2008 Expy 5.4L, 99K miles. Engine has idles like butter for it's entire life, almost hard to tell it's on. Now I am getting a slight vibration felt through steering wheel, etc. It's subtle and no engine light or codes. Happens only at 1,000rpms and is made worse by having the a/c on. Any ideas or steps to figure this out?

 

It sounds like a COP failing? Should be a code thrown but maybe not---do you have access to a code reader or scanner?

 

Yeah, no codes. It comes and goes. Do COPS fail intermittently or degrade? Or is it all or nothing?

 

Need a scanner with live data-stream.

 

In my case it was and still is intermittent, one condition previously due a bad harness connection, current another instance with a Granatelli COP throwing a misfire code and running rough when the temps drop below 36*. This latter misfire is "cured" by shutting down and refiring immediately, it doesn't recur during the entire day.

They have been known to die completely which would throw a code too but growing weak is possible too.