Help needed

[Anhquan1]

hi guys,

im new here and new to boating. I just bought a used boat with a 2008 Yamaha F90tlr on it, motor start right up and stay idle but if I try to trim up or down engine will kill and can't start again, engine alarm goes off after each time I tried to start. If I try to start it after 15-20mins it will start right up and if I try to trim it will kill again.
The guy I bought the motor from told me that the engine was running good with no issues, only thing he recommended was changing the lower unit oil since he didn't have a chance to get it done.
these are things that I did to the motor after purchase:
-oil change
-new spark plugs
-fuel filter
-lower unit oil change
-adding a water separate between the fuel tank and engine
-adding a different type of kill switch
can you guys give me any suggestions where and what to look for? Any help are greatly appreciate.

****the fuel bulb will get firm after few pumps but as soon as I stop it will soft again.

[99yam40]

kill switch activated while turning over motor should sound the alarm.

[fairdeal]

Engine dying when the trim motor runs is a bad battery / ground connection I believe

[Anhquan1]

Thanks, I'm gonna give it a try.

[rodbolt17]

lets see.
the ECU needs 12 v.
the ign system needs 12v.
the fuel pump needs 12v.
the injectors need 12v.
when you hit the trim something gets lost.
myself I would do a voltage drop test on the battery cables.