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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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23,857 Posts
Are they the correct Champion RN10PYP4 spark plugs? Treat this and solve as a P0300 fault.
Fuel pressure check OK? Sample some fuel from the fuel pump into a clear plastic bottle and make sure there is no foam or excessive bubbles along with the liquid fuel.
Any sign of fuel or oil carbon fouling on the plugs that you removed?
 

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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
Joined
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23,857 Posts
A cranking Compression test may not show a leak, where a static compression leak-down test would.
Bosch plugs have had issues in these. You will see a side-by-side difference with tip shape/depth (heat range) compared the correct Champion plug.
Has it settled on cylinder #4 as the misfire choice?
 
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