P Trap help

[Epfriis]

Im working on a old summer house and the drain was smelling awful. After a look below the sink, discovered the drain basically going from 90deg straight down to the drain. After I installed the trap, the sink won’t drain! It has no problem draining without the trap. There is an air vent but it’s directly under the sink/drain coupler and before the trap Any ideas?

 

[Diehard]

Im working on a old summer house and the drain was smelling awful. After a look below the sink, discovered the drain basically going from 90deg straight down to the drain. After I installed the trap, the sink won’t drain! It has no problem draining without the trap. There is an air vent but it’s directly under the sink/drain coupler and before the trap Any ideas?

Won't drain at all?
Or doesn't drain properly?
Do you know what happens after it goes "straight down to the drain"? Could there be another trap further down stream?
"an air vent..........directly under the sink/drain coupler" makes no sense at all.Is it actually connected to a vent pipe going somewhere?

 

[voletl]

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