My truck idles rough in or out of gear. In gear it idles at about 400 rpm and in park, 500. The symptom is: Smooth, Shudder, Smooth, Shudder, with smooth being about2 seconds, shudder, about one. (Is Shudder the right word? It's like when you go outside for ten seconds on a zero day, an involuntary shake of the shoulders kind of thing.) I get a consistent 12 mpg with combined city and highway when not towing or loaded. (I drive rather conservatively.) The tire pressures are at max, the air cleaner and plugs are new. the wires test good. I'd love to smooth the idle out and get better fuel economy. I bought this used a few months ago and have no service history to assist with a diagnosis.
A mechanic suggested running top engine cleaner through a vacuum line to clean the carbon out. He said it will also help with the piston slap noise I have when the engine is cold. I've read lots on the subject. It's either a miracle treatment or a really bad idea, depending on what I've read. Gm also is sending mixed messages. The local dealer sells the product, but I heard about a TSB that says don't do it.
This forum always seems to have the best and thoughtful advise. Does anyone have any input on this flushing treatment? Is there another suggestion to smooth my idle, obtain better economy and (maybe) shut up my noisy-when-cold engine? (It's amazing how many people think it's a diesel!)
Thanks for your help!

 

Get your throttle body cleaned. You should notice a pretty big difference.

 

Thank you. I'll try that.The throttle body is the part in the airstream after the MAF sensor, on its way to the intake manifold, correct? Also, should I clean the MAF sensor while I'm at it? I've heard Yes and No. Which product(s) do you recommend for each cleaning?
Do you think that is something I can do myself? I'm no mechanic but, I'm pretty good at taking things apart and usually only have a couple of pieces left over when I'm done.
Thanks again for the advice!

 

Yes, you are correct. I work at a dealership, so I had one of the mechanics here do it.

 

Thanks Dluv35!
I bought some Throttle Body Cleaner by CRC and some MAF Sensor cleaner, also by CRC. First I tried the throttle body cleaner. I am amazed how different it looks. There was a tremendous amount of black stuff around the butterfly edges and plenty on the body itself.
After that cleaning, I tried driving. There was a whole lot of improvement in the idle. Next, I got adventurous and tried cleaning the MAF Sensor. There was no real change beyond what changed from the throttle body cleaning.
I took good notice of the behavior of the truck during my next drive. The "In Gear" rpm is up to 500 -from 400 and the "In Neutral" idle went from 500 to 600. The truck idles so much more smooooothly, not perfect, but way better. (Not perfect is probably from cheap gas. I've had new and old cars that run lousy until I get gas away from the Detroit metro area.) The exhaust smell is less noxious. The dash lights don't flicker at idle anymore (due to, I suppose, the higher idle). The best part is, the truck actually moves when I push the accelerator! I'll bet my MPG improves as well. Time will tell.
Thank you again for your suggestion. I bought this demon monster about 6 months ago and your suggestion has it running better than it ever did during my ownership!