My guess based on what you have said is a vacuum leak. Check all your hoses and I am sure you will find a cracked hose somewhere. Also check your bleeder valves and ensure they are in good condition as well.

Do you have a Bentley manual or do you know what your vacuum lines look like?

Drew

 

Pull your speaker and look in there at the back of the turbo gauge and see if the vacuum line going to it has come loose.

 

Ditto to what Matt said...

Jitters

 

My guess based on what you have said is a vacuum leak. Check all your hoses and I am sure you will find a cracked hose somewhere. Also check your bleeder valves and ensure they are in good condition as well.

Do you have a Bentley manual or do you know what your vacuum lines look like?

Drew

I'll be under the hood tomorrow changing all the hoses and thermostat so I will have the chance to get a good look at all the vacuum lines.
Where is the APC pressure sensor located?
Bentley is vague and my book only covers '86 up.
Mine is an '84 8v.
The hissing really sounds like it is coming from the left corner of the dash.

 

I'll be under the hood tomorrow changing all the hoses and thermostat so I will have the chance to get a good look at all the vacuum lines.
Where is the APC pressure sensor located?
Bentley is vague and my book only covers '86 up.
Mine is an '84 8v.
The hissing really sounds like it is coming from the left corner of the dash.

Hey JW,

I'm not sure about your year but there are two sources of vacuum on that side of the dash, one goes to the APC gauge in the instrument cluster and the other is the overboost switch located on the left side kickpanel just behind the knee bolster...Both can be accessed by removing the left side speaker grill. Hope that helps some...

Jitters

 

Yes, it helps because the Bentley book I have is for 1985 and up.
My car is an 84 and so far nothing in the Bentley book has been very helpful.
Trying to locate a different manual but not having much luck.
Haynes manuals are usually worthless but I have seen one for this car and it is not too bad. Good wiring diagrams in the Haynes.

 

Glad someone asked this... my 89 has the same problem... when you hear the hissing, you lose acceleration for a sec. My friend says it sounds like someone is opening a beer can under the hood. When I tackle the head gasket I will get this fixed too.

 

Exactly. Can of beer opening then abruptly stopping.
Comes on at the exact time the boost gauge begins to show pressure.

 

There's a diagram here which may help.

Mine always used to pop a hose at the hub behind the radio, marked no. 6, which would cause me to stall on deceleration.

 

Does the boost gauge not show anything at all? Usually what happens to me is after I've replaced the hoses the new hoses still have a bit of a 'covering' on them so they don't pick up grit from everything. It also gives it less friction so the hose blows off of the gauge. A clip will remedy that. There are other sources of vacuum hose behind the dash on the left side, including cruise control elements. However, even if any of these hoses were just open to the air, it shouldn't cause acceleration problems. That would indicate a much larger opening. If it only happens on high boost and you can't hear a sucking sound at idle, it may be a torn intake manifold fitting or something else of the like.

li Arc

 

Gauge works fine.
Shows vacuum and base boost at the right times.
I had the speaker out today and the hose to the gauge is fine.
Problem is I can't drive the car and shove my hand or head under the dash to find the leak.
The only time I hear this noise is at the beginning of boost.
Sounds like when you put air in a flat tire.
Kind of cavernous/metallic.
Then just abruptly shuts off even with a steady pedal.
Revving the engine while parked won't do it.
No noise at decel or idle.
I'm gonna try the poor man's smoke machine next(cigarette+vacuum hose)and see if I get a puff from somewhere.
maybe a cigar for better density. :cheesy:

 

The cigar trick worked.
Fired up a swisher sweet, took a big puff and blew into the vacuum line that goes into the cabin.
The left side of the dashboard filled up with smoke.
The line to the pressure sensor is split.

 

Congrats Jdub...

For future reference, where exactly is the pressure sensor located?

Jitters

 

There's a diagram here which may help.

Stand on your head under the steering wheel and you might see it.