High oil consumption and alusil cylinder liners

[viperbluecerb]

Hi all,

I have been experiencing high oil consumption on my BMW 630i N53 engine and am in the middle of an engine tear down. Cylinders 1 and 6 are where the oil is being burnt. Head is being stripped and inspected by my local machine shop. Found some kind of scorching/carbon deposits on the cylinder walls in 1 and 6 which are not at all present in 2 through 5.

Couple of pics of bores in question here. thoughts/comments appreciated.

Edited by viperbluecerb on Monday 30th May 10:26

[viperbluecerb]

@Pumaracing, thanks very much for the feedback. I was curious as to the horizontal lines, they are present in all cylinders. Your comments make absolute sense, and I can only assume that the block left the factory this way, something I would like to get BMWs perspective on.

Here is a better picture of the bore on cylinder 6:

[viperbluecerb]

@stevesingo @Evoluzione, thanks for your comments too, much appreciated.

Searching around I haven't really found much at all on the N53 that has been much help in me diagnosing the problem prior to tear down. PCV system comes up time and again. Some people just accept the high oil consumption as BMW claim anything up to 0.7L per 1000km is acceptable. I wonder how many engines really do have quality issues that are down to unfinished bores if this really left the factory like it.

Edited by viperbluecerb on Sunday 13th March 23:00

Edited by viperbluecerb on Sunday 13th March 23:03

[viperbluecerb]

I will contact BMW to see what they have to say about it. Will keep you posted.

[viperbluecerb]

Nothing from BMW that is of any help. Thought I would share this photo just for info. This is a magnification of the bore surface. You can see the silicon crystals as the dark dots that show up. The horizontal rings that are quite visual on the photos cannot be felt by finger or nail in the bore where this magnification was taken. The rings are clearly seen in the magnification, but there is no evidence in the photo that these relate to any kind of ridge or step change in bore surface. The dispersion of the silicon crystals also seems consistent across these ring boundaries.

[viperbluecerb]

Not sure of the exact magnification but to give you an idea the spacing between the lines on the image is around 600-700um. The image is also at a slight angle as I couldn't get the microscope orthogonal to to the bore surface.

Measuring the bores reveals up to 50um taper in the most extreme bore (cylinder 6) with circular distortion (measured in parallel and 90 degrees to the crank toward the top of the bore, piston at TDC) as follows:

Cylinder 1: 30um (burn mark evenly split between thrust and non-thrust sides)
Cylinder 3: 40um (no oil burn)
Cylinder 4: 50um (no oil burn)
Cylinder 6: 70um (burn mark on thrust side)

Measured at bottom of bore (piston at BDC):

Cylinder 1: 10um
Cylinder 6: 20um


I am in the process of getting the injectors flow tested and spray patterns analysed. One thing that has crossed my mind is whether a bad injector that is leaking and/or has a distorted spray pattern could be causing some minor bore wash which is sufficient to thin out the lubricating oil on the bore surface such that it burns with the mixture (causing the consumption), but not enough such that metal on metal contact with ring/piston material to bore occurs (causing score marks in the same plane as piston movement). Just a theory, not enough experience with direct injection systems to understand if this is a possibility. Will be interesting to see what the injector tests reveal.

In the meantime, I got feedback on the head. Intake valve guides are fine, but interestingly exhaust valve guides are worn. So while the thing is in pieces it is getting a good clean up of all the coking around the intake valves and ports and new exhaust guides fitted. Which, BTW, you cannot buy as a separate part from BMW - in fact, the machinist can't seem to find out anything about them, so they will have to take an off the shelf item [and modify to fit if necessary]. Seems BMW just want you to buy a new head at ridiculous money :-S

Edited by viperbluecerb on Tuesday 22 March 22:02

[viperbluecerb]

Evoluzione said:

That's a very specific shape there, I wonder what caused that? If they bore and hone using tradition style methods I don't see how they could be left like that, it suggests the bore is being squeezed perhaps so that is a high point.

OP, what is on the other side of those shadows in the water jacket, anything?
Seems too obvious, but you never know....

That mark would seem to be where the outer wall of the cylinder meets the rest of the block, which is the bottom of the water jacket that is visible from the top of the block. So would make sense that this part would distort differently. I guess temperature differences could also come play.

Edited by viperbluecerb on Tuesday 22 March 22:05

[viperbluecerb]

Results back from injector testing are very interesting. 5 out of the 6 injectors are junk. These 5 failed the test due to leaking under pressure, with injectors on cylinders 1 and 6 in a really bad state and leaking heavily. Injector 2 was the only injector not observed to be leaking.

Now, when looking back at the cylinder bores, the horizontal marking only appears in cylinders 1 and 3 through 6 - number 2 shows no marking whatsoever, which is the cylinder with the only injector that wasn't leaking. It could well be the case that this leaking of fuel/rich mixture in the cylinders is causing some kind of staining or bleaching affect on the bore walls themselves.

The spray patterns on the injectors looked OK under test, but mixtures are obviously too rich on the affected cylinders, with 1 and 6 the worst. My thought is that this rich mixture is partially washing the oil film off the bore walls causing excess consumption due to it being burnt and replaced on each cycle.

The head should be back from the machine shop on Friday so I will get everything re-assembled. It then looks like a gamble of £1500 on a new set of injectors to see whether my theory is correct and there is no damage that has already been caused which is responsible for the consumption. I don't really have any confidence one way or the other at the moment aside from trying and seeing what happens.

But, one thing I would say - if you have any excessive oil consumption on this N53 engine and/or your OBD tools are reporting O2 range codes, get the injectors checked out - I have been told by the expert that tested mine that these piezo stack based injectors are prone to leakage problems like this due to pintle pitting/corrosion caused by ethanol content of fuel widely used in the UK. Testing cost me just just £35 each injector for those which passed, so in my case £35 for the lot. Well worth it in my opinion.

[viperbluecerb]

Engine is currently on 72K.

I'm not going to touch the bottom end at this stage as this is all still fully assembled in the car, and hence ring replacement would create another big chunk of work for me... so fingers crossed things work out!

[viperbluecerb]

helix402 said:

Good update. Don't forget injectors are coded to the cylinders, not sure if you're getting them refurbished or replaced. If refurbished put them back in the same cylinder or you'll have to re input the codes. New ones will need coding. (Some pedants may use a different term to coding).
Either way each injector needs a new decoupling element whenever it is refitted.

Thanks for the input. Yep, they are all new and will need re-coding, but I have everything I need to be able to do that. Decoupling elements are in the order I have just placed with BMW. £1356 all in. Ouch.

[viperbluecerb]

Got everything reassembled and running last night. With new injectors coded up and refreshed head it's running super smooth. It's going to be a couple of hundred miles before I know if there has been any significant impact to oil consumption, I'll report back as soon as I have some more info to share.

[viperbluecerb]

bmwmike said:

Cool!

What did you do about the oval cylinders? Or were they not that bad?

Any chance of a vid of it idling? Am sad like that

Cheers

I didn't touch the bottom end, and the measurements I took didn't indicate that there was any major distortion, as best as I could tell. Surfaces also didn't show any signs of scoring or abnormal wear.

Video of cold start idle at the bottom especially for you bmwmike :-)

Edited by viperbluecerb on Monday 30th May 10:26

[viperbluecerb]

bmwmike said:

Were yours coked up much?

They weren't really bad compared to some of the pictures I have seen, but they weren't good either, with fairly thick layers of carbon on the back of the valves and stems.

If I hadn't had the cylinder head off and I just wanted to de-coke, I would probably walnut blast the intake ports. From what I have read it seems the only good way to do it while the engine is still assembled.

http://blog.modbargains.com/get-nuts-feed-bmw-n55-...

[viperbluecerb]

500 miles and no drop in oil level on the electronic dipstick... before it would have consumed over a litre by now so things look promising!

[viperbluecerb]

My theory is bore wash through oil dilution causing oil that would normally be kept as a film on the bore surface to be partially consumed as part of the combustion process.

[viperbluecerb]

After 1K miles, it is unfortunate but not unsurprising that I must report that the oil consumption continues. It was always a strong possibility that the issue was ring/bore related and after all the work that I have undertaken, this has proven to be the case. Slightly overfilling led to a longer that expected time for the oil level gauge to report a drop in oil level and then it continued at the previous rate. Generally running very well with a stable idle and no visible smoke under any observed conditions.

The next step would be to completely pull the engine and rebuild/replace the bottom end. While my pockets run reasonably deep, I am not sure that for this problem they run that deep. Realistically it is another 4K+ to completely rebuild the rest of the motor with a new block and pistons and everything else that will be done should I choose to do so.

Hope the thread helps others in any way they choose to draw from it.

Edited by viperbluecerb on Sunday 29th May 11:50

Edited by viperbluecerb on Monday 30th May 10:28

[viperbluecerb]

Thanks. I am currently running 5w30 and have some 5w40 Liqui Moly sitting at home, LL04 approved. This coming week I will change the oil and filter for this, just in the interest of seeing if in my case it has any noticeable affect and report back any finding.

[viperbluecerb]

blade7 said:

Not BMW related but when I was thinking of building up a 944 engine I researched refinishing the Alusil cylinders and there weren't many places in the UK with a proven history of the correct boring/honing/lapping process. The rings and piston coating was important too. Serdi and Capricorn were the 2 companies that came recommended but they certainly know how to charge.

This was my worry, and also whether or not it is even possible to re-bore and re-hone them correctly. My understanding is that the process is extremely sensitive to tolerances to ensure the correct cylinder dimensions and exposure of the silicon crystals - as this has already been done at manufacturer, doing it again will surely involve firstly re-boring before exposing (otherwise you cannot get the correct exposure depth), knowing the process that was applied to create the Alusil bore in the first place - etched, arc sprayed, etc., how thick is this coat, will is sustain a re-bore, etc. Lots of questions I imagine it may be very difficult to answer with great confidence for different engines. And as you say, this will definitely come at a cost

Hence, personally, if I was going to go down this route, I would simply replace the whole lot (block, pistons and rings) to be 100% sure, I just simply wouldn't have the confidence that I could get everything reworked correctly.

Edited by viperbluecerb on Thursday 12th May 16:25

[viperbluecerb]

FordPrefect56 said:

viperbluecerb said:

This was my worry, and also whether or not it is even possible to re-bore and re-hone them correctly. My understanding is that the process is extremely sensitive to tolerances to ensure the correct cylinder dimensions and exposure of the silicon crystals - as this has already been done at manufacturer, doing it again will surely involve firstly re-boring before exposing (otherwise you cannot get the correct exposure depth), knowing the process that was applied to create the Alusil bore in the first place - etched, arc sprayed, etc., how thick is this coat, will is sustain a re-bore, etc. Lots of questions I imagine it may be very difficult to answer with great confidence for different engines. And as you say, this will definitely come at a cost

Hence, personally, if I was going to go down this route, I would simply replace the whole lot (block, pistons and rings) to be 100% sure, I just simply wouldn't have the confidence that I could get everything reworked correctly.

Edited by viperbluecerb on Thursday 12th May 16:25

What is mildly irksome is that the problem with your engine is so apparent and also easily fixed. Pumaracing pointed it out in his first post.

What is more irksome is comments like this. Totally presumptive and subjective.

[viperbluecerb]

FordPrefect56 said:

viperbluecerb said:

Totally presumptive and subjective.

Nope, 40 years experience of building race engines and diagnosing engine problems for people. Engines can only consume, i.e burn, large amounts of oil if it gets into the combustion chambers. The only two routes for this are past the rings or past the valve stem seals. Of these only the former can account for consumption in the order of a litre in a few hundred miles. A bit of guide wear itself is almost irrelevant as long as the stem seals are good and the size of the flow path past the valve stems pales into insignificance anyway compared to the size of the piston ring/bore interface.

The horizontal lines in the bores in your photos go right through areas of carbon build up but with none on the lines themselves. Whether or not you think you can feel these lines by hand they are clearly physical structures which the rings are skipping over. The rings are scraping the lines clean and the carbon is only building up between them. Piston rings can cope very well with a bit of bore ovality or taper but not with bouncing over stuff!

You then pinned your hopes on leaking injectors and "bore wash" but engines have been running rich since for ever due to people tinkering with them or not getting them set up properly and it doesn't make them consume vast amounts of oil. In any case I presume that despite what you were led to believe about the state of your injectors, the car was still passing MOT emissions, not throwing up fault codes and the catalytic convertor wasn't routinely on fire due to the hypothetical huge amounts of unburned fuel passing through it! No? Then it was never anything to do with the injectors.

It's the bores/rings, it always was and you really knew that anyway to start with from the thread title. You just tried to convince yourself otherwise to save stripping the rest of the engine.

Presumptive because you assume it can be fixed without all the necessary information being available, and subjective because an easy fix depends on many factors, not just technical. Having an opinion and sharing your knowledge doesn't have to be positioned with an abrasive attitude and a belittling two liner response. Everyone is here to learn and share and that is, after all, the entire point of this thread.

Edited by viperbluecerb on Saturday 14th May 23:16