Codes? Check engine light?
Sounds like a cylinder misfire, many causes for that.
1. Spark Plug fouled
2. bad or dirty injector on that cylinder
3. cracked coil bar or boot
4. bad valve
5. wiped out lobe on the camshaft for one of the valves on the affected cylinder, typically this would get worse under load.
Scan for codes...then...
1. Check plugs, replace if questionable post pics here ill review them and tell you by the look of them if the cylinders are firing correctly. If one plug appears cruddy, wet, black etc with carbon buildup then its likely the problematic cylinder. So make certain you note which cylinder they are removed from.
2. Compression test all the cylinders (if one is weak then we need to address why) could be as simple as a valve could be as much as a cracked head, or bad head gasket.
3. If plugs and compression test fine then we need to check injectors out. It might have one leaky or sticking injector or one not flowing well and causing that cylinder to run lean.
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