New to me 1994 LS400 has a BIG problem HELP

[DonyLingus]

I had and sold a 1990 LS400. Undoubtably the best car i will ever own. The 1990s have 18 gallon tanks and i got about 355-370 miles per 18 gallon tank on in town driving. It had 280,000 miles. My 1994 with 175,000 miles has a 20 gallon tank and i get no better than 265 miles to the tank. the 1994 has noticably less power especially in the high end.

I ran a couple tanks of gas with Valvoline synthetic complete fuel system cleaner, cleaned the throttle body and top end but nothing seems to help. Someone please tell me that my problem is something other than horrible compression loss and engine damage.

by the way, a steady grinding noise comes out of the air intake. I had a similar noise from the air intake with the 90 but with the 94 noise is twice as loud.

any ideas welcome PLEASE HELP

 

[JBrady]

Could be a number of things. Get the codes first to see if any of the sensors or ranges are failing. If all that checks out I would check the cats to confirm they are not damaged and creating massive backpressure. One of more of the injectors may be faulty.