Corrosion somewhere
Remove the wires on the starter post, all of them including the ones on the 90 amp fuse. Remove the negative cable on the rear of the block and clean them all up to shinny metal.
Also pull the engine harness 10 pin connector and clean the contacts
That is the circulating water pump item 10, gaskets item 11..
I most cases the pump needs replacing
https://www.mercruiserparts.com/bam/subassembly/31844/2704/200
If the starter was not turning the motor over very fast, that would give low numbers. If your numbers are correct then you need a rebuild, need 100 just to run
Think your good so far as temp
Being on a hose cal cause things to work slower. That said, none of my boats have ever done (or at least I have never noticed) what yours is doing
Everyone has seen prices go up for wood. I was looking around at some places like HD which was selling 2x12x8 red board for $250, that's $31 a board foot. Is it really that high? Don't see how anyone would pay that for one board
In the link, its the lock pin which is not engaging. When you squeeze the handle latch, does it spring back when released? If it does then its inside the rod or lock pin. If it doesn't then might be the spring
So the fuel pump turns ON when starting the motor, but once running the pump shuts off
If that is the case then it is the diode. The diode should be at the on the green wire. Most any diode will work and can be had from a local electronics store. Get one that can handle 1.5 to 2 amps
Or...
With a 160* thermostat the motor should stay right at 160*, it should not lower to 120*.
Start by using a IR temp gun and verify the of the thermostat housing is the same temp that your gauge is showing.
On manifold will most always be hotter the then other and more so on a hose, because the...
In the 60 and 70's premium was 98 to 102. Later in the 70's it went down to 95 to 98 and the 80's to 94 to 95.
In short the 91 of your motor equates to 87 today
Looks like this one
https://www.mercruiserparts.com/bam/subassembly/32078/1989/90
To take it apart, move the throttle forward then you can pull the center button out. Once out can remove the set screw in the bottom and should be something (bolt or screw) which can be removed.
In the pic there...
Got to the link and place the cursor over each listing. A pic of the control shows up. Find one that looks like yours. Find it and then select the link and explore the parts breakdown. This is how we would do it if you posted a pic of your control
Probably the spring or latch. Would need to know which control it is to figure it out. Here is where you can find parts breakdown
https://www.mercruiserparts.com/remote-controls-and-components-1986-th
Do think its the switch and temps need to get up above 200*
All the switches have a Tan/Black wire
The sender has a Tan wire and looks like it has a stud with a nut connection. On Mers they are both on the thermostat housing. Look on the intake
