AWR-FIX: N55 Oil Pressure Fix; Save Rod Bearings

[andrewwynn]

Within 2 months, both my wife's and my N55 motors self-terminated.

Doing dozens of hours of research found the very best thread on the internet describing the problem and a fantastic solution:

https://f30.bimmerpost.com/forums/sh...errerid=970772

So, a brilliant engineer did what they do and discovered that BMW stopped the nuisance rod bearing fails of the N55 in the S55 by changing the oil pressure mapping to use higher oil pressure, largely in the power. band but in general just a little bit higher everywhere.

It's a very involved process he found to fix the problem by pulling the mapping data from the S55 motor and pushing it into the DME of the N55 motor thereby cloning the oil pressure map from the M motor to the everyday motor.

It's for sure the best way to resolve this issue if you tune your N55 and push a lot of extra HP, but for the 'every day' guy that just wants to use their N55 powered car and not play 'Germain roulette' every time they punch the throttle, I've discovered a far far easier way to fix things.

One of my. best buds has TWO X5s a 2011 and 2013, I have My 2011 and wife has an F10; they all have had rod bearing failures 2 of the three shortly after engine oil change and 1 shortly after OFHG, the other 'unkown'. In any event there is about to be an 'epidemic' of spun rod bearings on N55 motors, due to the reasonable understanding that the problem clearly gets worse as the engine ages (time and miles). I've seen examples of failure as low as 60k miles and my engine which I pushed pretty hard at times lasted to about 196k.

I'm selfishly working to not ever have that happen again to mine or wife's car but unselfishly will be sharing with the world so they can copy me and hopefully save their FIRST N55 motor unlike me and wife saving their SECOND N55 motor.

Let me be clear: EVERY N55 motor is a ticking time bomb. BMW engineered the motor based largely on fuel efficiency; they run the temperature very hot to make the oil very thin, and run the pressure of the oil as low as possible.

It works very well until the engine ages and there is just enough wear on parts that you will have just a little too little oil film on your rod journals and in literally 2 seconds your engine seizes on you at 85mph when you drop throttle to coast and the DME drops the oil pressure from 65 psi down to 25!

That may work with a new motor and with worn in oil, but thicker brand new oil and a worn In engine, not so much.

I was planning to copy the genius mentioned above but I don't have the tools nor the skill for tuning so I'd have to find one, and also; it's over kill if you have less than 400HP. I only want to boost my oil pressure 8 or 12 percent.

So: I bumped into the exact specs of the pressure sender and it's very simple; 0.5 to 4.5V and a pressure range from about 7 to about 150 psi.

I developed a crazy simple circuit that will have a dial adjust where I can send any lower percent of the actual oil pressure to the DME and it will boost the pressure until it is satisfied; the theoretical values that are working good are about 8 to 15 percent boost.

It's going to cost under $40 and take maybe a total of an hour to build and install. (add another hour if you want to add a tee at the pressure sensor so you can confirm the actual vs. DME pressure.

I will be looking into batch producing them for sale and most likely with a pre-determined boost level (more reliable and can't mess it up).

I have followed over a dozen examples of premature N55 spun rods and I also spun the rods on an N63 motor, that happened at about 96K miles and again shortly after an oil change.

(that one unfortunately would be more difficult to adjust the oil pressure since it just uses a mechanical feedback from pressure downstream of the oil filter and not from an algorithm and voltage from the pressure sensor).

Fortunately the N63 (original no tu) are almost gone via 'natural selection' If I can find somebody with the need and an N63 tu+ I would definitely look into modifying the oil pressure feedback loop to bump up that pressure a little bit as well; I simply punched the throttle wide open followed by hard stop twice in a row (two stop signs each a block apart).

So back to my design and theory of operation: The prototype will use a ten-turn high resolution variable potentiometer with a 0 to 1000 dial for repeatable setting/testing. I will be taking actual vs. tuned pressure values and determined what setting will get what values (example 5, 10, 15, 20 percent boost).

I will leave mine with the dial so I can turn down the boost for long drives across country and turn back up when I get there and for cold weather and short drives in the city etc.

Next post: pictures.

[andrewwynn]

Pictures you say?




From the thread mentioned above, the DME map of oil pressure vs. RPM and load.

From that i developed this:


A spreadsheet with all the values from the two charts above so that i can generate graphs including n55, s55 and my n55 boosted pressure values.


Here is the first plot using 85% of pressure for an 18% boost in pressure.

The two sets are baseline N55 motor at the bottom three and boosted address the top three.


Here shows the N55 curves (at 15, 30, 100% load) (orange), my boosted by 25% (green) and stock S55 motor (pink)

It's higher than needed but you can see how it's nearly identical in the lower RPM range and almost a perfect compromise between n55 and s55 in the oak power band.

Once i can confirm the values that work i will share the BOM and the process so anybody that has an N55 but would like to not play German Roulette every time they punch the throttle, can copy me and possibly double the lifespan of their motor while using a reasonable oil change interval like 8000 not 5000 miles.

I'll be pulling the bearings from the 92k motor I'm about to install in my wife's car to get a sense of how worn they are. I'm confident they will be more worn than they should be especially with start stop.

The parts to do the prototype install and diagnostic come in a few days and I'm planning to get it installed next week.

More to come!