I have noticed that everytime I stop my truck to get out of it and go inside my house or shopping,a hissing kind of noise is heard from the engine compartment. Its not exactly a hissing but sounds that way,like a motor of some sort is relieving pressure. Anyone else heard this? Could this be the AC system discharging.Or something else powering down? Its a 97 f 250 light duty 4x4 with a 5.4 liter. It does it all the time and I havent run the AC in a month. Im not too concerned but the noise puzzles me and I would like to Identify it . Thanks

 

If you are running your defrost or almost any other heater option your ac comp. does run and could be the noise you hear. Myne makes slight hissing noise when engine is shut off after running ac comp.

I also have a 97 f250lt 4x 5.4 that I have owned for 4 years. I have always thought it to be under powered. When in over drive at 65 it will barly speed up unless I down shift. What do you think of your trucks power. I also get 10MPG in town and 11.5MPG freeway, what does yours get?

 

My truck does well, I installed a Banks Ram Air intake and it seems to have pepped it up a bit,on takeoff acceleration. I get about 16 hiway 14 city. I cant complain thus far. Im very happy with my truck.

 

JC,

I've got a 4.2l V6 in my 97, 2wd. I know it's not the same but I was noticing a similar issue with it when I first put a rebuilt in it. I live in the north east and travel a lot of hills. My truck seemed way under powered until I changed my wires back to original motorcrafts. The power issue was improved even more when swapped out my slightly oversized summer tires for stock sized snows. Just a thought if you've changed any of these things. I also noticed the MPGs went up with the tire swap, even more than the size differential would have accounted for.

 

My F-150 4.6L makes the same noise, has done it since the day I got it. That was 150k ago, so I don't think it is a problem. For me anyway.