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Hi
I've got a 1998 alfa 155 2.0 16v. I think its the later engine that was taken through to the 156 and GTV.
After the car is warm, at rest the revs rise from 1000 to 200 and back constantly. Also when I take my foot off the accelerator the car slows gradually but sometimes jolts slower still as though a switch has been pressed.

I'm presuming something is sticking a somewhere? throttle control body. Could this be affecting the idle as well.
Ive already replaced the idle control part.
thanks
 

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I'm guessing it has cable actuation for the throttle. If so, and it is the plastic cam cover, the engine is similar to the original CF1 as it is a CF2. Does it have the CF2 plastic inlet manifold? If so, it would be a good idea to wire it up.

As to the idle fault, try cleaning he throttle body and butterfly. It is probably just an accumulation of dirt which reduces airflow when throttle is closed.
 

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Throttle bodies are easy to clean. Remove the induction pipe first and disconnect breather from cam cover first. Ideally, to clean it really well, remove the throttle body itself. I think there are T30 x 4 Torx bolts holding it to the inlet manifold. The throttle cable and some wiring multi plugs would need to be disconnected but that is simple enough.

If it has the black plastic inlet manifold for the CF2 engine, ideally the car should be modified and have the correct map to run it. However, that is involved work and expensive. That said, due to the way it all works and if you have/can get hold of CF2 vacuum pipework for the vacuum actuator, it should work well enough if it simply shared a feed for the inlet camshaft timing variator. The changeover points do not coincide exactly but I think it would provide a bit more mid range torque.

Disregard that if it has the standard CF1 aluminium inlet manifold though as this does not have variable tract length runners.

I'm not aware of 155s having the CF2 engine as the 145 & 146 later had. Production of the 156 begun in 1997 and it replaced the 155. Your 155 may just be a late registered 155 or it may be a bit of an anomaly (which did happen with Alfas). Is the engine ECU mounted on the throttle body, or is it inside the cabin?
 
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