Vacuum Leak Trouble Spots

Vacuum Leak Trouble Spots

So you suspect you have one or more vacuum leaks. Based on my limited 928 experience, you are probably correct. Since different MY 928s have different vacuum lines and/or are routed differently, I will only talk about the system in general.

You should have a line diagram of the emission control system on a large silver decal located on the underside of the engine hood (bonnet). Most of the vacuum lines can be located using the diagram.

To check the system's integrity, you will need a vacuum tester (Myti-Vac). Each line to be checkd should be isolated.

1) Inspect all lines and connections. I have seen rubber connectors that have swelled and no longer make a tight fit.
2) Inspect the large oil return lines on the right cam tower (the left side of the engine as seen while standing in front of the car).
3) Inspect the oil filler "O" ring and the dip stick "O" ring.
4) Inspect the large oil lines to the filler and the plenum from the throttle body.
5a) For pre-87 928s, check the two large "O" rings at the base of the Mass-flow sensor. While that sensor is out, check the vacuum manifold that looks like a spider and all hoses adjacent to the throttle body that are now accessible. Also, check the rubber connectors between the intake plena, and the throttle body.
5b) For 87 to 95 928s, inspect the rubber boot the mass flow sensor fits into. (Since the area under the intake manifold is inaccessible without removal or the manifold, all you can do is hope all is well down there.)
6) Inspect the check valve that comes off the brake booster vacuum line.
7) Inspect the "T" or "X" connector below the check valve. One of the lines here goes to the AC/Heater control unit. (Be ready vacuum leaks here.)
8) Inspect the "Y" fitting that connects to that connector. These lines go to the cruise control and the vacuum reservoir.
9) If your 928 is an AT, check the line to the transmission.

My experience has found that items 2, 4, 5a, 7, and 8 have been areas where I have seen most of the vacuum related leaks.

I hope this information helps. Merry motoring, Ed.

On '80-'84 US cars using L-Jetronic, there are two 3" diameter o-rings that are prone to leak. One is at the base of the air flow meter where it mates to the plenum leading to the throttle body and the other is at the base of the throttle body.

Neither are particularly difficult to replace but will require a couple of hours labor. The one on the air flow meter is easiest and is replaced by simply removing the AFM and lower section of the air box as a unit after unplugging the harness to the AFM. The new o-ring will require a lubricant to make sure the AFM seats properly during reassembly.

The o-ring at the base of the throttle body requires that the intake plenum be removed from the runners and that various linkages and vacuum lines be unhooked to remove the t-body. Be careful when sliding the eight rubber mating tubes over the runners to remove the plenum since they will split easily if they're dried out.

Both o-rings cost less than $10.
Jim Stadter
'83 928 S 5-speed (U.S. spec)
'88 928 S4 5-speed

On my workshop manuals micro fiche #4 of 17 Feb '91 D 24-25 there is procedure outlined for checking the resonator flap function that also details a LOT of vacuum checks.

Hope that helps.
- michael
1988 928S4 A/T

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