[kwgreer]

I have been trying to figure out this misfire problem I have been having on the highway for quite some time. It happens over 80km/h and the rpm will drop and go back up within a second, it feels like a cylinder is misfiring but it will not throw any codes. I have replaced the MAF sensor twice, PCV Valve, Air Filter, Fuel Filter, Sparkplugs to NGK Iridium IV and plug wires to NGK as well. I also ran seafoam through the throttlebody and some injector cleaner through the gas tank. The problem happens in 2wd, and 4wd, I have also tried putting the truck in 3rd gear and it will still do this at highway speed. I tried everything I can think of and even took it to a shop which couldnt figure this problem out. Another problem which is possibly related is sometimes when shifting between 1st and second the truck seems to bog down as if it is running rich for a split second.

The truck is a 2003 Silverado 1500 z71 with aproximatly 226000km on it.

Any help is appreciated, thanks.

---------- Post added at 06:49 PM ---------- Previous post was at 06:31 PM ----------

also this problem only happens while cruising at a constant speed, if im accelerating or take my foot off the gas pedal it will not misfire.

 

[Bigg Ron]

Have you checked fuel pressure? sounds like a you could be dropping on fuel pressure or a injector starting to fail.

 

[silveradotrailblazer]

Welcome to the club. Did you have this problem before or after you changed plugs. Alot of guys on this site have had misfire issues after installing non-A/C Delco factory plugs. When they reinstalled A/C Delco's there issues stopped.

 

[kwgreer]

I have found these NGK plugs and wires to be of excellent quality with other vehicles, but will not rule them out as a potential problem. The problem was even worse when I switched the old delco ones to the NGK. How would I go about testing fuel pressure/ injectors when this problem only happens while cruising on the highway? I can't get the truck to missfire revving it at a constant rpm in neutral. Thanks