no power

[kurp]

Problem vehicle is a 97 explorer 4L OHV 4X4

Sometimes power is normal sometimes not. When it is not, rpm seems "stuck" at about 3000 and will not increase. Idle is a bit rough. Idle is also quite high when started initially but settles down after a few minutes. When throttle is about 1/3 open a hissing noise can be heard from under the hood but only when engine is under load.

I have changed:
fuel filter
air filter
spark plugs
plug wires
had engine scanned for codes - none showed
checked for vacuum leaks - found none. gauge read 20-22 in Hg and steady.

I give up! any suggestions?

 

[Alec]

Has your gas mileage gone down and/or can you tell if you're running rich at idle? If so, you need to replace your Fuel Pressure Regulator.