Allinkdupp

04 5.4, gryphon running custom 87 octane towing with 91 octane + octane boost ... On the way back from the hills I noticed an issue... At high RPM high speed like passing people on the highway my truck would stumble and jerk pretty bad? What do you guys think? This was the first time not adding octane boost. Bad gas? Plugs?

CSprain

Sounds like a misfire. I had a misfire that would happen only at like 45 mph. Fresh spark plugs and spark plug boots solved the problem.

jamesyarbrough

Yep sounds like a mis. How many miles are on it? Ever change the plugs?

Allinkdupp

112000, I got it used, the log in the manual said they were changed at 50,000

Allinkdupp

Correction, they were changed at 70,000

jamesyarbrough

If it were me, I would change the plugs and COPs and see what happens

Regulator

its not throwing you a code??? strange.... how often is it happening? if it isn't consistent I'd wait and continue to monitor b/f you drop big money on COPs and plugs....

Initially it sounds like a typical 5.4L misfire...

Allinkdupp

No codes, I ran the fuel out and just filled up, I'll take it on the highway tomorrow and see what's up.

Regulator

Let us know what you find out, curious to say the least....

05jakev

I had some plugged cats and it was doing the same.