P0300 only the dreaded random misfire

Braapdaddy

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20220731_135556.jpg
 
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Latest update did a compression test on all cylinders .....cylinder 3 or the third from the left has zero compression all others immediately jump to 180 and hold

Strange was I swear one time out of the 5 or 6 times I tried this cylinder it also jumped immediately up but now I cannot replicate

So has a lifter collapsed ?
Here are my new MB plugs with literal minutes of run time from right to left
Do a leakdown test on #3. I think you’ll find a valve sealing issue.
 

Braapdaddy

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Tore into the engine today while the outside temperature was in the mid 80s

Valve spring snapped in two on number 3 cyl

Now I am waiting on a MB camshaft tool kit to get here and a few seals / gaskets and I suppose a set of valve springs I'm about to contact MB parts now to see what a set of springs run

I did not see any witnesses marks via my borsocope on the piston top of cyl 3 hopefully there was no contact the valve seems to be smooth spinning and no wobble
 

Braapdaddy

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When I rotate the valve in a circle via the stem there no out of round motion
It can't fall very far at all due to the spring itself has let two coils touch it also doesn't have enough slack to get the keeper off yet without a spring tool ......also I can blast air in the cylinder with my compression tester and it will move up

I may be just wishful thinking but I think my only choice is to get the cam tool and at the very least one valve spring and put her back together and hope for the best

Any advice on what spring tool I should look for the clearance is almost zero it will have to be something slim in the grab area

(On edit )Also the rocker was still in place but was able to compress the lifter and the valve down with finger just enough to free up the rocker looks perfect also
 
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Mercedes has a special tool for nearly every task, so for the spring tool I would try to find out the one that Mercedes recommends for this engine and buy that (if it doesn't cost a fortune) or look for something similar in the aftermarket.
 
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Check the valve height with valves closed. If the one with the broken spring is lower than the others then the valve is bent. Likely below the guide if you can rotate the valve. I’d be impressed if it wasn’t bent.
 

Braapdaddy

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Check the valve height with valves closed. If the one with the broken spring is lower than the others then the valve is bent. Likely below the guide if you can rotate the valve. I’d be impressed if it wasn’t bent.
Thank you for the insight I do see what you mean by below the guide and the scenario of it being slightly lower basically one edge hitting the ring seal surface before the whole surface seals

Getting a accurate measurement will be a task but I will see what I come up with ..... If this is the case then I guess I have a bridge to cross on lifting the head or not

Hopeful but prepared for a bent valve
 
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Thank you for the insight I do see what you mean by below the guide and the scenario of it being slightly lower basically one edge hitting the ring seal surface before the whole surface seals

Getting a accurate measurement will be a task but I will see what I come up with ..... If this is the case then I guess I have a bridge to cross on lifting the head or not

Hopeful but prepared for a bent valve
You need to pull the head.
 
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You can buy a cheap spark plug thread air adapter to test with. With the other valve(s) closed on that cylinder, try to put air pressure in that cylinder and see if that valve will seal. If it will hold pressure, then you know the valve is straight.
 

Braapdaddy

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Yes I do fear with my luck and the way things go it has a bent valve like the other posters have said
And like Trav said there are fly-cuts in the pistons I can clearly see each set but can't see backwards with my scope tool but that only gives very little relief for stretching of the chain

I am buying the cam tool and a different style valve spring tool and just to keep from lifting the head will see if I can block off the injector hole and put #3 up on compression stroke and see if it holds if no then we all know what comes next

Thanks to all who have helped out Esp.Trav and Critic
 
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Not sure if you saw this:

Broken valve spring, misfire under load. Leakdown was perfect. Valve was bent, but it sealed perfectly.
 

Braapdaddy

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Not sure if you saw this:

Broken valve spring, misfire under load. Leakdown was perfect. Valve was bent, but it sealed perfectly.
Excellent Read and thank you for the info I read the whole post .... I will say this much when my valve is pulled up and twisted it shows zero signs of wobble or out of round and will slide down to the point it would drop in the cylinder if it was let to but I kept my magnet on it

I did take a measurement from the stem top to the machined sealing surface of my cylinder #1 at top dead center compression stroke and compared it to my cylinder in question same method and locations of measurement (with the cams still in until my cam tool arrives ) and it's so close to exact same

Will update the post as it moves along

Thank you

Mike
 
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That is a rare failure for a MB engine but like anything else it can happen. If you want to see the valves closed and look at the cylinder walls an articulating borescope is your best bet. This is a cheap one but will do the job you need, there is also a $50 coupon with this bringing it down to $170, cheaper than pulling a head and finding out you possibly didn't need to. If the valve moves freely you can check the installed height and guide clearence with a dial indicator, the spec is in the service manual, if its all good you should be okay.

 
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