Do Stock Trailing FC Coils batch fire above 4K RPM?
 

Just got my truck running with the E420D, starting to poke around at the idle/no load tuning, and I noticed my aftermarket tach takes off for redline above ~4000RPM, and under ~4000RPM it reads normal. I am pretty sure my tach is connected to the stock tach output on the trailing coils. I am assuming instead of firing each coil separately, it is switching to firing both coils at the same time, therefore doubling the tach output signal, making the tach reading jump to 8000RPM and up.

I am using the FCNA map on the link below:

FC3S Non-Turbo NA base map for S4 & S5 PNP and wire in 420 & 440 models.

Also on the base map above, I am at 8.9:1 at idle without reducing my master trim by -35%, but I am at about 50KPa of manifold pressure at idle, which puts the fuel table at a cell reading 74% when in VE mode, right where the fueling table stops rising drastically. I am going to assume I have less vacuum than what this map was made for. I get the day off tomorrow, I will play around, only done about 30 minutes of tuning/verifying sensors work.

Is your aftermarket tach programmable?

Answer to your question is NO.

I have not worked on a n/a car in over 10 years, and when I last did they were pulling around 17-19 inches of vacuum at a 900rpm idle. 14.5 or so( 50kpa) seems pretty low to me. I would just adjust your idle VE areas down 48-50 range etc. I made that base map out of thin air. Its always better to have the car run rich, then so lean it pops/hunts/stumbles or dies.

I doubt my stock tach is programmable, its a cheaper one, but has worked fine with the stock ECU for 2 years, and I have not touched its wiring.

I have my aux ports wired open and I have atkins sleeves in the ports, and a port matched intake. I bet the ports being open at idle makes a difference.

I have to poke around with it some more. If I lean up the idle to about right, then if I hold the engine rpm's at say 2500RPM, the AFT will climp up to over 17, then cut out, then be back in at 12 then go back up to over 17, and keep doing that.

Would a stock S4 TPS that shows 100% TPS at anything over idle be an issue?

You just need a 560 ohm resistor wired in like this diagram( this is also called the pull up resistor)

How to Connect a Tachometer to an Aux Output for Select ECUs

Wiring the ports open does make a big difference in the tune. I would personally use one of the aux outputs to control the OEM or an aftermarket solenoid to open the AUX ports. It is costing you power below 6000rpms.

Stock S4 TPS reads 100 percent at anything over 25 percent I believe.

It sounds like the car just need more fuel in the fuel map at that RPM point.