High idle after Intake manifold swap??
 

I switched to ls6 intake manifold and everything was fine. I decided to go back to stock LS1 manifold and when i did the swap i plugged the EGR hole on top of the manifold. I hooked everything back up and when i started the car it was revving to around 2.3 RPM's then idling at around 1.8 1.9 RPM's. I figured the gaskets were bad so i just put on some new manifold gaskets and checked all the vac hoses and everything was fine. STILL idling at 1.8 1.9 RPM's?!?! im stuck and have no idea what else it could be. I let it idle for a couple mins and drove it and was braking instead of hitting the gas pedal...

any suggestions???

Sounds like a vacuum leak to me....

- Justin

yup, vacuum leak, could be the manifold or maybe you missed a hose
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Rehab Discussion

the manifold cant get much tighter and i checked all the hoses like 20 times.

it could be torqued to eleventy-billion ft/lbs and if the gaskets leak it wont matter, if the hoses are all good, then its something manifold/throttle body related

Make sure you aren't over-torquing the bolts.....they are supposed to be done in 2 passes, first to 44 inch-lbs, second to 89 inch lbs (which is only about 7 ft-lbs).

- Justin

k thanks for the info. Why would brand new gaskets leak though? Im gonna re torque them down tommorrow and see what happens.

Yup, vaccum leak somewhere.

You can use something like brake clean or Gumout and spray all around the gasket and hose connection areas with the engine running. If you notice a fluctuation in the idle then you know you have a vacuum leak.

vacume leak

how can i spray the gasket area? i tryed spraying the plug on top of the manifold and it did nothing. i checked all the hoses. the only thing i can thing of is i torqued the bolts too much or too little.

Any area where the gasket was put on. Just spray along where the two parts come together. If there is a leak, the vacuum will suck in the cleaner and the idle should drop down for a moment.

Only things I could think of other than intake gaskets were injector O-ring seals, T/B seal and (not sure about LS1's) the booster hose. I probably got the booster hose confused with the 3.8l, they have an adapter on the manifold for the booster and it also has an o-ring. It does sound like a vacuum leak tho. Isn't there also some conversion you have to do for the PCV when you do the LS1/LS6 swap?

The only thing different when switching to the LS6 is deleting the EGR....the PCV stays the same. Booster is hooked up. I didnt change the TB gasket but if that were leaking wouldnt i hear it? Especially it im idling at almost 2k RPM's

Sometimes you can hear vacuum leaks, depends where there at tho. I usually do what the other guys were talking about with the carb cleaner, spray around the gasket areas. When your sprayin around the manifold and you hit a spot where the engine rpm changes that's usually where your leak is. Just hit the perimeter of the manifold, around the injectors and try and get around the throttle body. I'd lean towards the vacuum leak but the book lists a few other things. This was under diagnosis for the IAC valve so the valve itself is not listed, but I would still check it if the other ones don't work.

Visually/physically inspect for the following conditions:
• Vacuum leaks
• Throttle body tampering (throttle lever stop screw turned)
• Throttle plate or throttle shaft binding
• Accelerator and cruise control cables misadjusted or binding
• Faulty, missing, or incorrectly installed PCV valve

alright thanks for the help im gonna take a lot at it again tommorrow when its nicer out.

theres your answer...remove the manifold and look for cracks, sounds like you overtorqued it

Well after all that it was something i wouldnt have thought to look for. The little plastic peice on the cruise control cable got stuck and was just barely pulling the throttle open causing it to rev. Thanks for all the help guys

lol dont you just love tiny problems that cause BIG headaches.

i did my roller rockers 3 times over...turns out i dropped a spark plug or something and the gap was mostly shut so it ran like crap

yup they are the best!!!:banghead:

What did you torque them to, and in what sequence?

(if you didn't use a torque wrench, you're done troubleshooting)