2002 BMW 525i Change engine

Tiny
NSHERRIFF
  • MEMBER
  • 2002 BMW 525I
  • 175,000 MILES
I would like to change the engine to a diesel engine, will this fit and what else would I need to get to make this happen. Will thye gearbox also need to be changed or are they interchangeable. I am guessing most of the cableing will be surplus or an I wrong.
Any help would be a great help. Is this even possible to do.
Thanks
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Thursday, July 25th, 2013 AT 11:09 AM

4 Replies

Tiny
CARADIODOC
  • EXPERT
Your car was never available with a diesel engine so anything you shoe horn in there will be a custom fit, better known as a disaster. Even if there was a diesel option from the manufacturer, changing a car over is such a drastic project that the car is sure to end up with a "For Sale As Is" sign in the window after you finally give up.

You would need a larger radiator and fan, a much stronger suspension system to hold the added weight, larger brakes to stop that weight, custom wiring, custom exhaust system, the transmission won't bolt up to the engine or flex plate so you'll need to install those parts that mate with the engine. With the added weight on the front, the brake balance, front-to-rear, will change resulting in the need to heavily modify the brake hydraulic system to prevent easy rear-wheel lockup. None of the current dash gauges will work, the fuel pump, tank, and fill pipe will be different...

Instead of listing everything that won't work, go out and buy exactly what you want. BMW is one of the worst manufacturers in the world in regards to customer-friendly business practices so look at this as an opportunity to find exactly the car you want from a manufacturer that is more responsive to their customers' best interests.
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Thursday, July 25th, 2013 AT 11:47 AM
Tiny
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So you are telling me the BMW 525i cant be changed to a 525d, I am sure the wieght would be near the same so no need for biger brakes etc.
Very strange answer.
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Thursday, July 25th, 2013 AT 11:54 AM
Tiny
CARADIODOC
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Sorry to leave you hanging. I did some research and typed a really long reply and just noticed it got lost. This is getting real frustrating.

To try to recreate my reply, I'm sorry I wasn't aware of the letter designations that show your car model was indeed available with a diesel engine. At least that means all the parts you would need are available. Still, you will be in for a lot of surprises and unexpected expenses. I watched my neighbor try to have a diesel engine stuffed into his truck in the mid 1980s and it ended in a disaster. He got a demand from the shop to pay $5,000.00 for the labor to that point before they would continue with the job. He had supplied the engine that he already paid for. There were so many surprises, and after almost two years, the truck ended up in the salvage yard. He was out over $15,000.00. That's a real lot of money today, but those dollars were over 25 years ago.

I tried to compare some of the brake and suspension parts between the two car models. There are some brake pads for certain applications that are the same but the calipers are different. I can't see what the differences are but it suggests the engineers found the need to make changes with the different engines. The same is true of the front coil springs. All of them will interchange because they're the same length and fit in the same pockets, but there are different part numbers just for your model depending on whether it has a manual or automatic transmission. There's not much weight difference there but it's enough that the springs have a different spring rate to match the weight difference. Diesel engines weigh a lot more than gas engines so the springs are going to have to be stronger.

There's so many variables when choosing the correct brake pads but some part numbers are listed for both car models. The concern is not the additional stopping power needed for a heavier car. That is simply a factor of pushing harder or lighter on the brake pedal. The concern is the center of gravity and the weight transfer to the front during stopping. That is primarily an issue with the pressure-differential valve in the brake hydraulic system. His job is to limit rear wheel brake fluid pressure to prevent rear-wheel lockup during moderate to hard braking when most of the car's weight shifts to the front wheels. That valve typically is different depending on whether or not the car has air conditioning, it's a two-door or four-door, and things like that. Often there can be more than a dozen different part numbers available to match the weight distribution of the car.

You know the Engine Computer will be different because there won't be any spark plugs for him to fire. The instrument cluster is a computer that talks to the Body Computer that talks to the Engine Computer. It's anyone's guess which computers will work with the new Engine Computer and which ones will have to be replaced. My fear is there will be different terminal placement in the connectors meaning you'd need different wiring harnesses to mate with those computers, then what else has to be changed to mate with the rest of the plugs on those harnesses. I'm afraid of hearing you ran into a mess that couldn't be straightened out or more likely, will cost you way more in labor and parts than if you just bought the car you wanted. I DO know of a few newer cars where changing engine sizes is pretty easy but those all pertain to going from a gas engine to another gas engine of the same weight. That was easy to do on a lot of older cars too, but you're talking about some major changes that the engineers designed in to their products to give proper braking and handling. If you still want to pursue this I would look for someone who has already done it and find out what the results were and if they would do it again. If you can find that, more power to you and I wish you good luck. I don't want you to be the first person to try this when others have determined it is not feasible.
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Thursday, July 25th, 2013 AT 8:43 PM
Tiny
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Wow, I didnt realise it would be that difficult and probably would end up being more expensive than leaving it sas is. Would make more sense to sell and buy the diesel version. Thanks for your in depth reply to my question. Not being mechanicaly minded, there seems to be more than I expected, I didnt even think about the computer etc. Thanks again.
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Friday, July 26th, 2013 AT 4:30 AM

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