Slight whistle from under hood that goes away when accelerating

[Patriot Blue]

I'm getting a slight whistle coming from under the hood. I can hear it while driving and in neutral idling. It stops when you give it gas.
No other problems. Not sure if its even a vacuum leak? Exhaust?

2004 4.0 manual

 

[Chris]

Could be, I would start by doing a smoke test and seeing if it produces any vacuum leaks.

 

[Camdenst]

Try cleaning out the throttle body, specifically the idle bypass hole. Its super easy to take apart and you'll see a channel for air when the flap is closed if my memory serves me correct.

 

[CodaMan]

Mine was an intake manifold leak that was corrected by tightening the manifold bolts, but it could be a vacuum leak from anywhere.

 

[Jerry Bransford]

That's clearly caused by a vacuum leak since the amount of available vacuum decreases when accelerating. Has there been any work done recently on the engine that could have affected any of the many vacuum lines and connections?

 

[steelhd]

Did you or a previous owner install a cold air intake?

 

[glowell222]

Why does a vacuum line whistle? Because it doesn't know the words!

 

[LJBean]

I had something similar that was cured by finding a manifold bolt that had backed out. This also cured a recurring code related to inability of ECU to achieve the target idle speed.

 

[Patriot Blue]

I had something similar that was cured by finding a manifold bolt that had backed out. This also cured a recurring code related to inability of ECU to achieve the target idle speed.

Ok thanks for the response. I will have to check what codes have been thrown as well

 

[Patriot Blue]

Did you or a previous owner install a cold air intake?

No stock intake

 

[Patriot Blue]

That's clearly caused by a vacuum leak since the amount of available vacuum decreases when accelerating. Has there been any work done recently on the engine that could have affected any of the many vacuum lines and connections?

Nothing recently under the hood

 

[Patriot Blue]

That's clearly caused by a vacuum leak since the amount of available vacuum decreases when accelerating. Has there been any work done recently on the engine that could have affected any of the many vacuum lines and connections?

I'm not really sure what lines I'm looking it. Are they hard or soft lines?

 

[Patriot Blue]

Ok

Could be, I would start by doing a smoke test and seeing if it produces any vacuum leaks.

Ok I will have to look into how to do that thanks

 

[Patriot Blue]

Try cleaning out the throttle body, specifically the idle bypass hole. Its super easy to take apart and you'll see a channel for air when the flap is closed if my memory serves me correct.

Ok I appreciate that

 

[Jerry Bransford]

I'm not really sure what lines I'm looking it. Are they hard or soft lines?

Both, here's a diagram of most of them. It's missing the larger diameter rubber vacuum hose going to the power brake booster.

The box #8 is the vacuum reservoir to store a little vacuum for when the engine isn't running.

The vacuum hoses connect to a couple locations on the intake manifold since that's where all the vacuum is generated.

 

[GEEP4ME]

Intake gasket on at the back of the intake is probably leaking. Mine did the same thing. You could change the "tune" with the throttle pedal position. Bolts were tight, though, I did get one to turn slightly. Once it warmed up, the whistle went away. Take a jaunt down the highway, and it would return from the engine cooling slightly. My 4 minute drive to work, it would start to whistle about 3 minutes in and be whistling as I was pulling into the parking lot. Let it sit and idle while searching for the leak, and it would go away, LOL. Cooler/colder temps made it worse. Ultimately replaced the intake/exhaust manifold gasket. Thought I had a picture of the old gasket, but I don't.

 

[Patriot Blue]

Thanks everyone for the help

This gasket is the culprit. I can put my finger on it near the top bolt and it goes away

 

[LJBean]

Might not have to change the gasket. Have you tried tightening the header bolts? Check specs on torque - you don't want to bust one off.

 

[GEEP4ME]

If you decide to do this repair yourself, and you don't have rust issues on your Jeep, I recommend removing the driver fender for better access to all the manifold bolts. Only takes 15-30 minutes to remove it, and you can see pretty much everything right there. I had seen several mentions of that here and across the rest of the web.