Originally Posted by mattxr3i

I would suggest a Rolling Road diagnosis. They should be able to see what the engine is doing with regards to mixture and power and this can then start you off in the right direction for diagnosis. My friend had an ST170 and I fixed a few things which leads me on to:

As others have said, these are very sensitive to Lambda sensors but what can also happen is the IMRC (inlet Manifold Runner Control) unit can play up, as it had done on my friends:
* The gears inside can break up
* The gears inside can simply need greasing
* There is a switch inside (This was the problem with my friends - there is a switch inside the unit which can fur up, I got a nail file and cleaned the connections which fixed a lazy unit)
* The cable can snap so the secondary flaps are not activated
* The flaps in the manifold can be sticky and not return to closed

Sounds like your IMRC is not working and the ECU is expecting them to be open and therefore fuelling as if they are. This then causes the mixture to be too rich and for the shuddering you are experiencing. If you look down by the coilpack with the engine on, if you rev the engine you should see a little arm move. IF this moves the IMRC is working but, like what happened on my friends, it can be lazy.

Whereabouts are you