Whistling pressure relief on N* Intake Manifold???

[gdmarques]

Anybody ever seen this valve on the end of the intake manifold on a N*? After going nuts for a week trying to isolate the source of a strange whistling noise from my engine (which sounded like a bad bearing, and my mechanic was sure was my AC compressor) I found this spring loaded flap valve at the end of the intake manifold. After reading all the posts here about this type of noise coming from the manifold gaskets themselves I replaced them, but it made no difference. Using my auto stethoscope with the probe removed I found this valvew which sits behind the power steering pump reservior. I was shocked to find this since it seems odd to have such a huge opening which could allow unfiltered ait directly into the engine. I assume its a backfire relief or some other over pressure relief valve. After cleaning the gasket (which is molded into the flapper) and the seat, and reinstalling the manifold again the noise was gone. Anyone ever seen this and/or know what its purpose is?

[Ranger]

It is called an over pressurization relief valve. In the event of an over pressurization (backfire), the valve is forced open to relieve pressure rather than having the manifold explode.

[gdmarques]

Thanks for the quick response. Ever heard of one of these whistling or causing a vacuum leak before? I doesn't look like a replaceable gasket on the valve. Is the assembly replaceable or come as part of the manifold itself?

[Ranger]

It is not common, but I have heard of one or two leaking, causing a wistle and/or a high idle. Not sure if it is replaceable separately or not.

[submariner409]

There have been 1 or 2 "whistlers" in here a year or so back. My 02 FSM shows the valve removed in a couple of line drawings, so I assume it's replaceable. Sort of a screw-on collar retainer. I think there are a couple of different valves, depending on year.