04 GMC Sierra w/ 4.8l @ 134k miles. Stock intake tube into k&n cone filter w/ throttle body spacer. drive by wire throttlebody

Trucks been acting a little weird for some time now. On a cold start it fires right up, but when you start driving and let off the gas the RMP's slam down to idle, were talking snap of a finger. It only does it for a few minutes, until it starts getting warmed up, then it wont to it anymore.

Any ideas as to why it would do this? I took the throttle body off and cleaned it, sprayed down the MAF with MAF cleaner, and tightened intake bolts down. still nothing.

any bright ideas lol.

 

If the filter is attached to the maf that could be your problem..Its been shown before that the air is not stable enough going across the maf with a filter attached to it..Try adding a short section of pipe to the maf..

 

Also, how cold has it been in the mornings when you start the 4.8? I know that here in the mornings its just about 10-30* sometimes down below 0 during the night time and in the mornings my truck starts no problem but has a hard time idleing on its own and I think that is just due to the water in your fuel getting cold and trying to ice up in the fuel lines. Usually a can of HEET or Seafoam does the trick with a fill up to remove the excess water from the fuel system.


If you just did a good cleaning on the TB and MAF then the truck may have just been trying to re- learn the air flow to the truck since the settings have not been changed and now the system is reading more air flow and its trying to re learn the air fuel mixture on its own, but it sounds more like a cold day and water in the fuel system.

 

the truck has done it since I got it.. which was in the summer.


im assuming its my intake tube setup. ill try re-working it, or getting a diff setup.

 

Probably leaky intake manifold gaskets allowing a vacuum leak until the engine warms up.

 

i thought that to..

i tightened the heck outa the intake bolts tho and it didnt change

 

tightening the bolts won't fix a bad gasket.

try the intake tube suggestion first and if it still does it you can pull it indoors and spray some carb cleaner around the intake gaskets with it running and see if it starts to rev up. If it does then you need new gaskets.

 

i did that on a completely cold start yesterday morning..

 

can you hook up a scan tool to it and look for codes and take a look at the sensors. If so write down the rpm, maf volume, tps, intake temp, and long and short term fuel trim. At a few different times like idle cold, 2,000 rpm when driving, after you let off the throttle when the problem happens, then the same when warm and the problem does not occur.

 

ya right.

it only codes downstream 02's as the previous owner gutted the cats.