09-30-2009, 05:03 PM
Try this vacuum leak test if you have or suspect a leak ...
The Porsche 968 Workshop Manual Vol. 1, DME-Diagnosis, Test point 29, has this to say about finding vacuum leaks: "Remove upper air cleaner section. Attach Special Tool 9264/4 to the air mass sensor and build up a positive pressure of approx. 0.5 bar in the intake system. Leaks will cause the pressure ... to drop quickly". (no spec. is given, nor is there a picture of the tool)
Being unable to find a suspected vac. leak, I tried making what I hoped would be a reasonable equivalent to Porsche's special tool. Mine consists of a 3" diameter vitamin bottle, 2 vac. line pass-throughs (bulkhead connectors), two pieces of hose, my trusty 50 year old Penske vac./pres. gauge, and a ball valve with hose-barb and air-line fittings. The pass-throughs were sealed with rubber washers and cement in holes drilled in the bottom of the bottle. Alternatively, one pass-through and a Tee, for the gauge, would have worked just as well. In order to make sure there were no leaks, I put the cap on the bottle and pressurized to 6 psi (but I had to put an old o-ring in the lid in order to get a good seal). At first I had tried an air gun with a barb to control the pressurizing, but it didn't hold the pressure.
Using the homemade tool ...
In addition to removing the air box top, also remove the mass airflow unit and hang it out of the way, being careful about protecting its innards. Stuff the open end of the bottle into the rubber boot still attached to the throttle body and tighten the sealing band. Pressurize to 4 or 5 psi (per the manual, don't exceed 7 psi) and shut off the air supply. Watch the gauge. If it drops rapidly, repressurize and start listening and feeling for escaping air somewhere.
I found a short piece of vac. hose with a small split, one connecting one of those black plastic vac. pipes to the manifold. After replacing it, there's less than 2 psi loss in 60 sec., which I'm counting as a good result compared with the 4 psi loss in less than 15 sec. before.
The Porsche 968 Workshop Manual Vol. 1, DME-Diagnosis, Test point 29, has this to say about finding vacuum leaks: "Remove upper air cleaner section. Attach Special Tool 9264/4 to the air mass sensor and build up a positive pressure of approx. 0.5 bar in the intake system. Leaks will cause the pressure ... to drop quickly". (no spec. is given, nor is there a picture of the tool)
Being unable to find a suspected vac. leak, I tried making what I hoped would be a reasonable equivalent to Porsche's special tool. Mine consists of a 3" diameter vitamin bottle, 2 vac. line pass-throughs (bulkhead connectors), two pieces of hose, my trusty 50 year old Penske vac./pres. gauge, and a ball valve with hose-barb and air-line fittings. The pass-throughs were sealed with rubber washers and cement in holes drilled in the bottom of the bottle. Alternatively, one pass-through and a Tee, for the gauge, would have worked just as well. In order to make sure there were no leaks, I put the cap on the bottle and pressurized to 6 psi (but I had to put an old o-ring in the lid in order to get a good seal). At first I had tried an air gun with a barb to control the pressurizing, but it didn't hold the pressure.
Using the homemade tool ...
In addition to removing the air box top, also remove the mass airflow unit and hang it out of the way, being careful about protecting its innards. Stuff the open end of the bottle into the rubber boot still attached to the throttle body and tighten the sealing band. Pressurize to 4 or 5 psi (per the manual, don't exceed 7 psi) and shut off the air supply. Watch the gauge. If it drops rapidly, repressurize and start listening and feeling for escaping air somewhere.
I found a short piece of vac. hose with a small split, one connecting one of those black plastic vac. pipes to the manifold. After replacing it, there's less than 2 psi loss in 60 sec., which I'm counting as a good result compared with the 4 psi loss in less than 15 sec. before.
