P0300 Code

Hello everyone I have a 97 buick LeSabre custom 105,000 new engine 8/09. I got a check engine light two days ago car drives great except two times rpm was below the normal and car had slight rough idle and it corrected itself no check engine light. I used my AC and got a check engine light my father checked the codes and it read PO300 random multiple cylinder misfire. This happend to me last year and the code was PO300 misfire cylinder 3: we changed the spark plugs and the coil pack just one though, and car ran great, now this. does any one have any ideas i take all over over opinions seriously thank you.

Most common cause of a P0300, would be a vacuum leak. Areas to inspect include UIM/LIM gaskets, vacuum fittings and hose, injector seals, or vacuum dependent accessories, such as the power brake booster, or EVAP system purge valve. With the engine idling, you can use a propane torch with a rubber hose inserted in the end, then open the valve and hold hose end close to suspected areas. If the engine ingest the propane through a leak, engine idle will increase accordingly.

The leak can happen on car that the engine has been replaced? Is this costly?

Vacuum leaks may appear on any engine, but can be especially troublesome when the engine has been replaced. Most of the time, when removing existing vacuum fittings & hose from the old engine, they will crack & split and if reinstalling them without a new replacement, leaks are more common. Check each and every line & fitting on the engine and EVAP system, one at a time. Each fitting must be tight & free from splits & cracks. This procedure is very inexpensive, especially if the problem is only a hose or fitting.