I had all the COPs in my 1997, 5.4L engine changed today because I had a slight misfire and noticeable power loss. However the problem is still there. When I start the truck and its idling, you can hear a misfire and when driven it has lost some of its power. I changed the plugs also.
What other things could be causing this power loss/ misfire? My mechanic is saying a vacuum leak probably inside a valve, which he is saying couldn't be repaired. What do you guys think? Its not throwing any codes at least not yet, will check it again tomorrow. Any suggestion is appreciated.

 

Do you have a CEL? Have you checked the fuel system, changed the fuel filter?

 

No CEL and its not throwing any codes at all. I haven't changed the filter for this maintenance. I will do that.