Hello,

First time posting to the forum but I am having issues with an RB25det Neo swapped 93 240sx coupe. The car was purchased from the previous owner who blew a headgasket. The previous owner had the head decked and replaced the head gasket. After assembling the car he said it wouldnt run right. He believed i was it was a vaccuum leak. Once i got the car i have been trying to diagnose and fix this thing for the past 5 months. Im still learning alot and have gone through so many forum thread its ridiculous. The car itself has an issue with running super rich (9.50 stoichiometry). It also idle hunts really badly and unless my foot is on the throttle it wants to die. Below is everything i have replaced or added.

Replaced or added:
- Injectors (Still Stock)
- TPS Sensor
- CAS
- MAF
- Timing Belt w/ water pump and idler pulley (Was off by one tooth)
- Alternator (The new one stopped charging the battery so i need to look into this...)
- Coil Packs (Savage performance)
- Spark Plugs (NGK BKR7EIX pre gapped to .8)
- Added a Fuel pressure reguator w/ Gauge to watch fuel pressure. (Keep it at 43 PSI)
- New ECU (AA502) w/ nistune type 4 board
- Added a AEM UEGO X wideband
- Intake manifold & Collector Gasket.
- Wiring Specialties Harness (Purchased by the Previous Owner but is in great shape)

I have vacuum tested the intake with 25 psi pressure and their does not seem to be any leaks. I have donw a compression test and it was 140 psi on every cylinder (Cold). I need to fix the alternator issue because the voltage is at 11.6 with the car running but what else could be causing it to run like it is...
Attached is a video of it running with some gauges up. ECU is only throwing an ABS & Traction Control Code.

https://www.youtube.com/watch?v=VWK3fsNI4qw

I'm not familiar with those engines, but is there any adjustment to the idle air control valve? You might want to induce a vacuum leak just to keep it running. You could also adjust the fuel pressure down to get it to run better as well.
At least at that point you could start doing diagnostics, check ignition timing, etc.

Sounds like a vacuum leak, I went through hell trying to find all of mine. check all couplers, vacuum lines, etc etc. Even check the exhaust for any leaks, can cause it to run rich and throw off wideband numbers. I know you checked it to 25psi, did it hold that pressure?

Reason why I go to vacuum leak, because its the most likely culprit. Youve got everything else checked off ...timing, fuel pressure, maf etc etc. So start w/the easiest/cheapest thing to repair.

Good luck! Ive got one of these w/HE351CW and it is super fun !!