Wich wire”s” do I disconnect when checking compression

[Hugomartin]

My truck still won’t run after trying turning the cap all sorts of ways. What wire do I need to disconnect to check for compression?

Question 2 is that silver nut I’m pointing to called an oil pressure switch?

 

[Hugomartin]

Closeup

 

[Hugomartin]

Wich one is the fuel pump relay I only have one tank what is the other relay for?

 

[Snoots]

1. You can just take the cap off.
2. That's what it appears to be.
3. Don't know about the relays.