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1999 Ford Expedition Engine Hissing/Sucking
It's a 4.6L. I have inspected all of the vacuum lines. It's not coming from the brake booster. I thought it might be the egr. When I get on it or let off the gas, the noise goes away, but when you're cruising or stopped you can hear it. At idle it will flux between 500 and 100 rpm. When I shut it off It makes one last huge sucking noise when the engine stops. There is an electronically controlled valve on the top of the intake and when I unplug it the problem multiplies by 10x. Not sure if this is a choke of some sort or air flow sensor? or if that is even the problem, maybe it could still be the egr?
Any help would be great.
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