I've been noticing a strange sound when shutting the engine off lately. This is on a '90 F150 302/auto with 160k mi. The sound is a "whoosh", that sounds like either a rush of air, or like something spinning to a stop - the sound slows to a stop, if that makes sense. At first I thought about the fan clutch. However, once I had someone else shut the truck off while I was listening under the hood, the sound appears to be coming from the rear of the upper intake, or the rear of the motor. I recently re-routed the PCV and "T'd" into the brake booster line. The sound was not present before that that I can recall. Could that have anything to do with it?

Any ideas??

TIA,

Wade

 

I have heard this exact noise.


I changed the spark plugs on my friends 98 expedition and accidentally pulled the PVC out of the head and forgot to push it back in. first time I shut it off, I hear a loud WHOOSH. Put it back in... .I still heard it a little, but it was a lot more muffled afterwards.

-Chris

 

Change the hoses back to the way they were.

 

Are you suggesting that as a temporary test, or are you saying return them permanently?

Wade

 

The brake booster may have a leaking check valve. When you kill the manifold vacuum by shutting off th engine, the booster may create its own residual vacuum on the brake booster hose, which would rattle the PCV valve as it backflowed, thus creating the noise.

If the noise repeats on every shutdown, try removing the hose from the brake booster and plugging the end. Fire the truck up, then shut it down. If the noise does not happen, then the brake booster is implicated.

If the noise is unpredictable, try testing the brake booster after shutdown. Wait 20 seconds, the depress and release the brakes a few times. There should be enough residual vacuum to retain power boost through at least 3 or 4 cycles. If you have "hard pedal" right away, the booster is not holding vacuum.