Your readings are still useless. There is no such thing as 120 % duty cycle. As in life 100% is all you get.

Your vacuum gauge readings are just a reflection of the way the engine performs. Engine dies vacuum dies. Lots of possible reasons, but if you wish to use electronic testing for monitoring and calibration you need to get you tool calibrated. There are a couple known quantities to line your tool with. The best is key on engine off. You should read 70%. If you don't chances are your tool set-up is the likely reason, especially if you ever read over 100%, which is impossible. You have to understand what you are measuring. You are measuring duty cycle otherwise know as on-off ratio. On all the time is 100% on duty cycle. You can't be on more than all the time.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician