Just picked up a blazer and after buying it the Ses light came on. Misfire on cylinder #4 code. But the truck feels completely fine while driving. Fuel mileage is 15mpg ish

Checking with my sites scanner it’s showing missing on cylinder 4 and occasionally on cylinder 1. I noticed it’s only around 1500 to 2500 rpm. Idle and full or heavy throttle it doesn’t miss. Previous person said they did new distributor, plugs and wires,spider injection,cps, mass air and idle air sensor and valve.
Side note at idle when warm and stopped in drive it runs about 15-20 psi. Is that normal ALL HELP APPE

 

Also found out it has a jasper engine

 

Could be it never had a crank sensor relearn after installing the Jasper engine or a new crank sensor. This could cause a false misfire to be detected. You will never get to zero misfires on the scanner's counter. As long as the SES light stays off you are golden. Key here is do you feel a misfire? If you need further help, please post in Engines and Transmissions or Second Generation Forum.

15-20 psi at idle is normal oil pressure for 5W30 oil and a warm engine. Are you hearing noises?

Best Wishes