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!

[Sschwab]

Great work!

Would you say that running thicker oil contributes to the rod bearing failure? If so, would using a thinner oil resolve much of the issue?

[Clavurion]

The same thing as every modern engine. They don't care about the longevity but the minor change in measured consumption. Everything is over-optimised to the edge without any margin and they just don't care anything beyond 100 tkm. This is an easy fix as the oil pressure on petrol engines is regulated by solenoid so the pump itself has the potential power to deliver more. A different thing with diesel engines where the pump itself determines the oil pressure. M50d oil pump produces more pressure on low revs but that involves a lot more than a change in mapping.

[andrewwynn]

Originally Posted by Sschwab

Great work!

Would you say that running thicker oil contributes to the rod bearing failure? If so, would using a thinner oil resolve much of the issue?

Nope has been pretty throughly tested. Haven’t seen any objective evidence to support using other than the factory spec oil.

Today driving around and coasting downhill noticed pressure dip to about 20 psi. That’s the problem. BMW decided to allow way too low of dip in pressure when not under load.

It’ll work ok until it doesn’t. I suspect parts wear enough at rod bearing interface. Example: the crank opening around the rod journals wears a bit or the sides of the bearing shells wears and the slightly bigger opening gets you a thinner oil film on the journal.

There’s also a known issue on initial fill of the oil circuit that BMW developed a first start procedure of cranking for 3 sets of ten seconds with no gas or spark to prep the oil circuit before starting after any job that interrupts the oil circuit.

It's a band aid for something intrinsic to certain motors that introduced a problem. This procedure is new to the automotive world.

I discovered the feedback on N55 is a simple pressure transducer that maps 0.5 to 4.5v and 15 to 150 psi (numbers may be slightly different from memory)

Anyhow a simple voltage bridge will tell the DME pressure is lower than reality and it’ll make more.

Ideally adding a little extra on the bottom end I want minimum pressure to be at least 30 psi.

For several of the n55 motors I've been connected with that failed, some situation developed that caused a disruption in the oil volume flowing though the rod bearings including: boost pressure in the crankcase, OFHG seal blowout.

There's no room for error. When some disruption happens, a small safety margin could be saving many motors.

There's a whole "cottage industry" insisting that preventive replacing of rod bearings is the solution. It's a band aid. When things are working properly rod bearings are not a wear item.

The ones i recently replaced at 92 kmi do show a wear pattern I'm confident is from start stop and I'm glad i replaced them (and disabled default start/stop).

Certain BMW motors are known for rod bearing issues and to me it's no surprise that aging motors with DME controlled fuel pumps are at the top of that list.


–awr–

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[andrewwynn]

Originally Posted by Clavurion

The same thing as every modern engine. They don't care about the longevity but the minor change in measured consumption. Everything is over-optimised to the edge without any margin and they just don't care anything beyond 100 tkm. This is an easy fix as the oil pressure on petrol engines is regulated by solenoid so the pump itself has the potential power to deliver more. A different thing with diesel engines where the pump itself determines the oil pressure. M50d oil pump produces more pressure on low revs but that involves a lot more than a change in mapping.

While researching the fix I'm developing, I've found add-on systems that use an electric pump that kicks on if oil pressure drops below a certain level. Designed to be added in parallel to the mechanical pump. That would be the way to go vs. swapping in a full pump and often sump to go with. I was considering going that route before i discovered the feedback loop is very straightforward.


–awr–

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[Clavurion]

Originally Posted by andrewwynn

While researching the fix I'm developing, I've found add-on systems that use an electric pump that kicks on if oil pressure drops below a certain level. Designed to be added in parallel to the mechanical pump. That would be the way to go vs. swapping in a full pump and often sump to go with. I was considering going that route before i discovered the feedback loop is very straightforward.

What kind of pump and connection where you thinking? Would it be activated from the moment ignition is turned on, engine would be pre-lubricated even before the engine is started?

[andrewwynn]

I thought of that but the DME i think sets a fault if oil pressure over ambient when RPM zero.

It's an issue i may have to deal with doing my fix.

I want psi to register "negative 5-10 psi" when key on engine off. That may set an error so i might need to have to add some electronic parts to engage after the car starts.

The pump that i saw was designed to monitor psi and just pump into the output of the main pump any time psi dropped below a preset.

For motors with pure mechanical pump might be the only way.

It's theoretically possible to play with the spring inside my oil pump to set up some minimum pressure but setting up some jig to test and design that is outside the scope of what i can do.


–awr–

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[andrewwynn]

Progess being made finally!

Still very busy working on a big house addition in Pennsylvania, but got inspired to tweak my circuit last night.

I need to get a measurement of the dme impedance to fine tune the circuit but i updated the circuit to match the oil pressure sensor.

FYI: pressure = sensor voltage * 36 - 11

Right now the design is set to boost psi by 6.6 psi at idle and 9.1 at steady state.

The primary goal is to introduce some safery margin of which there is none.

I'm about to replace a friend's S63 motor that failed out of the blue at 42 kmi. That's crazy. That motor uses much higher oil pressure but if it keeps similar to S55, it will be just as low at idle and i suspect since most BMW motors I've seen fail it happens not long after anb oil change, they really need to have oil circuit primed even after an oil change.

I know any modern BMW motor I work on will get the oil ckt primed after oil changes.


–awr–

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