My 2009 Fit hit 100,000 miles recently. I bought a can of Seafoam to put in the vacuum line.
I've searched the site but limited info on how to do this. Mostly posts with 1st generation and then it is more a debate if it's even going to help.
This is the best post on it I can find.

1st generation:
"After the car is warm , turn off engine. Use a pair of pliers and take off the tube in the center of the back intake manifold. The other side of this tube goes to the master brake cylinder. Pour about 5 ounces of Seafoam(which is the recommended 1/3 of the bottle ) into a cup and have it ready nearby. Take the clear vynil hose and put it over the inlet where the stock hose was. This is the place where the Seafoam will enter the engine."
-@jchcmax 2008

Are these still good instructions for the 2nd gen?

 

Why are you using Seafoam? What does it do?

At 100k miles, I would look at the official Honda maintenance schedule. Maybe include these things:
- Valve adjustment
- New spark plugs (see updated instructions here)
- New engine air filter
- Clean MAF (only MAF cleaner)
- Clean throttlebody (see special instructions here)
- New PCV valve
- Consider flushing key fluids (coolant, transmission, brake, oil)

 

Seafoam is used to clean carbon deposit in the engine. Lot of people swear by it. Often increases both horsepower and mpg.
I did most of things on the list already. Tranny fluid is due at 120k miles. I was actually looking at ordering a PCV valve today, but waiting to see what else I want. Likely going to change the fuel filter also.

 

Assuming you avoid the MAF entirely, one thing I worry about with seafoam is potential impact on cats and o2 sensors. For me, risking those expensive parts is not worth it.

For sure direct injection systems have problems with carbon but throttlebody systems are less problematic.