4.2 I6 wont start after engine change???? HELP!!

[nitrorocket]

I have a 2002 trailblazer LS with a 4.2 I6. Truck ran perfect, but developed a rod knock.

So I purchaced the same year engine from a salvage yard with lower miles and installed it. Now it cranks but will not start.

I hooked up my EFILIVE tuning software and get these codes even immediatly after they are cleared.

-P1681-Control Module throttle position(tp) system
-P1635-5 Volt reference 1 circuit

Truck cranks but will not start, I smell fuel from the tail pipe, and both the CEL and Reduced engine power lights stay on.

Keep in mind, the truck had no lights or peoblems at all before I swapped the engine so I am not sure what could be the problem.

I swapped throttle bodies between the motors and still nothing.

Any help and ideas would be appreciated.

[Taz]

This may sound crazy ... but check the electrical connections to the engine cooling fan. If memory serves its on the same 5V circuit as the APP. Damaged wiring / defective fan may cause a fluctuation (or spike) in the voltage.

In the back of my mind I recall there was a GM service bulletin on this ... not positive.


Regards,
Taz

[nitrorocket]

I tried unplugging the cooling fan, no change, so I guess I will have to check for voltage now...