Hello All, been lurking for a while and a lot of the info I have found here has been really helpful. My first project car and I'm loving it. However I'm stuck with this idle issue on my '86 N/A S4.

Some info:
- Car is pretty much stock. All emissions, thermowax, BACV are all present.
- P/O added a aftermarket radio + bazooka tube sub (I have the sub disconnected, radio is always off)
- P/O installed aftermarket power window switches, they work fine.

Issue: Car idles OK...No hunting, no pops. I have it set to 750RPM, but my idle adjustment screw on top of the throttle body is backed out A LOT? If I have it 1.5 turns backed out from closed, the Idle will be really low. It might be 2-2.5 turns out right now... I have a rubber cap on it so it doesn't vac leak through the screw. Car drives good in the daytime, no hesitation on acceleration. However when I'm driving at night (headlights on) and I come to stops (holding brake pedal) my idle can drop down to 400-500RPM. The car never really wants to die, but it always makes me nervous. The idle+voltage will also drop a lot when I turn the blower on to MAX (no A/C, just blower on max). I don't really see the voltage drop with just the brakes lights coming on, but once I turn the blower to MAX it goes down to like 12v. Other little electrical loads (power windows) will cause the small dimming of dashlights.

Things I've Tried:
TPS - I've set my TPS to 1 volt, at the connector. I made sure to warm up the car, and made sure the thermowax has completely pushed the fast idle cam off the roller. Tried dual light method as well.
BAC - Since the idle drops under electrical load (from what I think..), I thought it may be the BAC but haven't had much luck. I've spliced in a new connector plug, cleaned BAC, checked opening/closing when connected to battery, Ohm readings OK. tried 2 other BACs. One thing I noticed is if I have the BAC unbolted so I can see internals but still electrically connected. From what I've read when the car is in the ON position the BAC should vibrate. I can definitely hear the BAC vibrating. But I don't really see the plunger moving all that much, definitely WAY less than how much it moves when connecting directly to battery. Is this normal? faulty BACs? BAC not receiving correct signal from ECU?
ECU Transistor - I've replaced the transistor in the ECU that can go bad and send a bad signal to the BAC
Grounds - I've cleaned up/redone the standard grounds, battery to chassis, engine to chassis, ecu to engine (from Aaroncake's write up). Maybe I'm missing one of the smaller grounds from when the lights/brake lights come on? Or a switch connector/ground?
Alternator - This car used to have a stock S4 alternator. I installed a reman'd Ford Taurus alternator thinking the stock alt just wasn't putting out enough amps anymore to support the electrical loads. I get 14.4v at the battery with car idling with lights off. Idle drop still present when head lights + step brakes on.
Timing - I've checked timing, lining up with marks on the pulley. One thing I find a little strange is when I reset/restab the CAS (have top open to make sure it's not turning), the car will start but when I check with a timing light, the pulley mark is off by quite a bit and idles very low. Once I realign the marks the engine starts to sounds better and idle goes up.
Vacuum Leaks - I've put new silicone vacuum lines. I made a pressure tester and connected to the intake neck. No leaks.
Brake Light Switch - I wanted to test if it was a vacuum leak/brake booster or electrical load. I had the car running with lights on, press brakes idle drops. Disconnected the brake light switch (by brake pedal) so that the brake lights don't turn on. No idle drop. I installed some LED bulbs to try to reduce any amp draws but not much luck with that either.
Fuel System - Replaced fuel filter, new fuel strainer. Gas tank was very clean. I sent out my injectors for cleaning at RC Fuel Injections and they are functioning WAY better now according to the test reports. This definitely helped on start up, but the Idle drop still exist when I switch on headlights+press on the brakes.

The injector servicing was the last thing I've done and was hoping that would rid the problem, but it's still there. I may try to reset timing and idle this weekend. Since more tweaking may be necessary after the injector servicing.

Anyone have any insight on this? I'm wondering if my alternator still isn't putting out enough amperage/faulty. If there is a ground that has poor connection. Or I've just got a streak of bad BACs.


I'd love to hear your guys' thoughts, Thanks!

 

**Update if anyone comes across this**

I redid timing, reset idle, rechecked connections to alternator, and tightened the alternator belt. I also set the TPS using Ohm method. Maybe my TPS is a little funky since I normally use the 1volt method, but I had to adjust it quite a bit to get 1k ohm reading at idle.

Idle drop seems to have gone away (yay, for now? lol) when I press on the brakes and have headlights on. I am going to monitor and update if I see any changes.

I still get a voltage drop when turning the blower to max and/or operating the sunroof open/close.