Cylinder not firing (dont think its a coil issue) - Mercedes Benz SLK Forum

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#1 Old 05-23-2012
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Cylinder not firing (dont think its a coil issue)

Hi, hope someone can help me! I have a 98 SLK 230K. the forward most cylinder (not sure which number it is) has decided to stop firing. I was overtaking someone and it suddenly started firing on 3 clylinders.

I swapped over the coil packs to try and eliminate this, it made no difference to the fact the forward cylinder wouldn't fire. Also I had the engine running and removed the HT lead from the plug and it was earthing when it touched the cylinder head so there was definately a spark there.

The spark plugs are also less than a year old.

MAF sensor is a cheapy one but quite new.

Are there any other common causes that would stop a cylinder firing. I realy hope it is nothing serious!

Any hints would be much apreciated
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#2 Old 05-23-2012
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#3 Old 05-23-2012
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When you removed the plug lead did you remove and test the plug? When plugs go it is usually sudden.
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#4 Old 05-23-2012
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Pull the plug....


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#5 Old 05-23-2012
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I haven't removed the plug yet as I didn't have the tools available at the time. The car is now sitting outside my house and I am sat on an oil rig for 2 weeks. I didn't suspect the plug as it was less than a year old. There were no horrific sounding noises from the engine when It was firing on 3 cylinders.

Will pull it when I get back, if it is that I will be over the moon!
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#6 Old 05-23-2012
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If the plug checks ok it may be the fuel injector as it will give the same sympton's .
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#7 Old 05-23-2012
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Definitely check the plug, and also the fuel injector as well. The plug is the more likely to fail. I don't know your engine but in general the injector will have two wires, one will be constant +12V, the other will be to ground via the Motronic. (The idea is the Motronic will ground the injector briefly during each intake stroke to inject fuel, the longer it's grounded the more fuel's injected). So, disconnect the injector electrical leads and check for +12V on one of the wires. If you have an electrical meter that can measure duty cycle, that might be good value to compare as well. Compare the cylinders to each other and post back your results.
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#8 Old 06-08-2012
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Hi all, well...been tinkering with the car all day with no joy, here is what I have tried.
Changed plugs
New plug leads
Swapped coil packs about
Replaced maf sensor
2 replacement injectors
Compression tests are all good


Faults showing up on little diagnostics are multiple misfires, one for each cylinder, definitely only one cylinder not firing,


Help! I'm seriously stuck... Only thing I can think of now is a broken rocker arm?
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#9 Old 06-09-2012
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Were you able to test the injector voltages per my last post?
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#10 Old 06-10-2012
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Originally Posted by efair View Post
Were you able to test the injector voltages per my last post?
Oops forgot to mention in my last post, I checked these two and everything seemed ok with them, all 4 giving good readings...totally baffled as to what it could be...hope the engine is not terminally damaged
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#11 Old 06-10-2012
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Does it misbehave at idle, or only at higher RPM/load?

Did you replace your AMM before or after the misfires started? (I've seen cheap AMM's give all kinds of problems).

If it's due, I might go ahead and replace the fuel filter proactively. Then I'd measure the fuel pressure.
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