[Jrs3800]

First things first...

How long was the car driven with a Miss? If it was driven a while with a miss the PCM will see the extra added oxygen in the exhaust stream and its correction is to add more fuel to the mix.. In some cases this can be a really bad thing..

How many miles did the car have on it? Was the upper intake replaced due to an upper intake failure?

Generally these engine are pretty rock solid, the flaw of these engines is in the lower intake gaskets and the upper intake design..

My 2nd 3800 II suffered an intake failure..... Later down the road, it spun the #4 rod bearing... When i tore the engine apart I found the rod bearings to be pitted to different degrees...

I can't say that this is the Mechanics fault, nor can I say that it isn't.. But I will say if I have to work on a 3800 II that has had an Intake or lower intake gasket failure the first words out of my mouth will be " I can't promise nor gaurantee the engine or work I perform on this engine as it may fail down the road due to the recent failure... "

Its a sad thing, But I have no trust at all in a 3800 that has ruptured an upper intake or the lower gaskets.. I have seen a battery of these engines fail after a gasket or upper intake failure...

Best of Luck on the motor you use as the replacement, and best of luck to your sister..