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This^^^^. It pops into a grommet on your air filter housing. It lets you know if the filter needs to be replaced. Look for an empty hole with a rubber grommet in it on your air box.I believe it's an air filter monitor. When connected between the filter and intake, a dirty filter will restrict airflow and the increased vacuum will pull a spring-loaded indicator down showing that it is time to replace the filter.
I found this device today in my engine bay while doing another project and it appears to have been left there from an oil change perhaps? Can anyone tell me what this does?
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Yep, those were used quite a bit on the Detroit Diesel generators that I worked on for a time, in another life.