Idle issue ?

[crate]

i searched the board but couldn't find anything for my truck on this issue.

 

First i have a 99 NBS 4.8 v8 Gmc Sierra 140K

 

When i go to start in the morning, the tach runs up to around 2500-3000 for a moment, then drops all the way done to 500, truck is shacking and feels like it will die(never has though). After awhile of warming up(temps here has been in the zero to 10 degree range for the past few days.

 

After that and i drive it runs fine in all speeds, but when i go to slow down at a stop, apply the brake the tach starts to jump again going from around 800 to 1100 and truck is jumping..go to take off from stop, it lunges forward(like i had my foot on gas and brake then let off)..then back to normal...

 

if i stop the truck while warm, then restart put in any gear(reverse/drive) it lunges forward again.

 

I just picked up some NGK sparkplug and will try that first, if that doesn't work any other thing to check. I had to clean out intake due to carbon buildup about 2 months ago. I look in my haynes manual and talk about how to check the IAC and whatnot, do that test tach doesn't move at all when i turn on AC..doesn't go up or down?

 

***UPDATE***

well this is weird. When i got into work i wrote this message(been doing this for about a week or so). today i had to go over to main campus for a meeting, was in office for about 15-20 mins. When out to truck started it, idled fine, no jumping...drove to campus first stop light no weird lurching at stop, or bad idle. the eninge was not at normal temps a little bit lower, so i seems this only happens when cold or at running temp....

**************

 

Sorry for the long post, Thanks in advance for any help

[Bish]

i searched the board but couldn't find anything for my truck on this issue.

 

First i have a 99 NBS 4.8 v8 Gmc Sierra 140K

 

When i go to start in the morning, the tach runs up to around 2500-3000 for a moment, then drops all the way done to 500, truck is shacking and feels like it will die(never has though).  After awhile of warming up(temps here has been in the zero to 10 degree range for the past few days.

 

After that and i drive it runs fine in all speeds,  but when i go to slow down at a stop, apply the brake the tach starts to jump again going from around 800 to 1100 and truck is jumping..go to take off from stop, it lunges forward(like i had my foot on gas and brake then let off)..then back to normal...

 

if i stop the truck while warm, then restart put in any gear(reverse/drive) it lunges forward again.

 

I just picked up some NGK sparkplug and will try that first, if that doesn't work any other thing to check.  I had to clean out intake due to carbon buildup about 2 months ago.  I look in my haynes manual and talk about how to check the IAC and whatnot,  do that test tach doesn't move at all when i turn on AC..doesn't go up or down?

 

***UPDATE***

  well this is weird.  When i got into work i wrote this message(been doing this for about a week or so).  today i had to go over to main campus for a meeting, was in office for about 15-20 mins.  When out to truck started it, idled fine, no jumping...drove to campus first stop light no weird lurching at stop, or bad idle.  the eninge was not at normal temps a little bit lower, so i seems this only happens when cold or at running temp....

**************

 

Sorry for the long post, Thanks in advance for any help

 

<{POST_SNAPBACK}>

 

 

 

First thing I would do is pull the Throttle Body and thoroughly clean it including removing the IAC(Idle Air Control) and cleaning and reinstalling. Use a TB safe cleaner. My Tahoe had a really erratic idle all of the sudden. When I pulled the IAC, it was covered in crud. Cleaning it brought the idle back to normal.

 

If that does not solve it, I would look into a vacuum leak somewhere.

[crate]

when you pull out the IAC what do you clean on it?

 

I have already clean my TB(while on truck). and i know where the iac is, just unscrew, and spraw the crap out of it with tb cleaner? then place back. do you clean the ports it goes into as well(no knowing if there are any?)

[Bish]

I used the same TB cleaner I use on the throttle body. If the TB has been recently cleaned, just pull the IAC. On my Tahoe, standing in front looking into the engine compartment, it is top right on the TB. You can remove it with 3 screws IIRC. I sprayed it and then wiped it down with a clean rag. I also sprayed into the hole where the IAC installs.

 

There is also an "Idle Re-learn" procedure of you are interested. Get truck up to temp, set parking pbrake and shift into D. Let it idle for 5 or 10 minutes (can't remember of the top of my head), then shift into N for 5 or 10 minutes. Done.

 

Good Luck.