The engine in my BMW 325 is really bad and l am thinking of replacing it with a 318 low mileage engine in a DIY with a friend. My girlfriend prefers the 318 engine as she says it will cost less to fuel the car since she is the one that drives the car more often.

Please what parts aside the engines will require replacement.

Please help as l will begoing to pay for the new 318 engine by this weekend.

Unless you also have a engine control module engine wiring harnesses new dashboard for 4 Cly engine and necessary hard to get items like different torque converter and probably transmission too. Do It yourself l think not. Replace the 6 and start looking for a complete car with the small motor.

You never mentioned the age of the car.
My BMW is the E30 era 3 series and l went the other way, from a 318is to having the 325 engine in there.

It all works on these older cars - needed replacement gearbox, clutch, exhaust etc, but l do not know if it would work the other way around. Ca not see why not?

You will need new:

1) Gearbox
2) Prop shaft
3) Differential
4) Wiring Loom
5) Loads of Money

It will be far cheaper:

1) Sell the car & buy a new one
2) Get the engine reconditioned - it will run better and improve economy.

l hope this helps,

Andy

to start off you're not giving us that much info to help u out. What year is the 325 & what year is the 318 out of? Are u doing a swap in an E30, E36?
Typically some people with an E30 325 will swap out to a E36 318(M42). others with just swap up to higher hp engines.
To be honest u will find better information on swaps on bimmerforums.com There r a bunch of guys on there who have done alot of swaps & will be able to get u detailed instructions on how to get it done.