2004 6.0 suddenly running rough - no codes

[ktg]

My 2004 GMC 2500 with 6.0 suddenly started running rough several weeks ago but there have been no codes.  It starts perfectly and seems to run a little better when it's cold. When hot, it idles like a dragster.  When accelerating, it suddenly will start running fine around 2500 rpm and then back to rough when it shifts to a lower rpm.  Going slightly uphill around 45 mph with cruise on, it shakes pretty bad.  Mechanic said there's nothing he can do with no codes.

Fuel trims are close to zero at idle and jump to 8-10% under load.  I thought I found the problem on scan tool.  It said it was calling for 117 on IAC but position was at 1 and didn't move.  I then realized that it doesn't have an IAC.

Installed new plugs and wires about 2 weeks before this started.  Anyone have advice?

[greatmizzou]

I’m no pro here, but you would think the shaking would be easier to diagnose first?

 

No codes thrown would just mean all electronics are throwing info to the PCM that is within spec?.  Since tons of things are not connected or monitored by PCM, start there.

 

I was surprised when my U joints failed how crazy it manifested into vehicle shaking.   

 

I don’t understand how idling issues would manifest as vehicle shaking unless the transmission is not handling the engine power and torque appropriately.  Low rpm maybe engine is struggling to burn fuel?

 

It totally sounds like my old Saturn that had crazy idle because my coolant temp sensor needed replaced and thermostat was stuck open.  After replacing the pump, thermostat, and sensor it purred like a kitten.   (I think i also replaces o2 sensor for good measure)

 

Engine mounts could be shot too.

 

 

wel I’ll be quiet and see what the smart guys have to say lol