I just bought a 2002 Z28. Got it at a used car dealership. The car came with full exhaust and a FAST intake manifold and what looks like a matching throttle body. I don't know what size the intake manifold is 90mm, 92mm, or 102mm. The throttle body has no writing on it. Intake Manifold says Wilson FAST no numbers saying the size.

 

Looks like the 78. The stock coupler is still on it.

 

Yea it is. I am having a idle issue. When i am driving and i push in the clutch and throw it into neutral..to roll to a stop sign or red light..my idle won't drop it will stay at around 2000rpms..when i come to a complete stop after like 5 seconds it will drop back down to normal idle. The idle in this car is 1000-1100rpms.

It doesn't happen at a cold start.

 

My car did the same thing after I put a new fast intake and fast throttle body. My throttle cable was way too tight and I also had to adjust the set screw on the throttle body. Originally I thought it was in the tune but I asked my tuner and he said the tune was good.

Hope this helps.

 

sounds like a idle air control(IAC) problem.

 

Try posting this in the PCM Diagnostics & Tuning section. You will get more accurate answers.

 

I heard that its a common problem on the LS1. The throttle cable doesn't feel that tight. It feels a little loose when i pull on it. I am taking it to the race shop on friday to have it looked at. how much Hp/tq u pushing with your mods?

 

396/350

 

Can't you take it back to the dealership?

 

IAC is what i originally thought.. i heard that if it was the IAC is would be happening all the time...maybe the IAC needs to be pulled back a little in the computer

 

You could also clean your MAF.

 

i could take it back...the car runs great otherwise..i don't mind dealing with this problem..its not my daily driver. i don't know if the stock coupler is on it.

 

do u have a stock computer in the car?

 

Gotcha. You mentioned taking it to the speed shop Friday. I'd take it back and save yourself some cash

 

lol...i really don't want to take it back though....its a solvable problem right?

 

Definitely. Re post your question in the Diagnostics & Tuning section. There are a ton of people that can help you more than I can. Don't wanna lead you down the wrong path

 

will do. So u think that the intake manifold is a 78mm? and how could u tell the difference between them>

 

The 102 would have different fuel rails on it and the 92 only comes in black I think, so that makes me think it's the 78...

 

That doesnt matter. The stock couple fits a 92mm tb if you stretch it.
This can be a lot of things. It could be the tune, or a vaccuum leak. The iac going nuts would cause a very erratic idle. Either you have a vacuum leak, blade too far open due to the cable being too tight or something in the tune is telling the iac to stay open for too long. does it do it in park? If so let the idle settle and then unplug the iac and retest, you can do the same thing driving around but it may cause the car to stall

Not a serious problem just a bit of a pain.

see above answer. the iac is fine.
The 92 used to come in gray when it was first made but they all say 92 on the top of the intake. The 102 looks completely different than all of them. Fast also made a 90 which is exactly the same as a 92 minus the 2mm difference in the opening but there is no power difference between the 2.

At any rate that's a 90, the tb is an older nw who never made a 78. Thats good for you op since the 78 was only marginally better than the ls6 until you ported the opening to an 85.

 

So what you have is known as the cruise control effect. Too much base running airflow, a throttle blade that's open too far, or the follower and cracker tables in the PCM haven't been tuned for a larger TB can all contribute to this.

I would recommend taking it back to the dealership and getting more details on the modifications done and whether or not they know a reputable tuner who can adjust the base running airflow or follower/cracker tables and if they will pay for the adjustments.