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Vacuum Diagnosis 'Diagnostics Master Guide'

Started by Neil4speed, 28 August 2022, 10:40 AM


Hi all,

Just wondering if there is a 'Diagnostics master guide' for sorting/identifying vacuum leaks. My locks unfortunately don't work on my 6.9 and I am looking for the starting point to start fixing any/all vacuum leaks that might exist. I imagine it starts with a Smoke machine, but not sure of where to start.

Any support would be very helpful :)


Get a Mitivac. Start at the firewall and make sure the one way valve works first on the yellow line.

Then start in the front door, make sure the 4 way valve doesnt leak. connect to the vac actuator and see if any diaphragms leaks.  Move on to each door and check the same.
Also check to make sure the vac tank in trunk and in left front wheel well (behind the panel) dont leak.
Also check the gas door and trunk lock actuators.

The technical section of site has instructions for removing the door panels.

While the door panel is off it is a good time to lube the window regulators and door handles and door locks.

'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,