Cold weather vacuum leak

[evolve991]

I recently replaced the intake on my 350 and made damn sure it ,and all lines, were sealed well. This "arctic vortex" comes along and suddenly I have a vac whistle at part throttle. It stopped when temps went up for a day and it's back with this latest blast. Anyone ever have this problem? Any ideas before I go get purple fingers?

 

[dyeager535]

Any chance it's the THERMAC? The valve on the air cleaner? It has a temp switch in the air cleaner IIRC. Only thing I can think of that might be influenced by air temp short of something metal that is shrinking when the engine is cold, but that should go away with an increase in engine temp, especially since it goes away with warmer ambient temps.

Depending on what you have on the motor there might be some vacuum switches, but those should be coolant temp dependent, not influenced by ambient temps.

 

[evolve991]

Guess a little info on the motor would help. It's a 73 block, no vac switches I can see, no Thermac on breather, simple vac T's and a ball reservoir. It's hard to locate where the noise is coming from too...seems to be centered and near the firewall but not a definite,I was wondering if it may even be somewhere in the heater controls.

 

[dyeager535]

Is it an AC truck then? The blend doors are all vacuum operated, would seem very weird that cold would affect that, all of that stuff is plastic isn't it?

 

[evolve991]

Yes it has (had) AC. Yeah it is weird that cold makes this happen. Its just too damn cold to take my time and track it down but since it doesn't do it if it ISN't frigid I guess that's my only choice.

 

[dyeager535]

If you manipulate the controls, it doesn't change the noise? IE switch between defrost, floor heat, etc.?

Believe the temp controls (lower control of course) were always cable, but the upper controls are all vacuum.

 

[evolve991]

No,no difference with heat/defrost. I appreciate the ideas though

 

[dyeager535]

Inside the cab though?

On the back of the heater control is a vacuum control setup. I removed mine long ago, can't recall exactly what it is, for some reason a clear, soft piece of plastic or rubber comes to mind, and it is rotated as you move the switch. Held on with spring clip kinda things. If the sound is coming from that area, I'd almost bet that is it.