exhaust like a stopped up cat converter. I doubt if it is a torque converter,that may make it slow to get to speed but shouldn't effect hwy speeds.
You can check this vehicle for an exhaust restriction very easy by going to the DPFE sensor and measuring exhaust back pressure with a vacuum/pressure gage. It will be either silver or black in color will have 2 rubber hoses hooked to it going down to metal tubes that hook into the exhaust.It will have a 3 wire connector with the color of the wires being BN/WH,BN/LG,OG. Disconnect one of the rubber hoses from the sensor and hook the vacuum/pressure gage into the hose.You should have less than 1 psi at idle,then press accel to wide open throttle and take a reading,it should be no higher than 3-4 psi. Anything more is an exhaust restriction.

SOURCE: I have an 1989 Dodge Dakota with the V-6 TBI

have you tryed to pull a code besure the plug wires are in the right place i mean fireing order