Noticed Strange Sound From Gas Tank At Startup

[Salonika]

I have a new sound that has become more frequent and I'm taking notice now. When I start the truck up I hear a whooshing sound, almost like a fan running, when I start up and it is definitely coming from the far rear of the truck. It doesn't last more than 5 seconds or so and varies in intensity. I'm thinking it could only be the fuel pump as nothing else is back there that could make noise when sitting. I need to try and listen to it from outside but haven't had time yet. Anyone have any ideas as to what this is, or if I should worry about it? Thanks!

 

[Sabre]

If it's new and strange it's most likely something beginning to fail. I agree that it sounds like the fuel pump. On my Gen III there's a nifty access plate under the rear seat to make such a job easier.

 

[Salonika]

Running fuel pressure test tomorrow. My current fuel pump only has about 15,000 miles on it, non-Mitsu, but hopefully that's what the sound is and maybe this could clear up a chronic minor rough idle I've been battling unsuccessfully too.

 

[Salonika]

Fuel pressure test passed, it was in spec. Unless it is intermittent. I have no idea what that sound is. I'd have to come up with a way to monitor fuel pressure for an extended period of time to see what it reads when I hear that sound. Unfortunately there is no easy way to do that.

 

[Salonika]

Updating this.......I replaced the fuel lines on my sending unit (described on another thread) a couple months ago and haven’t heard this strange sound since then. I went from rubber lines with hose clamps to plastic lines using compression fittings. I have no idea why this stopped the sound. I never had a fuel leak with the old hose and hose clamps so I don’t understand why switching to plastic compression would have cured this problem, but it did. It is still a mystery to me even though it has been solved.

 

[coffeegoat]

I noticed a similar "weird noise" when I opened my gas cap (whoooshing). I found an evap leak on my carbon canister that when fixed seemed to stop the noise. In my case I think it had to do with the fuel vapor purge on startup that created a vacuum, but something similar and obscure may have been happening with yours.