V8 Rebuild problems

[diesel-smoke]

Had the head gasket go on my 3.9, so stripped the top end down, heads checked skimmed and rebuilt, and all back together now with new gaskets and bolts.

However, when I come to start it, it immediately goes to max revs and stays there.

I have checked all wiring connections, hoses etc have been put back correctly, and all seem fine.

Anyone else experienced this, or have any ideas what might be causing it.

[Devon-Rover]

Even with the Idle by pass screwed open it'll rev to 2 maybe 3.  I would go over basic's like the throttle is fully closed and there isn't a leak on the pipework and the plenum is sealed. does it do it with the Idle control unplugged?

[diesel-smoke]

Yes, have backed of the accelerator cable so it is complete slack, and checked the flap inside to make sure it is closed.

There is no difference whether the idle control valve is connected or not

[Budgie]

I had something similar on a 3.9 Disco. This was a few years ago now but IIRC it was the Stepper Motor on the back of the Plenum Chamber which was at fault and needed replaced.
You can remove the motor and clean it, then test it's working. You'll need to search for the testing side as I can't remember what to do.