Up and Down idle, seems worse in colder temps, need advice.

[knever3]

It seems EVERY Renix system has had this problem, mine is no different.  Well here goes, it's a Renix run 4.7L stroker with a BBK billet throttle body, 21lb injectors if I remember right and it has run like this since I put it together.  The idle chases up and down and never settles into a normal RPM.  I have the Snap On scanner and nothing seems awry, the temperature comes up, goes into closed loop runs fine.  I would like to fix this mostly because it's so annoying I don't like driving it like I used to.  I have checked the vacuum and it goes up and down with the RPM, so I don't know if I have a leak, can't remember what the value was off hand.  I have went through the harness and got rid of the splices and added a few grounds, nothing has changed it.  I have a family member that offered to help, but I want to get a handle on it myself since he lost almost everything from the flood in 2019, he lives in Sanford near where the dams breached.  I don't want to put him out for this.

 

So where do I start? I have checked the manifold bolts, but haven't sprayed it with anything flammable to see if I have a vacuum leak.  My mind works better bouncing ideas off others in person, but the forum will have to do I guess.  Any ideas will help.  I will upload a video tomorrow if I get a chance.  I should hook up the scanner and a vacuum gauge so you all can see what I see, run it until it goes into closed loop and you will have a better idea where to throw the darts.

 

Thanks everyone!

[cruiser54]

Maybe with this?

 

http://cruiser54.com/?p=94

[knever3]

https://youtu.be/keSWerL3qPY

 

https://youtu.be/RDy_Tm0VoVQ

 

Okay, made some time to take a couple of short videos of it running with the scanner, vacuum gauge and fuel pressure gauge in the mix.  To me it looks like the vacuum at idle is good, I tried using MAP gas around the intake manifold and the engine didn't move off idle, so it's safe to say I don't have a vacuum leak.  The fuel pressure stays solid.  As for the throttle body I have the BBK newer style one.  It seems the idle air control motor isn't working like it should, that should not change at colder temps though.  I have replaced the IAC motor before.  I changed the TPS as well while putting the motor together.  I have not replaced the intake air sensor or knock sensor or temp sensor.  I wouldn't think my cam would be the cause of this.  Here is a picture of the specs of my cam as well, stock valve train.

 

 

 

 

 

 

 

 

[cruiser54]

If the throttle butterfly is too open or too closed, the IAC has trouble controlling the idle.

[knever3]

What is the normal idle speed?

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[Ωhm]

750 ±50rpms.

[knever3]

Well, that did the trick. It was dipping to 540 RPM's and then chasing the idle. The set screw was loose, it probably worked its way down over time. I started with some shims to determine how much to adjust it. I was able to adjust it from the bottom even though the screw is upside down to prevent tampering.

I will drive it this weekend to see how it feels, thanks a ton guys!!

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[knever3]

That didn't last long, left the truck outside overnight to see how it would do in the cold.  Wife gets up and says "fixed the idle eh?"  Back to the drawing board.  Any other ideas?  I took a small video this morning of it.

[cruiser54]

drive it awhile.