So due to cylinder no 4 misifiring, slightly low compression on that cylinder, and carbon fouled valves, I removed the cylinder head. End goal was to clean valves well. But then I noticed this brown ring in the misfiring cylinders wall. Anyone know what this would come from? Being in the middle of the pistons stroke? The piston may have been sitting at that spot for the last several days while I worked on it, and moved from that spot when I put the flywheel timing lock pin in.

Obvious carbon fouling, but the brown ring slightly above the top of the piston


Am I looking at a damaged piston ring? It doesn't wipe off with a finger.

 

I think more concerning is what’s all that other gunk in there around the bore and top of the cylinder?!

 

Pretty much all carbon. They all look a bit like that.


Guessing it's dead space above where the piston skirt rides?

 

Yeah the stuff at the top of the cylinder looks a lot worse than it is from the camera flash. Once I took the head gasket off it looks like you'd expect, plus a little carbon. Nothing actually built up.

The rust ring mostly cleans off with a bit of pegwood and pb blaster. I'd still like to know if that's an indicator of a failed ring, oil feed ring maybe? Not doing its job? Perhaps unrelated to the original misfire though as compression on all cylinders was 160-180psi.

I just noticed something odd though, I can rock the pistons by finger. In the below image, it's maybe hard to see so I've put an arrow. That shiny thing right where the piston and wall meet, you can SEE the top piston ring. That seeming alarming to me, I pushed the piston, and it wiggles. All of the pistons can be pushed and move.






I've rebuilt a chevy 350 and a Jeep straight six...both of those engines had piston clearance of a just a couple of thousandths of an inch, you could absolutely not see the rings or visibly wiggle the pistons.

Maybe just how these engines are?? Compression good, ran ok except that single cylinder misfiring constantly.

 

My first impression is that's discolored from rust from the piston remaining stationary for a while. But I think it would require more than a few days. And I would expect to see it in other cylinders, too.

That you can wiggle the pistons suggests a good amount of cylinder wear. The persistent misfires can be tied to wear.

Can't eliminate ring problems but you'll have to remove the piston and check the rings. Also, while you are there check piston ring land wear, piston wear and cylinder wear up and down the cylinder.

 

Thanks RockC. Sure wish I didn't have to go that far but I guess you're probably right. Have to pull the engine out the rest of the way... lucky me.

This is from the now-paid TIS site. Don't know if it is the right measurement.


Piston running clearance, translates to .0012" - .0026" of clearance between piston and cylinder wall, again, if TIS is telling me what I'm looking for. I don't think I could move the piston like that if it was within spec. U.S. toolmaker, I think in inches, sorry.

This next link is to another forum (sorry, inform if not allowed). His pictures show the piston ring visible like mine, but doesn't discuss rocking the piston. But his look the same or worse from a gap standpoint, and nobody indicates to him this is abnormal.
R56 Piston rings? Is the block salvageable? N18 | MINI Cooper Forum (mini2.com)

 

How's your motor going? Are you doing a full rebuild and have it out of the car yet?

 

Rebuild yeah. Piston bores were right on the edge of being in spec....so I chose to just hone them. Compression was ok when I started so I don't think I'll make it any worse doing this versus sending it out to be machined .25mm over (the right way). New rings, will probably get the pistons back into the block tonight. I left the block bolted to the transmission and took everything else off from there.

I hope you're not asking because you're in the same boat!

 

I sure hope to never be in that boat! However, it seems that over time the odds point to various failures imminent. So trying to understand and anticipate the potential mechanical issues and learn how smart people here on the forum are handling/fixing them. Sure makes me want to get rid of the Mini ticking time bomb...

I assume you've decarboned the pistons and head, etc. etc. Reusing the metal parts with new rings and gaskets/seals/etc.

 

I don't blame you for that feeling. If I get this running good again, I'm seriously considering selling it after I'm sure my work will hold up.

Yep. Getting the carbon off the pistons was a lot of elbow grease and parts cleaner. I originally bought new stock size pistons, Sneed4Speed via ecstuning. But I was sent 81mm pistons instead of stock 77mm.....so those got sent back and I just decided to rerun the original pistons with new rings.

Not done with this yet either, but yes, remove and clean all valves, clean head, new valve seats, relap valves to their seats, etc. If I was smart or wealthy I'd send the head out to be magnafluxed to check for cracks.
Not sure if stated here, but this all started because of a cylinder 4 constant misfire. Assumed valves, the rest of the work has been "well, while I'm in here..." Found another thread here a couple nights ago, someone had a random cylinder constant misfire, and it was just a new ECU for them to fix the problem. .....if it turns out that's all I needed to do, I'm going to be very, very angry.

EDIT: Sadly I'm not replacing the oil sump while I'm in here. Can't find one. So just new chain and sprockets on that end. My chain was VERY loose.

 

Thanks for all the information! How did the valves look and end up? I think there was another thread about using a borescope and seeing some questionable things in the valve area as the root cause to the misfiring?

 

My pleasure. I don't have the intake valves out yet. Looking at them through the intake port, not good, but not as bad as some posts I have seen. Cylinder 4 (the misfiring cylinder) is marginally cleaner than cylinder 1 which is the most gunked up. Exhaust valves all look fine. Fine carbon dust coating came right off with just a brush, not cooked.

Cyl 4 had lowest compression of the cylinders and came up to pressure slower than the other 3...that's why I blamed valves, Piston bore, piston, rings, nothing notable between cyl 4 and the others. I still have not found a "AHA Gotcha!" item to point my finger at for the misfire yet. Injector doesn't look worse than the rest, but all are getting replaced too. Maybe when I get past the special springs on the intake cam and get those valves out, a root cause will show itself.

That other post was me. I was a post ...rhymes-with-door... for a minute there really looking for help. This forum is pretty hit or miss in that regard. I suppose most are. I've debated deleting them as they ended nowhere useful to anyone. The images of foreign matter in the cylinder from the borescope were indeed just carbon flakes.

OH, and I should mention, I discussed pistons rocking in the bores up above. Apparently that's not all that abnormal. Lots of youtube videos talking about it. The head of the piston expands a lot more than the skirt during engine operation, the heat of combustion against it. Piston rock isn't an indicator of anything necessarily. The piston has to be removed and micrometered to know if it is a problem. Which in itself is a problem. Above I posted the piston measurements, spec says to measure pistons at "point A" but I have never found where point A is. Most manufacturers designate a spot on the skirt for micrometer measurement. I went 10mm blow the oil control ring and my pistons measured within the above spec. Can't know if it's right, but it's the best guess I have, that they are still ok.

Reassembling the stuff on the outside of this short block and front end will not be fun. SO MANY sensors, clips, order of ops, it makes my head spin.

 

Thanks for all the details. It is a daunting task all the way around. Makes my doing the head gasket on my 1996 Camry (long ago) seem real easy now lol.