[Sameen_619]

Hi, car started knocking and turns out it’s something internal and not then injector which I assumed at first.

Currently the cars in the garage being stripped and investigated. I did a compression test and the compression was 0 on cylinder 5 and between 260-280 on all others.

Just wondering what should the compression be? I got told it should be around the 400 mark.

Please can someone confirm what the compression should be on all cylinders.

The car is a LCI E90 335D, M57D30TU2

Thank you.

[therealdb1]

This is really a hypothetical question in this case.
If you have zero on one cylinder and similar pressures on the other 5 you have a serious fault which will need an engine rebuild especially as you describe the sound as a "knock".
400 might be what it should be as a new engine but you can expect it to drop a bit as the engine wears and of course it depends upon the calibration of the gauge you are using and the seal that you get when turning the engine over.
The important thing with a compression test is that all of the cylinders give readings that are close together.

[Sameen_619]

So i went to see the engine and piston is scratched down the side and also some light scratches on the cylinder wall.

Car also had an intermittent low oil pressure warning light. Can the scratches/scuffs between the cylinder wall and piston be caused by the lack of lubrication due to the low oil pressure?

Also the top of the piston crown is scuffed and seems to be melted but doesn't look as severe as some of the pictures I've seen online of melted pistons. It seems more like its been gnawed at by a rabbit or something.

Also wondering what material are the piston rings made off? if the piston crown has melted which is cast aluminium alloy (i think) surely the piston rings would have melted which are chrome steel (i think). I have some pictures on my phone which i will try to upload.

Thank you in advance for any insight. I really appreciate it all the help.

View post on imgur.com

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View post on imgur.com

[nomiS330d]

Are you sure the swirl flap hasn’t been ingested by that cylinder?

[Sameen_619]

Originally Posted by nomiS330d

Are you sure the swirl flap hasn’t been ingested by that cylinder?

Swirl flaps been deleted. Got the blanks on there. Literally every other piston is perfect and the cylinder wall is perfect. Wondering if there is an oil feed that squirts onto the piston which could have been faulty or something.

Had the injector checked but its absolutely fine. Got told it could potentially be over fueling but i didn't notice any abnormal consumption, got around 48mpg driving between 60-80mph on a 300 mile trip.

Got told it could've been the DPF clogged causing heat to go back into the engine but that i don't think is the case either as after it started knocking i noticed clouds of white smoke so clearly DPF isnt clogged and also smell of fuel BUT NOTE the smell of fuel was only after the knocking started. Never smelt fuel before or seen car smoke like this. The smoke also settles down after a while, only smokes for a while after starting the car.

Thought it was the injector but also had that replaced which made no difference and the original injector is currently being tested which i'm positive will return fine.

I can only logically put it down to the oil/lubrication in that cylinder which also ties in with the low oil pressure warning light.

Also for some unfathomable reason: the compression test showed no compression in cylinder 5 but after stripping it down, its apparently cylinder 6 that is scratched on the cylinder wall and apparently piston from cylinder 6 which is melted/bitten/gnawed :S. No idea how to explain that one. Sorry did not take any pictures of the cylinder wall but they were verticle scratches the length of the cylinder just enough to stop a finger nail as its run over the scratch.

Any suggestions?

[therealdb1]

Originally Posted by Sameen_619

Also wondering what material are the piston rings made off? if the piston crown has melted which is cast aluminium alloy (i think) surely the piston rings would have melted which are chrome steel (i think).

Yes the rings would need to be made from some kind of steel but they will not melt.

Melting point of aluminium is around 660 degrees C whilst steel is somewhere above 1400 degrees.

Even in the days of the dreaded Nikasil engines, aluminium pistons running in aluminium block with no liner the rings were still made of steel because aluminium is far too soft and melts at too low a temperature.

[therealdb1]

Originally Posted by Sameen_619

Any suggestions?

You are into the realms of a serious rebuild now anyway so what happened is for interest only.

I would think that for some reason you have had meltdown on cylinder 6 and besides the bits and pieces falling off your piston causing the knocking and bore damage the local overheating may have blown your head gasket around cylinder 5 allowing water to get in causing the white smoke. You then have no compression on cylinder 5.

You really need to give the cylinder head a thorough inspection too since with such localised heat and detritus in the cylinders the valves may be damaged. Could it have been a sticking valve that caused your problem in the first place?

How many miles were on this engine because you might be better to cut your losses and get a complete replacement which will probably be cheaper than getting someone who really knows what they are doing to rebuild your one to a confidence inducing standard?