Engine revs up when starting

[johnyoung]

Fellers I have a real problem on my hands. 09 - 5.3 out of a 2 wheel drive chevy , brand new motor and trans with 900 miles on it. Install went great in my 67 SB pickup. Problem is when I start it up it revs to about 3000 rpm's then slowly comes back to idle. It even does it when I stop and put it in park and rev her up a little - I called the guy that does my harness's and computer and he has never heard of this. Replaced the throttle body twice - no change. Any ideas will be greatly appreciated.

[aknovaman]

Tune air flow startup tables are not correct for your engine and need adjusted. Another possibility is you have a vacuum leak that the initial startup opens the iac then closes it to compensate for the added air from the vacuum leak to bring the rpm down to the commanded idle speed.

[johnyoung]

Thanks for the help. I will check that out tomorrow. I need to show some pictures of our install as it is really nice and clean. Looks like it came in there from the factory. Anyone else have any ides just put her on here. I appreciate the forum as it's everybody helping out. One person here told me Chevy had problems with the 08,09 model motors with PCV valve and another guy said EGR valve. Motor sat since 09 until started about 4 weeks ago.

[aknovaman]

Is there a valve on the back of the intake that pops out? Some of the older ones had it. Looks like a turkey button that pops out if there is a backfire. It's on driver back driver side of intake near top. Another thing to try is clamp shut the pcv valve hose and the brake booster hoses. If the issues goes away, you have isolated the problem.

[dayj1]

Originally Posted by johnyoung

Thanks for the help. I will check that out tomorrow. I need to show some pictures of our install as it is really nice and clean. Looks like it came in there from the factory. Anyone else have any ides just put her on here. I appreciate the forum as it's everybody helping out. One person here told me Chevy had problems with the 08,09 model motors with PCV valve and another guy said EGR valve. Motor sat since 09 until started about 4 weeks ago.

'08 and '09 shouldn't have a PCV valve. Somewhere around 2003/2004 GM went to a fixed orifice that is cast into the truck valve cover. An '09 shouldn't have an EGR valve either. That went away on the truck motors in 2002. You also don't have an IAC valve since that is only for DBC setups and your Gen IV motor should be DBW. The vacuum leak theory is a good one and that's what I'd be looking for.

[aknovaman]

Missed the year. Actually the ecu will open the throttle body servo motor to maintain commanded rpm.

[solidaxel]

Originally Posted by dayj1

'08 and '09 shouldn't have a PCV valve. Somewhere around 2003/2004 GM went to a fixed orifice that is cast into the truck valve cover. An '09 shouldn't have an EGR valve either. That went away on the truck motors in 2002. You also don't have an IAC valve since that is only for DBC setups and your Gen IV motor should be DBW. The vacuum leak theory is a good one and that's what I'd be looking for.

Thanks for the information on the PCV system on my engine!
I have been wanting to ask the question on PCV"s

[68c10airstream]

My thoughts on vacuum leak would be throttle blade would be shut and idle would be above target set point and long term fuel trim would be on the plus side. To be honest I haven't dealt with this scenario in a while but a guess as said earlier I suspect a software issue, but I always worked on stock applications.