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Help* Hissing sound under the back box

11717 Views 11 Replies 8 Participants Last post by  Leroy Holthaus
Hello All,
In the last week I have noticed a sound that I have never heard/noticed before. It sounds like pressure building and hissing out at a constant pressure under the box of the truck. I can make to sound start/stop with the key turned to run, but it only runs for about 20 seconds then turns off, but back on when the truck is started, and continues to hiss.
A buddy and I got under the back and could not see fluid dropping or locate the source of the hissing, almost sounds like it is in or around the fuel tank. We then thought it could be the fuel system since it requires so much pressure (diesel in general), we thought it could be that it is pressurizing the lines. However I can turn off the truck and it will reactivate with the key every time.
What made me notice this sound is that it can hear it over the sound of the engine when standing by the doors of the cab, it is that loud.
ps: I dont have air bags, so there is no air lines in the truck.
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Anyone own a truck that is making this noise or sound?
No. My truck makes no sound like that.

Obviously, since it turns on with the key, it's electrical related. Only thing I can think of was mentioned. That would be the fuel pump inside the fuel tank.

If you have air-bag suspension then it could be that air pump.

There is an injector for the DEF and that is located before the muffler. Do not know exactly where or how that is injected/pumped.

Sounds like a dealer issue as it does not sound normal.
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Mine has made a noise since day one, I wouldn't call it a hiss though, more like a whine. It's from the DEF pump figure, I wouldn't say mine is that loud though, I'd have to be by my filler door and maybe crouched down to hear it over the engine running.
There is a barley audible hissing sound from underneath I figured it was fuel pump, but never contemplated that it could be the DEF pump (not having delved into it's workings). I do not think it is anything to be concerned over unless is unusually load as compared to other EDs.
@gadfly2 you been able to scope out another ED for comparative purposes?
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You might want to stop by some local dealer that has them just to see if what they have on the lot is also like this. it's what i'd do.
@gadfly2 you been able to scope out another ED for comparative purposes?
Update: So I went back to the dealer I got the truck from and they let me check out one of their other ED's on the lot. Same sound, same location, however my truck made more sound quality (could hear it louder, but not by much). It seems that the sound is normal, but might get louder with mileage? However, the sales rep and I could not pin-point the exact location, but he believed it was around the fuel tank as well. He mentioned that Eco Diesel is a high pressure fuel system like all diesels (running around 3000psi I believe he said), so he thought it was the fuel pump “priming” or pressurizing the lines to maintain pressure.
**diesel guru's: I am new to diesel vehicles, so if any of this sounds wonky to you, this is what the sales rep told me, he is not a mechanic.
The high pressure fuel pump runs at 29,000 psi, but is located on the engine not in the rear of the truck. What you could be hearing is the low pressure lift pump, in the fuel tank, that delivers fuel to the high pressure pump. Salesmen will tell you anything, fact or fiction, to satisfy you.
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^ Very true point on sales people. At the end of the day all they want is their commission, best thing to do is to just get your info from someone else. Sales managers might be of better help.
Same sound and issue here. 13,000 miles. seems to have gotten louder in the last 2000 miles. crawled under there last night definatly from the top of the fuel tank. no leakage. .............Question is there a return line coming back to the tank with a pressure relief valve by the motor????????? I'm going to fill my tank today. Questioning if the sound might be less if there is no air space for a return line to blast fuel back in the tank and echo the sound around. (true feeling is if there is a return line it should be fastened to the bottom of the tank but has came loose and sprays across the top of the fuel tank) Thoughts? any one ever have a tank off or know how the lines are set up in there? Thank you in advance.
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