Kerr

So, you are sure the cats are fine?

The miss fire could be lots of things... these trucks seem to throw that code a lot. what you can do is rotate #6 wire to say #1 and see if the miss fire moves.

By chances could a brake be dragging just a little. My suburban did this.. just a little bit.. i never noticed a smell or anything just slower, bad fuel mileage, etc.. Then it got hot on the highway cought on fire and i found my problem lol..

If everything checks out i dont know what to tell you.. Im still guessing that its cats.. and yes stock mufflers dont flow for crap. Im supprised it did help by cutting it open.

If you have a saw and welder just hack the cats off. look at them if you can see threw it its good if not its clogged... ( then "dont drop them on the ground because the stuff will come out... what doenst come can be knocked out with a scoket extension.. i mean if you happend to drop it on top of a socket extension ) then weld them back on lol...

BlackGMC

iTrader: (31)

get a tune, the HD 6.0l trucks are insanely detuned from the factory, they need some timing...

2manycars

iTrader: (7)

He said the misfire was only a stored code and did not come up on test drive so that is not the problem, also the exhaust back pressure checked good so hackin' cats will not solve the issue either. I'm a firm beleiver in checking the mechanicals first, even though this sounds like it could be a sensor/ecu/tune type of issue those electronics are only functioning correctly if the mechanicals are ok. Check your mechanicals first easiest to hardest, exhaust backpressure is a great start then go to vacuum and compression. After that it may be time to check the lift on the cam(could be wiped out). Once you have eliminated mechanicals then you can go to the tedious task of isolating and checking sensors (MAP, TPS, etc.) and have someone look at the tune in the ECU. Good luck.

TJ

iTrader: (10)

it hasnt always been a dog. this is a current issue.
the only thing i can do now is a leakdown or a compression check, which still may not be the answer, but its something i need to do.

i was told to unplug the MAF and go drive around and see what it does. if the ecm is getting fed crap, it will run like crap ??
its something to try.

philntx

iTrader: (8)

Read this: http://www.aa1car.com/library/converter.htm

TJ

iTrader: (10)

thanks for the link. ive read that site in my searching. truck is still down on power a bit but running. i checked the exhaust before and after the cat with a pressure gauge. no clog in the muffler or the cats. it doesnt even read a pound of backpressure at 2500 rpm.

Tootall

iTrader: (79)

any miss fires ? Checked the coils ? . . . Battery still good? Grounds ok?

TJ

iTrader: (10)

yes. everything good. 147K on the truck and its never had a code other than an abs sensor that went out at 80K. buddy at the dealer said there was 300 misfires logged on #6, but he said one or two cylinders usually show up like that. said its pretty normal?? when you get into the thousands is when you have issues. truck actually has a brand new battery. cold snap got the old one.

turbo lcc

Im interested in what you find, I recently bought an 01 suburban 2500 with a 6.0 and its a little low on power too.

TJ

iTrader: (10)

im going to change the tranny fluid and filter and see what the fluid looks like.
tranny going south, converter fins bent strait is the only thing i can narrow it down too.
going to try and get a compression check tomorrow to see if its off. if the compression checks good, im going to pass on a leakdown, but if the compression is off, im going to leakdown the worn cylinders.

i have seen more than one 5.3 eat cam/lifter, but it usually makes a tapping sound. my buddy JC at the dealer used to do one once a month it seemed like. i doubt thats the issue. motor is super quiet for 150K.

8mpg is killing me when im used to getting 10.5-10.9. i drive between 80-130 miles a day. my gas card is on fire.