Need help !
 

I have a Buick Lacrosse 2005 ,3.8 with code P0350.
I change spark plugs (iridium),wire,3 ignition coils,ignition module...
But Check engine light is ON after 20 minutes.
RPM is 850,and the gas per mile is more that before...
I have two mechanics look on ,but nobody can find the problem...
Please help

The engine uses the waste spark system. Did you change the plugs before the check engine light came on or after? Are you sure the plug wires are installed on the correct terminals of the coil. The engine will run , but not good. A bad injector will cause an intermittent misfire.

Before I change the spark plugs the code come on ,the mechanic check the plugs wire, everything is ok. but the car is run high, sometime I do 18 /21 miles per gallons, any suggestion ...Thanks

Please help me understand what you mean by the car run high. Thank you

RPM is little bite high (850) and I make 14 miles/galon instead 21. Thanks

Sounds like you have a vacuum leak. That could be anything from a broken hose or a bad sealing gasket that is allowing extra air to get into the engine. When this happens the computer tries to add extra fuel to balance all that extra air coming in. Normally all air that comes in is measured and fuel is added according to that amount of air. When air comes in that is not measured the computer tries to add the extra fuel because the oxygen sensor tells the computer add fuel. If the system cannot add enough fuel the engine runs lean and starts to misfire also causes idle speed to go up.

Thank you ! Today I go to the mechanic to check those things,appreciate you answers. Thanks

The mechanic check the vacuum ,everything is ok no leaking ,sealing gasket ok, no broken hose...but same problem. The computer show 14 miles/galon... any sugesstion ???

It's time to get what we call serial data from the computer. One of the readings we need is called short and long term fuel trim. if there is a fuel problem that reading will show that. Also need to examine oxygen sensor activity and performance. This cannot be done with a code reader but must be done with a good quality up to date scan tool. If the mechanic does not have these tools he cannot be sure what he is checking is actually working. Your problem could be something simple , but without the proper tools to find the problem you both are wasting a lot of time and working blind.