I have a 99 navigator that has a total loss of power. It starts and stays running but top speed is about 10 mph. What could it be

carb needs overhaul or high speed jets need ajustn--hav a pro du this cause if u lean it out too much-u can fry the engine--clymer makes a mechanics manual for motorcycles and atv

Your description sounds more like a torque converter clutch problem. It would explain the poor mileage, having to floor it to go. I suggest having the transmission scanned, looking at the slip speed of the torque converter clutch.

I have seen this problem before and the cause could be TCC Staying in Lock-up or an Electrical Issue within the Engine itself or a Fuel Delivery Issue.

If the Transmission TCC is the issue, try driving at slower speeds (below 30 and come to a stop, does it Shut off then? Increase Speed by 10 and try again. Does it react differently once you go above 42-45 MPH?

The only things that hold you back are:

A totally clogged gas filter
A blocked exhaust converter
A slipping transmission

Rare to fall back to a slower speed

this car is dissolve in ramp

I would check the fuel filter, it should be changed every 20,000 miles or so. Also I would check the fuel pressure at the test port. I would also check for a weak ignition coil.

Check for codes. Sounds like a bad coil

check /replace the fuel filter and check the fuel pump pressure before and after the bleed valve to the throttle injectors

your in limp mode have the vehicle scanned for trouble codes most auto stores offer this service for free --

You might want to check the MAF sensor(Mass Air Flow) and clean it with MAF/MAP cleaner. I am also kind of puzzled with this problem as I have a 2004 Corolla also, I only had power loss when my MAF went bad.