Flickering headlights
 

Been putting lots of miles on my Ford build. Still fine tuning a few things and next thing I want to check off the list is my flickering headlights. My engine speed fluctuates slightly at idle and the lights slightly dim or brighten. Not a lot.
I installed a 3g alt and bigger battery during the build. There are no other electric issues that I'm aware of and all electronics seem to be functioning properly.
It's not bad enough that I can't live with it, but am concerned some part of the electrical system is getting strained.
Suggestions? Anyone else have this issue?


edited to add:
I have a heavy ground from the engine block to the body and another from the lower intake to the firewall as well as the DS head to the body. The ECU and harness is grounded to the firewall as well, so I would think I'm good for grounds.

Is it just that when engine RPMs drop below certain point the 3G alt is not putting out the max voltage causing the dimming?

For example when above 700 RPM the alt is putting out a full say 14V and at 600 it drops to 12.5V.

I think so. I really only notice when I pull into the garage at night. If I raise rpms a bit they brighten up. I actually raised the rpm a bit (redid the base idle set), but still get a bit of dimming. I notice it in the interior lights too.

Probably unrelated, but I also have 29(vss) and 31(evp range) trouble codes. All the egr stuff (valve, solenoid and pos sensor) are new as I was chasing a 34 code a few weeks back. Could a bad IAC be triggering the 31 somehow and contributing to the small amount of bucking in 1st at 2k rpms?

Codes are gone. Weird things happen when you have a vacuum leak that doesn't happen at idle- hose was leaking from the evap cannister and would only happen at speed when it was being purged. Anyways, all good. No codes other than the air pump deletes (81, 82, 94, 44) and it runs great.
Anyways, I'd like to raise my idle but had read on some Mustang forums that regardless what speed I set the idle to, the computer would drop it back to around 650rpm after a few cycles. Is that true? What do the guys with big cams do to keep their idles up to keep from stalling?

The EEC wants to control the idle and has its "target" RPM it tries to get to. But you can adjust the throttle blade stop so that mechanically it stays open enough to maintain a higher idle (the EEC can only fully close the IAC valve and mess with timing maybe but won't be able to get it any lower due to the throttle blade being open further). After doing this the EEC may throw a code "can't control idle" or something like that.

If you have a chip installed and get it tuned, the tuner simply programs in a higher target idle speed for the EEC to manage to (the proper way to raise idle RPM I suppose)