allthrottle&nobottle

I have a 1999 GMC NEW STYLE SIERRA 1500 2WD REG CAB SLE 4.8L V-8 AUTO 3.73 POSI EVERGREEN AIRAID RAM AIR, FLOWMASTER FORCE II CATBACK and Predator.
Over the last month at 85K it started to misfire under load, buck in OD at 50-60mph while trying to speed up and would not like to go over 4500rpm.
Started reading threads on here, LS1 & CF.
No codes, LT/STFT looked ok, O2s ok, but 8 of KR!
Regular maint is done, and I put in Rapidfires 30K ago.
Started looking for issues...........nothing obvious!
Put in new front 02s........a little better(originals any way!)
Cleaned the MAF......a little better.
Plug wires are original but read ok.....(getting some OEMs due to age & mileage)
Put in 32oz of decarb........after 2 days all is ok & no bucking
Put in fresh TR55s last night.........even better and revs normal now
KR max of 4 which is about what it always had.
Good to read prior posts..........stuff like this has usually happened before

Steve Bryant

iTrader: (1)

Have you changed your fuel filter? The plug wires are a good idea too. Can you hook the truck up to a scan tool and look at any misfire history that you might have on any of the cylinders? You could also be getting some form of knock retard due to bad fuel, but I doubt that.

I'd change the fuel filter and plug wires just for grins right off the bat. See what happens and then look deeper.

All my best,

Steve

GM Muscle

iTrader: (9)

I was having the exact same issues with my truck as you where, and it seemed to come and go. If the misfire and KR comes back check out your cats. I pretty much did everything you did plus alot more and it turned out to be the cats where dead. I had them replaced yesterday under warrantee and what a differance. It's a whole new truck. Funny thing is, I've been fighting this issue for more than 3 months and the truck never threw a cat code until this past Monday.

allthrottle&nobottle

Thanks guys,
Forgot to mention the fuel filter was the 1st thing I put on. Usually do it at 50K but I wanted to see how far I could go on OE parts..........
I though bad gas too, but it wasn't.
Seems fine now and will rev to red line w/o issue.
I had no idling problems or codes....I disconnected some stuff to see if my scan tool was ok, it was!....carbon will do that!
Muscle, I read your old thread as I searched..........I checked my cats but they are ok. Codes do not always show up mechanical issues