: WTF? Intake Manifold Noise?

05-20-04, 12:59 PM
I have a very loud strange whine/whistle kind of noise coming from behind the power steering pump on my 98 Deville (Northstar)

It looks like there's some sort of 2nd throttle body on the intake manifold.
Have any of you guys seen problems with these? I've removed the belt so I know the noise isn't coming from anything like the alternator or PS pump.

Unfortunately I don't know how to isolate the noise to the Intake or Timing Chains. The noise temporarily goes away when I snap the throttle closed (Dropping RPMs) so I'm afraid it might be a temporary slack in a noisy timing chain.

Any advice on where to go from here?

05-20-04, 01:00 PM
just wanted to add..... I just finished putting Head Gaskets on this vehicle.

05-20-04, 01:19 PM
It's probably the seal on the little flap-valve on the back of your plenum...... I think cadillac put that there in case of backfires. At high-vacuum (idle), any leaks could manifest themselves as a whistle.

05-20-04, 01:45 PM
That or, more likely since the manifold was off for the repair, there is a vacuum leak at the intake sealing surface. I would pull the intake off to check the seal and/or look for any sign of debris under the seal or the seal being cocked or twisted.....

The little trap door at the end of the manifold...with the spring loaded hinge...is a pop off valve to protect the manifold in case of an overpressurization event....it usually seals fine but maybe it was unseated and some debris got caught under the seal lip.... You can flip the valve open (engine off) and check the seal and clean the mating surface.

05-20-04, 07:24 PM
I found the problem!!!

It turned out to be the intake manifold and fortunately that Flapper valve is servicable separately from the intake manifold! :) It's only $25 but it had to be ordered. I swear it sounded like a bearing noise but I'm VERY HAPPY that this is all it was.

Thanks for the advice!!!

05-20-04, 07:27 PM
Also thought I'd add.......

I removed the valve and cleaned the seal / sealing surface, only to have a minor improvement. Then I tried adding a little pressure on the flapper and the noise went away. It appears that the springs are weak, so I just ordered a new Valve.

05-20-04, 10:21 PM
The springs aren't "weak"...they are specifically calibrated and checked to allow a very small amount of pressure to pop the valve off....they are supposed to be very soft...not weak...springs.

Chances are the seal is damaged or there is something caught under the lip of the seal causing it to not seal with the pressure of the spring. Normally the valve is under vacuum so it doesn't seal due to the spring pressure the suction against the valve seals it....the spring just "seats" the valve and holds it closed...vacuum actually seals it. If there was extra pressure necessary to seal the valve then the molded seal on the valve was failing or something....

05-21-04, 03:12 PM
Did you also order the O-ring that goes around the flapper valve? When you get the valve use it to make a tool to unscrew the old one and screw the new one in.

When you get the old one out look inside to see how much oil/liquid is at the bottom of the intake or plenum.