M115 engine misfiring - Mercedes-Benz Forum

 
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post #1 of 6 (permalink) Old 09-14-2011, 09:01 AM Thread Starter
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M115 engine misfiring

I have a '77 230 that is misfiring in cylinder 2,

i checked the spark, its definitely sparking the color of the spark is blue, I've already changed the plugs.

It seems to smooth out when it warms up.

I've done a compression test where all cylinders read about 100psi,

they should be giving 130psi so thats low,

Anyone have any ideas as to what could be causing this misfire?
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post #2 of 6 (permalink) Old 09-14-2011, 04:55 PM
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How is the rest of the ignition system, wires, cap, rotor? How is your starter, sometimes when they're weak you get an artificially low compression test result?

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post #3 of 6 (permalink) Old 09-14-2011, 06:08 PM Thread Starter
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Wires cap and rotor are fine, the plugs are new.

the starter seems fine, the ignition module died a while back and it would just crank and crank and crank but not spark, so my feeling is that the starters in these are pretty damn sturdy.

There is kind of a fluttering sound when I accelerate, hard to tell whether its just something shaking around, or the sound of power escaping or something or that sort.

My feeling is that the spark is definitely getting to the cylinder, but there is an intake valve sticking or something like that going on.

I am planning on doing a head gasket job/ minor rebuild with new valve stem seals, pistons rings, headgasket, valve cover seal, and whatever other gaskets come with the kit (intake/exhaust, thermostat, etc.), however the one website I found that carries parts for m115 engines (autohausAZ.com) has them out of stock and back ordered.

That being said, anyone know where I can get m115 engine parts?

Any other ideas as to whats causing the misfire? Is my stuck valve theory a plausible one?
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post #4 of 6 (permalink) Old 09-14-2011, 06:43 PM
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You haven't mentioned the miles on the car, but sure, anything is plausible in an older engine. That being said, a sticky valve would likely not be helped by the car warming up, so...

Try germanstar.net for engine parts, failing that try Metric Motors.

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post #5 of 6 (permalink) Old 09-14-2011, 07:02 PM
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Ugghhh. "Fluttering" sound? Most likely a valve guide has come loose in the head. Not a common problem but one which does occur on the M115's.

When/if you have to pull the head, take it to a COMPETENT machine shop and one experienced in this engine. There is a minimum thickness to the head and the most common cause of guide failure is a machine shop which automatically mills the surface flat below minimum.

A rule of thumb on the thickness of the head. On the left side towards the rear is a row of numbers cast in the head. A paper matchstick laid sideways (flat side against the metal) should fit between the lower head edge and the bottom of those numbers with just a smidgen to spare. If the matchstick touches the head's edge it has been taken below minimum.

The best way to deal with those heads is to heat and straighten them. It takes an "old school" machinist who is familiar with the technique. Expect to pay for his knowledge and attention to detail.
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post #6 of 6 (permalink) Old 09-14-2011, 07:58 PM Thread Starter
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just rounded 115,00 miles,

I know germanstar I'm in the process of emailing them, will try metric motors.
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