I have recently had my engine rebuilt after a big end failure. It has a misfire at 3700 rpm only on full throttle in 4th or 5th gear. The car splutters and back fires. I am told the rpm needle flicks to zero for a split second during the misfire. The ECU is not reading any error codes and plugs/coils have all been changed as well as the throttle position sensor. Earthing points have been checked as well as any electrical contact points (apparently the injectors have been checked as well). I am told the backfire is similar to switching a bike engine off while running at speed then turning the ignition off. The car was supposed to have been checked on a gas analyser and ruled to be OK. Am I right in assuming it must be an electrical fault? Should I get the immobliser removed before a rolling road session (to rule that out)? After the rebuild they forgot to connect the engine block earth, could that have damaged the ECU? Has anyone had a similar problem like this? Would the backfire be damaging to the turbo?

The people who did the rebuild are telling me it is beyond their expertise and that they are not prepared to pay for the rolling road session despite the car misfiring since the rebuild. Any advice would be greatly appreciated before a costly rolling road session with an electrical expert.

Yes definately electrical problem. The speedo's are electronic, so when you turn the car off while it is moving the speedo will drop to zero instantly.

It will most likely a problem in the ignition circuit.

If you ECU was damaged it most likely not work at all.