2003 maxda 6 3.0l engine replaced
 

Hi guys, I need help
I recently had a mechanic replace the engine on my 2003 Mazda 6 6cyl 3.0L
He installed a 2006 engine with 66,000 miles on it. The car runs great but the only problem is that in the mornings the car has trouble starting. Once it starts the is no problem starting it again even after sitting at work for 8 to 9hrs. Is not the fuel pump as it has been check.
The mechanic seems to think that the new engine is not compatible with the ECM computer.
I don't know what to do, neither does the mechanic.
Any advise with helpful at this time.
Thanks

Could be lots of things. Injectors for an automatic car vs a manual trans car have different flow codes. Auto is heavier and requires more fuel. It could be that engine has a lot of carbon buildup on the valves and soaking up the additional fuel required for a cold engine. Add a bottle of Techron to the gas tank to decarbon it so you don't chase your tail forever. Could be the spark plug he used is not hot enough. Use only NGK...I like the NGK G- power plug. It is a platinum plug and cost only 3 bucks apiece. They are good for 100k miles. Check the fuel pressure regulator too. It should bump up the fuel pressure on cold starts. Cycle the key on and off 3 times without starting the car and see if it helps. This is fuel pump prime and if it helps then the check valve in your fuel pump is bleeding down.