My lt1 (GM847, LT headers, 1.6rr, Mad Z28 tune) has started to misfire when it is cold. Opti, plugs, wires have been replaced, and I've swapped the coil and ICM with the ones off my friends car with no improvement. The car feels like one or two cylinders are not firing when it is cold.

Once the car goes into closed loop, the problem is gone. Also, if I press the pedal down, the problem goes away. Does not matter how much load is on the motor or what gear. It doesn't miss at idle, but as soon as I go in 1st gear it starts until 2500 rpm or so.

Any ideas?

 

Bump. Anybody have any input now?

 

Have you scanned it for codes?

 

No codes present other than a 36 high res pulse failure that started months after this. I put a new Opti on it today and it cured DTC 36, but the misfire remains before closed loop. This one really has me stumped