[__G__]

Hi All,

I thought I'd document this repair journey because I've found inconsistent or patchy details when it comes to repairing these engines while researching it myself. As they get older, hopefully, more people are willing to take on the challenge of DIY repairs, so the more info out there, the easier that will be.

I recently purchased a 2010 E90 335 with N55 for a great price - the catch is that it had a misfire on Cyl 5 (error code P0305). I was on holiday visiting family interstate at the time, but we were already planning to move there ourselves. So I bought the car, ran a handful of diagnostics, and then stored it. Fast forward a few months, we have moved, and I can work on the car properly.

So far, I've tried replacing 1x injector, and 1x coil pack and I did a compression test. The injector and coil pack made no difference, but the compression test showed very low (70psi) or no (0psi) on Cylinder 5. Low compression is an obvious problem, but inconsistent reading is interesting too. I've put a bore scope down the plug hole, but the field of view isn't huge, so I can only from the centre of the piston out to about halfway through the valve recesses - in any case, there are no obvious signs of damage.

At this point, I suspect a valve train issue - potentially a bent or burnt valve or something similar.

I've removed the plenum so far, and will hopefully remove the valve cover today. One thing I noticed after removing the plenum is that intake valves for Cyl 1 and 5 are open slightly, but it seems that cylinder 5 is open a fraction more than cylinder 1 - approx 1mm - I guess I'll find out what that's about when I get the valve cover off.

I expect that I'll need to remove one, or both camshafts for some head/valve train repairs, so I've been researching that process. I realised that there are quite a few special tools required, some are expensive, but they also take a few weeks for freight, so I'd like to find alternatives where I can. I've ordered a few ahead of time, but I just learned of a valvetronic retaining spring compression tool (LINK) - Is this a necessary tool, or a preferred tool? What's the alternative if any?

Thanks in advance.

[wheela]

Thanks for documenting this repair journey! I'm afraid I'm not much help on this, but am excited to follow your progress and learn some things.

[__G__]

Quick update. Leak down test shows ok (some leakage, but low on the scale), confirmed that compression test is 0psi. We actually tried removing the gauge and feeling for pressure with our finger to confirm the gauge was working.
Valve Cover came off today to visually inspect for anything unusual but it appears ok. There's not a lot we can see, so that's not a great test. Head has to come off next.
Seeking advice for removing the turbo from the head while it's still in the car.

[Sethorion]

Before pulling the head off and committing to buying a new head gasket and bolts, I'd get a cheap borescope (one with 2 cameras, with one facing down and one to the side) and take a peek down through the spark plug hole into the cylinder. Hopefully you'll find the piston/cylinder walls are fine and you just have a stuck valve.

[whyzee125]

Really interesting thread here. Have you confirmed the intake valves are not caked up from carbon? This has been known to lower compression though I'm sure not to 0psi. Also in the states 2010 is N54 but you seem to know your stuff, so it's definitely N55?

Subscribing to see how this one turns out. Good luck!

[Xalithar]

Have you done a wet compression test? Puting a teaspoon or a bit more of oil into the cylinder will help with narrowing down the lack of compression. if the compression goes up with the oil it means your piston rings are the culprit. If not it means you have a leaky valve or head gasket.

[__G__]

Hi All,

We've now removed the head from the engine while it was still in the car - for anyone considering doing this in the future, it's possible, but it's a LOT of effort. It was a solid 12 hrs work for 2 guys not having done this before. There are a number of tricks we discovered in order to access all the hardware for the turbo manifold. This is a RHD car by the way, so the steering column runs past the turbo too - as if it wasn't cramped enough already. I'll be removing the rest of the motor and gearbox to re-assemble, it's just not worth the effort of leaving the bottom end in the car.

Anyway - onto the diagnosis.

We visually inspected the valves for carbon build-up - there is some, but it's no out of control, but keep in mind that the leak-down test passed - which means the valves seal at TDC. But the compression test failed - that's a dynamic test, so it could be leaking at any point on the crank cycle.

The wet compression test showed no change - this usually indicates a top-end problem or a catastrophic failure (hole in the piston etc).

Now that we have the head off, it visually seems OK, I can't see any defects - no signs of piston/valve contact, no damaged springs etc. I'm in the process of testing the valves by adding some water to each port with the valve shut - so far no leaks. I'll test further with compressed air when I get the chance.

I did notice a patch of what looks like corrosion on the cylinder wall (see attached image) - some of it cleaned off fairly easily with a fingernail - so it clearly isn't rust. It is raised slightly, so it's possible that it's enough to cause wear to the piston rings, and seeing as though the engine is slanted, the wet compression test would not have helped because the oil would settle to the low side.

I need to find a test for the rings without having the cylinder head on, otherwise, I'll need to drop the sump and remove the piston to inspect - probably not a bad idea in any case.

Questions:
Does anyone have suggestions on ways to test/inspect the cylinder head? I'm thinking using a dial gauge and incrementally rotating the cam to ensure it's moving lifting the valves as expected.
Is there any way that a fault in the Valvetronic system could cause low compression on 1 cylinder?
Does anyone have a suggestion as to what the brown deposit on the cylinder wall might be?

[__G__]

Originally Posted by whyzee125

... in the states 2010 is N54 but you seem to know your stuff, so it's definitely N55?

Yeah, I noticed that when looking for parts on FCP Euro. I thought I was buying an N54 but I was pleasantly surprised to discover it's an N55 - Injectors are MUCH cheaper.

[whyzee125]

Originally Posted by __G__

Yeah, I noticed that when looking for parts on FCP Euro. I thought I was buying an N54 but I was pleasantly surprised to discover it's an N55 - Injectors are MUCH cheaper.

But the engine failures are much more common. Not a great tradeoff IMO but regardless it'll be a fun car once you get it sorted out.

[wheela]

Anybody notice anything about that block? Does that have a closed-deck insert? I have a used ebay n55 block and head I purchased for some turbo kit mock-up, and the open-deck is completely open around the cylinders - see pic.

[wheela]

Or is that just how the head gasket is?

[Spooly]

Originally Posted by wheela

Or is that just how the head gasket is?

headgasket, seals to coolant ports on head.

[__G__]

Yep, that's the head gasket. The deck is completely open once the head gasket is removed.

Quick update - I think I've found the issue - but first, the process: I checked over the cylinder head fairly thoroughly (without disassembling). I moved the valvetronic system through 3 positions (min, mid and max lift) and in each position I rotated the intake cam through a few revolutions, hoping to find some indicator of a stuck valve. In the minimum lift position, I used a screw driver, to gently move the top roller bearing away from the valvetronic cam so I could check that its still rolling nice and smooth (side note: bearings work best in continuous rotating applications. In rocking/oscillating applications they wear out much faster because defects tend to roll back and forth over the same spot hence accelerating wear). There was not signs of wear in any components on the intake, and everything seemed to move smoothly.

I did a similar exercise on the exhaust (without messing around with the valvetronic setup), and the exhaust seemed to move freely as well. I did a water leak test on both intake and exhaust sides, but there didn't appear to be any issues there.

Finally, I poured 100ml of oil into the #2 and #5 bore to compare the leak rates past the rings. And while I don't have any numbers, it was pretty obviou that #5 was leaking much faster. After maybe 2 hrs, the oil in #5 was long gone, and #2 was mostly still there. So this is looking like the most likely culprit.

Next weekend, I'll flip the motor over, and remove #5 piston and rod - hopefully I'll find some pretty solid evidence of the problem.

[Will_460cs]

Good work! be interested to see what you find..

[__G__]

Well I finally found the problem.

I'll let the pictures tell the story.

One minor correction to an earlier observation - a wet compression test won't be different from the dry compression test if there is a chunk missing from the piston and the oil escapes.

Speaking of oil escaping, the test I did with oil in the cylinder was very helpful. I've seen it suggested elsewhere, but it was followed up with not knowing what the standard leak rate should be. Well I can add some information to this for future reference. I put about 100ml of oil in cylinder 2, and 1 week later I removed at least 70ml probably more like 80ml. So insignificant leakage.

Anyway, here are the pictures.

[Will_460cs]

Wow thats unexcited, I wonder what has caused that?

Maybe worth get you injectors checked. Possibly a lean condition and knock as a guess?

[yupetc]

If you're going to sell this car after you button it back together, just get a used piston and check your piston to cylinder clearances. Install it and call it.

If you're going to keep this car, then by God you've already gotten this far, at least get some CP pistons and ACL rod bearings in there. Those will last and give you a peace of mind going forward. My ACL rod bearings are taken out after 20k miles, inspected with zero scratch. CP pistons showed slight skirt wear, but I had it coated with Cerakot's Microslick, so altogether, it's held up fine and will go back into the motor. These upgrades will last.

[yupetc]

Also, by only observing that piston close-up picture, here's my opinion:

I don't think you have a lean running issue to have caused this piston failure. If you observe the witness marks left by the piston, you'll see this is simply a failed cast piston in which the metal chunk came clean off the piston. It simply cracked and pieces fell off. In any case of piston failure, consider yourself lucky because most failures are more brutal than this. If you have a lean condition caused failure, you'll see a much different result, and much worse event than what you've gone through. In lean-condition caused failure you'll see some torched marks or some wash-out trail as if there's a hole being burn through the piston, that's not seen here.

Judging from the fallen pieces, I'm guessing this is the result of a combination of two factors: 1) detonation and 2) poor quality piston.

For a piston to crack, typical causes are detonation, overheating, oil starvation which also causes pistons to overheat, improper piston-to-wall clearance, or poor quality piston. There are likely other causes, but my list includes most that I've seen in my days.

On your piston, I don't see discoloration to indicate overheating. Piston-to-wall clearance isn't very likely the issue because you would have had this failure long before today if that's the case. Since the stock N55 pistons are cast, that leaves a lot on the table for slip planes to occur and all it takes is some good repeated beating on the poorly casted spot, or a good set of deep detonation; then that's all she wrote.

Other small observations I have:
1) I don't see any piston ring deformation. So that's a big relief.
2) Could be a coincidence that maybe you've moved the top piston ring as you pull it out, but the top ring gap is right where the failure is. Assuming ring gap is there for certain, then a small chance that a ring-flutter event could have taken place between top ring and second ring. If this is the case, then you'll have piston and ring instability which can cause the piston to have irregular contact to the wall, which further disturbs the originally designed piston motion. Another side effect of this fluttering is the repeatability of shaking motion in the rings in which if it hits the right resonance, it'll cause more erratic force between the piston and the wall. I don't see very terrible witness marks on your cylinder picture, therefore, I don't want to pinpoint flutter as your cause for instability-lead piston failure. Also, there are very seasoned race engine builders who will attest that ring flutter does not exist. So there you go. all of what I said is up and welcome for further debate.

I also want to go on and say that the N55 pistons are relatively weak in design, to begin with. I know this has been argued by many and running 500whp seems fine with these motors. But just look at the thickness of each ring land and space in between. There aren't much meat to take the beating in the first place. If these were forged then the same exact design may work simply because the strength of the piston is on an entirely different classification. But cast? with these thin ring lands? One bad quality piston get past the manufacturer's warehouse into an engine like this, then this fun stuff begins.

[__G__]

Good observations there. I think your right for the most part. The piston was probably flawed, but only enough that the tune exposed it. Its a 150000km engine, so extremely unlikely that something would just fail due to inherent weakness without a little extra provocation...
Regarding the piston ring gap you observed, I think that's a broken ring, not the ring gap... It's a bit unclear in the image, but there are chunks of the ring in the sump, so it didn't get out unscathed. That's to be expected considering it was still exposed to compression, but didn't have any ring land to support it.

In any case. The engine is back together and planned for reinstalling next weekend.

[yupetc]

Originally Posted by __G__

Good observations there. I think your right for the most part. The piston was probably flawed, but only enough that the tune exposed it. Its a 150000km engine, so extremely unlikely that something would just fail due to inherent weakness without a little extra provocation...
Regarding the piston ring gap you observed, I think that's a broken ring, not the ring gap... It's a bit unclear in the image, but there are chunks of the ring in the sump, so it didn't get out unscathed. That's to be expected considering it was still exposed to compression, but didn't have any ring land to support it.

In any case. The engine is back together and planned for reinstalling next weekend.

I am quite sure your ring landing broke off first, leaving no meat left to support that top ring, and henceforth the whole piece of top ring broke off and went into the oil sump. Anyway, ring gap was mentioned to guess a root cause for ring flutter, since your ring has a piece that broke off, I'm leaning towards no ring flutter, simply bad quality piston and a chunk came off causing whatever is weak after that point to also come off. Consider yourself lucky because failure like this usually don't come this single-file in the total damage department. Glad you got it all sorted out and ready for the build to go back in. Happy motoring, bro.

[whyzee125]

I don't mean this as a knock against your thread, car, or project at all. I'm excited to see how this turns out. But my goodness the N55 is a nightmare of an engine.

[__G__]

Engine went back in over the weekend - now I just have a bunch of stuff to plug in, then try to start it!