Help understanding random stumble at idle

[Shaman]

After changing the supercharger on my Saturn Ion Redline (stock to ported), installing bigger injectors, and installing a supercharger spacer(comes with gasket), I have picked up a stumble at idle.

Vacuum doesn’t seem to change much, usually hovering at 20 in/hg, then dropping to just a hair above 20 on the gauge when idle drops to 800 RPM. WBO2 shows a drop when the stumble hits. A few days previous, I had the header changed to a better one with a new gasket. The downpipe was clamped, not welded in. I will get it welded in the future, as it does sound like there is a leak.

With the car at warm idle, I sprayed around the supercharger spacer, around the “new” injectors (rebuilt be Deutschweks in OKC), the vacuum lines that are visible, and the intake to TB connection with brake parts cleaner with no noticeable change in idle speed. I think there is one more hose I need to check. Where else should I be looking? There are no driving issues that I am aware of when not at idle.

 

[HangFire]

Check if EGR actuator is moving (if it has one.)

 

[hansj3]

Are you running too rich? What kind of engine management are you using? Are the injectors spec'd for your fuel pressure?

 

[CT8]

Spark plug gap??

 

[Shaman]

Hangfire, no EGR, only PCV strainer (think small wire mesh coffee strainer).

Hansj3, I drop down to 16 on the AEM WB when the stumble happens, but usually am around 13-15. I have a new narrowband that will get installed when the exhaust gets fixed. So unless the WB is set up wrong and the NB is offest somehow I don't think so.

CT8, I did get spark plug blowout once at the top of 3rd when I hit 18 psi, but I am at around the generally accepted heat range and gap. Iridium plugs that are only about 15K miles old.

I am thinking/wondering if this would be a DBW dirty throttle body problem; sticking etc. Does a DBW TB open and close like a Idle Air Bypass valve would normally do?

 

[GMBoy]

Just a random thought - have you considered using copper plugs? With the performance mods you have, a copper plug may be a better choice and since they're easy to change and cheap you shouldn't mind doing them every 30k miles. The reason I say this is because I have an 87 Buick GN that is slightly modified. The usual bigger fuel pump, injectors and turbo. OEM plug was copper, which was not unusual for that time peroid, but I ran platinum plugs with good use until I upped boost to 18-19psi where I had stumbling. I went back to copper core plugs and the issue was solved.

 

[Shaman]

GMboy - People will similar mods are running the same plugs. My stumbling is only at idle 15+ seconds after coming to a stop.

 

[The_Eric]

What are your fuel trims doing when it stumbles?