2010 tundra starts/dies instantly 5.7 flex fuel

Hello, I have a 2010 tundra starts/dies instantly 5.7 flex fuel. Truck has 147,000 miles on it and has never had one issue. The battery died recently and the truck would not even crank over. Battery was about 6 years old so I replaced it. Now I have a crank and start but instantly die after half of a second. It will not always start also. Sometimes you have to cycle the key two to three times for it to start / die within the half second. First thing I did was scan it and found a b2799 code. The key in the immobilizer are working. When you install the key the red security light goes out. Also using a scan tool I can check the immobilizer and it shows the key is linked and registered. I have tried using the spare key. I also reprogrammed both main and spare key. Fuel pump is running. It does have spark. I checked the fuel pressure at 50 lbs at the rail. I've also checked the maf sensor and check for carbon in the intake at the butterfly.

 

Fuel pump relay?

 

No, fuel pump relay is functioning and I have fuel psi. I have also determined after it dies, then I lose spark and injector pulse until I cycle key a few times, it will start then instantly die. I believe the engine immobilizer ECU has failed

 

This could be the alcohol sensor in the gas tank going out. As a Flex Fuel owner I had my reservations about that sensor going out and having to drop the tank to replace it.

 

REPAIRED.

Immobilizer ECU was the failure. Service procedure claims you have to remove the dash and the AC/heater box. Immobilizer ECU is mounted to the front side of heater box right above the hard ac lines extremely tight to fire wall. After removing lower dash panels on both sides and unbolting steering column but not removing you can gain limited access. I also removed left side lower HVAC vent for driver's feet. I used two door airbags place between firewall and heater box for lockouts to gain extra space. I use a 90° hose pick tool to unsnap ECU. Once loose I was able to retrieve ECU and ECU harness out above heater box through glove box hole. New ECU has to be programmed and keys programmed. Once this was done still had to ground out pins 4 and 13 on the OBD2 plug for about 45 minutes to resync ECU and immobilizer.

I hope this helps someone else cause I have a ton of time wrapped up in this repair

 

Thanks for follow up and
the info.