2000 F150 4.6 Misfire started 2 months ago.
...Changed plugs no help
...Had autozone check code, misfire #6
...Installed new COP...No help
...Rechecked code, missfire #6 and #3 Added injector cleaner.
...Misfire stopped and Light went out after 2 tanks of gas
...Misfire back after 3 weeks, but much worse
...Autozone code check missfire #2,3,4
...added injector cleaner, no help
...Changed fuel filter, no help

How do I check for vac leaks? Cant see or feel lines in back of engine. I have a feeling I have a vac leak. Test gauge?

What is a logical procedure to diognose this problem?

 

Vacuum leaks usually don't cause misfire codes. Take the plugs out and make sure none of them are cracked.

 

ALso, make sure when you pull the COP's that there is no moisture on them. It will cause them to short out is water gets into the spark plug pocket area.

 

Water was the problem. I tapped the COP on the table and drops of water came out. Water must have evaporated, entered the COP from the bottom and condenced inside.

 

Have you washed the engine lately?