Impeller part no. for a 1987 force outboard model 507y7c

SOURCE: Mercury 20Hp impeller/water pump location

It is located inside the lower unit housing. They are fairly easy to replace. Most Mercury authorized service stations will charge around $120 for this task.

Triton72

SOURCE: 4hp Force Outboard motor overheating

Its possible it sucked up weeds or a plastic bag it into the intake and dropped off when you shut it down.

SOURCE: Force 90 outboard locked in forward gear after impeller replaced

i have fought with mine in the past also.
put lower unit and the control in neutral
remove all spark plugs and have a helper turn the fly wheel clockwise while you install the lower unit i had to use channel locks to put the lower unit to neutral
good luck i hope this helps

Testimonial: "Got it together. Thanks"

SOURCE: how to replace impeller in a 1959 evinrude

Put engine in fwd gear. Then disconnect the shift linkage. Remove the bolts that hold the gearcase on and slide gearcase off of the tower. Remove the 4 bolts or screws in the waterpump housing and lift the housong off. Replace the impeller and reassemble in reverse order. Make sure that you get the water tube lined back up and you can twist the prop to line the splines back up in the powerhead.