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Discussion Starter #1
Ok summer is over for the M5, CPS sensors and spark plug change became a bit more......

Pics show the remains of the plug and shows how the piston is chipped. Head is also beat up but valves seem to have escaped damage.











The background is that the car has been bad on low rpms, rough idle and lack of power. All symptoms indicated vanos/cps. Also it would loose power after like 5km and then after a few seconds it would come back. So at the end of June I order CPS´s and other stuff. Was going to change them out and if they were not the issue, double check the vanos soldering.

So on July 7th I was driving on a local track, not really pushing the car yet since I was learning the track. Then I got a really bad vibration from the engine, sounded like bad vanos rattle for a few seconds, then went away. I went directly into pits and then home afterwards without stressing the car. Thought it was the old issue getting worse and did not want to drive the car until it was fixed.

What I believe now is that the engine ate the plug at the track.

Need to find out why.

So.....

Ordering a new piston from VAC, it´s a forged CP 10.5.

Then engine out and head off, rebuild this winter.

Annoying as hell.

Almost as annoying as the fact I just replaced all CPS´s with the engine in the car.

But to look at the bright side, this build has been rock solid for 13 fun filled years & 80k km :smile
 
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What cylinder?

Hello, sorry for your loss.

What cylinder #? Is this possibly the result of fuel starvation?

Best of luck with the rebuild.
 

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Your M5 has always looked good and been kept in good shape. Definitely worth the time and money to get it back on the road.
 
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You’ve also got a problem with Cyl. 7.
As Krauthammer said, you seem to have a fuel starvation issue and what you heard may have been detonation which would explain the plug and piston.
 

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Discussion Starter #5 (Edited)
Yes it seems that 2 cylinders have been running hot/lean, detonation signs on piston.

Plug melted on #7 and #6 looks like it´s been running hot.

I will be replacing the injectors.
 
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I would not even bother trying to rebuild that engine. the cylinder wall and head are going to be all messed up. I would run another set of plugs and run that motor into the ground, with a spare s62 in the garage.
 

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Discussion Starter #7
I would not even bother trying to rebuild that engine. the cylinder wall and head are going to be all messed up. I would run another set of plugs and run that motor into the ground, with a spare s62 in the garage.
I´ll pass on that advice ;)
 
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Discussion Starter #8
Hope to have it running before the summer, engine out:

 

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Good luck with the rebuild, I hope it works out. This might be an optical illusion but what's going on with the thermostat housing outlet in your last pic? Looks crushed.

EDIT: I need to wear my glasses more, it's just an electrical connector.
 

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Discussion Starter #10
Covid providing some time for projects - taking engine apart.



Use the opportunity to detail/restore/refurb everything in the engine bay, got a small sandblasting cabinet and some very fine glass media:



Oil filter housing was looking a bit shabby after 20 years:



Cleaned up nice :)



 

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Nice progress! Wishing you the best.
 

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Wondering what the head gasket looks like between those two cylinders with the bad plugs......
 

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Discussion Starter #13 (Edited)
Engine teardown continues....

Cleaning and refurbing parts as I go along.

Note the bracket holding the wire tube:


Now all shiny and nice:


Alot of parts to remove....


and refurb!


Almost there:


Cleaning pistons to see status/damage:


As expected, piston 7 was totally gone:


Has been detonating like crazy towards the end.....


Unfortunately there turned out to be a pair of deep scratches that did not go away when honing:



So I have ordered a new piston set from CP/VAC, 93.5mm instead of 93mm. Sleeves will be bored over to match.

I´m also sourcing a set of S62 heads from the UK that have been refurbed and have some upgraded springs from VAC.
 

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Is the working assumption still clogged injector resulting in fuel starvation in the affected cylinder?
 

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Discussion Starter #15
Is the working assumption still clogged injector resulting in fuel starvation in the affected cylinder?
I will have the injectors tested soon.
And for good measure, I will have the new injectors tested too!
 

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I've been doing a lot of work on 4.6, and the s62.
I thought the 4.6 was a 93mm bore, and the s62 was 94mm.
You might want to double check what you're getting, and the current bore.
 

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Discussion Starter #17
I've been doing a lot of work on 4.6, and the s62.
I thought the 4.6 was a 93mm bore, and the s62 was 94mm.
You might want to double check what you're getting, and the current bore.
This is a sleeved block ;)
 

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I had seen no mention of the engine being sleeved or rebuilt.
Only the mention of the sleeves being bored to match. I did think the piston was way too shiny, as factory pistons are dull.
I'm a firm believer in the benefits of the aluminum bore. Rebuilding problems not withstanding.
Carry on.
 

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Man that #7 piston...OUCH!!

Glad to see you soldier through with the rebuild. Keep up posted.

Sent from Germany with Love.
 

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This thread makes me want to cry! JEEEZ!! Is it cost prohibitive, or you just don't want to make this nightmare worse that you are not replacing all the pistons with forged ones?

now you're making me need to go get a media blasting cabinet as I cannot handle the cleanness of your parts!
 
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