928 Vacuum Line Routing
05-20-2008, 08:04 PM
#2
Rennlist Member
I'll see if I can dig up a picture that I found. It took me 3 days of various searches to find one. I went through several incorrect versions and there is not a diagram in the service manuals. Apparently they forgot about '82!
Is there something specific you are trying to resolve?
Is there something specific you are trying to resolve?
05-21-2008, 12:05 AM
#3
Rennlist Member
Have you checked the underside of the hood? There should be a placard with the hookup points.
05-21-2008, 01:42 AM
#4
Rennlist Member
Just as a caveot I only have the engine vacuum diagram. If you are looking for the HVAC diagram someone else may be able to help
I found all this information helpful and it was lifted from several posts on the list... so I won't take credit.
On the top of the intake plenum (what Porsche calls the "air distributor") is the vacuum source for the auto transmission. (manual, p. 26-3)
On the left side, the venturi tube attaches to the front of the plenum, down low, large hose. That's the upper left connection on the diagram. The second-from-the-bottom left connection on the diagram is where the venturi enters the intake plenum (y'd to brake booster) at the top.
On the front of the throttle body, near the idle adjust screw, is the port for the ignition vacuum advance. (Manual, p. 24) There are actually two ports there; it is my understanding that the second should be covered with a red cap unless your car was manufactured to California spec (I heard this once, I don't know for sure but mine's capped).
On the back of the throttle body, the driver's-side port goes to the decel valve (vacuum limiter, on right side of TB), the pressure regulators and the pressure damper. (Manual p. 24-14 again).
Back of the TB, passenger's side, is the port for the vacuum check valve for the carbon canister (you guessed it, same pages) and to the distributor vacuum retard, if so equipped.
There are additional vacuum connetions to be made from the above lines for the control for the air injection regulation, which I have lost familiarity with since they went away. Sorry.
Excluding the emissions equipment and presuming the venturi system is attached and all other intake ports capped, if you make the connections to the fuel system components and the vacuum advance you should be able to run it for troubleshooting.
Check all connections from the brake booster also, to the HVAC and cruise control.
*****************
A rythmic idle up and down might be a tired vacuum limiter (UFO shaped thing on the side of the throttle body.) It can be replaced with a 5/8" and 3/4" rubber heater hose blockoff plugs with no ill effect.
I then pulled out the might vac and tested the decel valve. The WSM claims that if you add vacuum, the idle should rise. This indeed happened after I added a number of pumps, however then it went into the idle oscillation that I had seen before. There is one smoking gun!
I found all this information helpful and it was lifted from several posts on the list... so I won't take credit.
On the top of the intake plenum (what Porsche calls the "air distributor") is the vacuum source for the auto transmission. (manual, p. 26-3)
On the left side, the venturi tube attaches to the front of the plenum, down low, large hose. That's the upper left connection on the diagram. The second-from-the-bottom left connection on the diagram is where the venturi enters the intake plenum (y'd to brake booster) at the top.
On the front of the throttle body, near the idle adjust screw, is the port for the ignition vacuum advance. (Manual, p. 24) There are actually two ports there; it is my understanding that the second should be covered with a red cap unless your car was manufactured to California spec (I heard this once, I don't know for sure but mine's capped).
On the back of the throttle body, the driver's-side port goes to the decel valve (vacuum limiter, on right side of TB), the pressure regulators and the pressure damper. (Manual p. 24-14 again).
Back of the TB, passenger's side, is the port for the vacuum check valve for the carbon canister (you guessed it, same pages) and to the distributor vacuum retard, if so equipped.
There are additional vacuum connetions to be made from the above lines for the control for the air injection regulation, which I have lost familiarity with since they went away. Sorry.
Excluding the emissions equipment and presuming the venturi system is attached and all other intake ports capped, if you make the connections to the fuel system components and the vacuum advance you should be able to run it for troubleshooting.
Check all connections from the brake booster also, to the HVAC and cruise control.
*****************
A rythmic idle up and down might be a tired vacuum limiter (UFO shaped thing on the side of the throttle body.) It can be replaced with a 5/8" and 3/4" rubber heater hose blockoff plugs with no ill effect.
I then pulled out the might vac and tested the decel valve. The WSM claims that if you add vacuum, the idle should rise. This indeed happened after I added a number of pumps, however then it went into the idle oscillation that I had seen before. There is one smoking gun!
05-21-2008, 01:49 AM
#5
Inventor
Rennlist Member
Rennlist Member
Nice writeup!
The vacuum limiter was factory deleted in '83, for automatics. Some have said it may be needed for 5-speeds to stop from stalling. I think turning the idle up by 50 rpm would fix that.
I've also had no problem deleting the venturi-vacuum dealy-flopper, on both my '81 and '86s, by running a hose straight from the plenum to the brake booster (like on most other cars). It is there, AFAIK, to keep vacuum even during extended full throttle driving - on the autobahn, for example.
'84 auto:
The vacuum limiter was factory deleted in '83, for automatics. Some have said it may be needed for 5-speeds to stop from stalling. I think turning the idle up by 50 rpm would fix that.
I've also had no problem deleting the venturi-vacuum dealy-flopper, on both my '81 and '86s, by running a hose straight from the plenum to the brake booster (like on most other cars). It is there, AFAIK, to keep vacuum even during extended full throttle driving - on the autobahn, for example.
'84 auto:
05-22-2008, 01:37 AM
#6
Instructor
Thread Starter
Man, thanks a bunch for this info! You're awesome.
No, there wasn't anything particular I was looking for. I recently bought an 82 off of ebay that is not running to try to restore. (Not like totally restore to mint or anything....just to try to get running and have some fun as a project.) And checking/fixing all the engine vacuum line routings is on my early list of things to do.
So this info will help tremendously.
Thanks!
No, there wasn't anything particular I was looking for. I recently bought an 82 off of ebay that is not running to try to restore. (Not like totally restore to mint or anything....just to try to get running and have some fun as a project.) And checking/fixing all the engine vacuum line routings is on my early list of things to do.
So this info will help tremendously.
Thanks!
