2002 SCab 4.6. 148,000 miles. Replaced Motorcraft spark plugs and new Delphi COPS. Replaced due to misfires under load. Starts and idles fine. Runs fine on highway. Stumbles when revved while in park starting at about 3800-4000 rpms. I did use dielectric on the boots. Actually went over the replacement to check for loose connectors or vacuum hoses and couldn't find any problems. No codes at this time.

Any suggestions?
John

 

Don't rev it in park. Don't understand why that high of rpms matters if it runs fine and no codes.

 

It might be the rev limiter.

 

Thanks for the quick response.

John