Miss-fire on 2003 6.0L

[rhumfeld]

Hi, i am looking for help on my 2003 Silverado 2500 HD. It has the 6.0L with 115,000 miles on it. I noticed a while ago, (2 months), that the truck was idling kinda rough. It got worse after a while and the engine light now comes on at an idle or when coasting or city driving. It also chuggs on starts from stop lights/stop signs. When sitting at the stoplight, the whole truck will shake and idle rough. It has not very much power on lower RPM (less power than I believe it should have. I can't spin the tires if i tried). Once the truck revs up it seems to be better. I have had a few different mechanics look at it. There are many, many miss-fires on cylinders 4 and 5 it is constantly racking them up when the scanner is connected. We have tried new plugs and that does nothing. We swapped injectors on 3 and 5 (one a good firing cylinder and a not good one), and it does nothing, swapped the coil packs on 3 and 5, does nothing. We finally decided to try a compression check and on cylinder 5 at idle it read 45 pounds. When i rev it up some, it reads 65-70. On the good cylinder (3) it reads 80 (i think) at idle, and 125 or so when it is revved up (2500 RPM.)

Basically i am just wondering if there is anything i can do to fix this (and what is the cause of the poor compression, or do i need to find a different engine to swap. Thank you in advance for any advice/ideas!

Randy Humfeld

[jyount]

"We finally decided to try a compression check and on cylinder 5 at idle it read 45 pounds. When i rev it up some, it reads 65-70. On the good cylinder (3) it reads 80 (i think) at idle, and 125 or so when it is revved up (2500 RPM.)"

HUH?
You don't check compression with it running. As for the problem, maybe a vacuum leak in the intake tract between 3 and 4 ports? That should throw a lean code though. I don't know....Mechanic talking cylinder contribution maybe? If so they should all drop rpms the same, the ones that drop the least are obviously the worst cylinders, but you probably knew that.