Surging idle
Thread Starter
Teching In
Joined: Apr 2019
Posts: 11
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Surging idle I have a 96 camaro lt1 I just put afr heads and Howard's cam in it along with a 58mm throttle body roller rockers and lightweight flywheel. Pcm for less tuned my comp, my problem is my idle is everywhere sometimes it will be about 500rpm then next it will be 1900rpm it changes at every stop at a red light when it idles low it will continue for about 5 seconds then jump up to normal idle I blocked off the elbow that comes from the valve cover to the throttle body and plugged it at the throttle body is this my problem? Or something else
Most likely you are experiencing the problems caused by aftermarket throttle bodies. The three biggest problems people experience with them are:
-Idle set screw not properly adjusted
-Lack of idle "bleed hole" which allows idle circuit to have air flow regardless of IAC position
-Incomplete idle circuit due to gap between intake manifold idle circuit port and IAC port on throttle body
It can be any combination of the these issues, I know this is what happened with me. Now I am able to have great idle with the a/c on regardless of engine temp.
I have a 230/242 cam just for reference.
-Idle set screw not properly adjusted
-Lack of idle "bleed hole" which allows idle circuit to have air flow regardless of IAC position
-Incomplete idle circuit due to gap between intake manifold idle circuit port and IAC port on throttle body
It can be any combination of the these issues, I know this is what happened with me. Now I am able to have great idle with the a/c on regardless of engine temp.
I have a 230/242 cam just for reference.
my money is on this, common issue with aftermarket TB's as they are engineered to work on TPI and lt1 motors so the bleed hole isn't quite right for an lt1. what brand TB do you have?
TECH Regular
Joined: Nov 2011
Posts: 471
Likes: 14
From: Tucker, GA
You can also plug it up and watch the IAC counts. That will tell you a lot. (Can you do that on a 96?) Do you know if the builder adjusted the TPS correctly? If not, as others have said - that's most likely the source of your issue. A massive air leak will also cause a bouncing idle.
if you can't datalog the car, do not try to fix the idle because you will be working blindfolded.
nice thing for OBD2 is you can get one of those bluetooth connectors and run datalogging software off a phone app.
nice thing for OBD2 is you can get one of those bluetooth connectors and run datalogging software off a phone app.
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From my experience you need to either completely remove the pcv system or completely retain it. Every piece of it is necessary to it's function and you handicap it when you take parts off you don't like.
Thread Starter
Teching In
Joined: Apr 2019
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Thanks for all advice it is a trick flow TB. The cam is 585 lift with 308 duration 113lobe seperation and I built the motor myself and my TPS doesn't have. Slotted hole for adjusting idk if there is Nother way to adjust it but it deff dosnt have the slotted hole.and yes I ported the intake to fit the TV gasket which was the same size of the tb.
Slotting the TPS is a modification some due to help with aftermarket throttle bodies. I was able to make mine idle great with the A/C on using the "bleed hole", adjusted idle screw, and completing the idle circuit. I'm not a fan of slotting the TPS because this will increase the likelihood of split BLMs.
When the air at idle enters through the idle circuit (the IAC passage from the tb through the intake) it is evenly distributed to the cylinders. If it is overwhelmingly going through the plenum you can't control the distribution.