Pontiac Bonneville '96, runs rough on low RPM's until it warms up?

[Eki]

Hello, I have a problem with my Bonneville '96.

When the engine is cold, it runs rough, feels like diesel boat motor.

I've changed air filter, fuel filter, and spark plugs.

Occasionally it lites up Check Engine code P442 (EVAP Small leak), but it happens only with near empty tank, and I think it is not related to this problem.

I did't notice ant significant change in fuel consumption, and once the engine reaches work temperature, it runs steady, but still feels underpowered.

What are the next things to check / replace?

Thanks!

[Rob E]

Is it a 3.8l 0r a 3.3l? Either way im gonna bet you have a vacuum leak in you intake manifold. It runs rough till the heat expands the manifold and then seals itself up. As it cools it shrinks again. Remove the oil cap with the engine running and see if the idle changes if it does then you need to get the intake gaskets replaced. A clogged catalytic converter would also cause the same symptoms but it would set a check engine light and a converter code P0420.