W108 M130 Misfire on seemingly random cylinders - Mercedes-Benz Forum

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post #1 of 38 (permalink) Old 05-18-2015, 12:16 PM Thread Starter
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W108 M130 Misfire on seemingly random cylinders

Hi again, Benzworld.

So I have this intermittent misfiring issue. My mechanic originally changed the spark plugs to Bosch W7DC0 back in April which caused a massive misfire (loads of pinging, etc). He fiddled with the spark plug boots and that made it run properly again for around that time. Fired it up a few days later and it ran on 5 cylinders. Thought it was probably the plugs not working correctly, so I changed them out to NGK BP6ES. Then it ran perfectly fine for another day. Fired it up a few days later and it'd misfire again, just not as bad (still driveable, but you can feel the car is not firing properly).

What does this sound like to you people? It seems to do this randomly, though I've not been able to make it run without misfiring for the past 3 times I've started it. It'll hold idle just fine and it fires on all 6 cylinders. It also fires on all 6 cylinders when driving, but it sounds like it's misfiring on one or more cylinders.

Note: I've changed all HT leads, rotor and cap today and it still misfires. The only one I have not yet replaced is the lead going off the center of the cap and to the coil, it apparently didn't show up on the desk today. I've not cleaned the points since I have electronic ignition (is there something I can clean underneath the rotor, even with electronic ignition? I totally forgot to check!) I'll buy it and replace the center lead once it's here.

Last edited by Jackpody; 05-18-2015 at 01:09 PM.
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post #2 of 38 (permalink) Old 05-18-2015, 01:17 PM
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Are you using original style copper wires on the HT? this was just on another thread. http://www.benzworld.org/forums/vint...l#post12047961

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post #3 of 38 (permalink) Old 05-18-2015, 01:19 PM
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Sounds like it would be easier to keep all info together in one thread.
http://www.benzworld.org/forums/show....php?t=2353873

Did you determine if it is spark? Not fuel? I assume it is spark, but you should troubleshoot systematically.

Do you have a timing light? You can check spark as I mentioned without disturbing anything when you use the method I suggested in your other thread.

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post #4 of 38 (permalink) Old 05-18-2015, 01:35 PM Thread Starter
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Sounds like it would be easier to keep all info together in one thread.
Parked W108 in working order, now runs on 5 cylinders - Mercedes-Benz Forum

Did you determine if it is spark? Not fuel? I assume it is spark, but you should troubleshoot systematically.

Do you have a timing light? You can check spark as I mentioned without disturbing anything when you use the method I suggested in your other thread.
Ah, I honestly forgot I had created that thread in the frustration of it not running after changing a lot of the ignition system. Anyway, I do not have a timing light but it's definitely something I could invest in. A friend of mine suggested that I check the coil and that it's grounded properly, and that I should replace the lead going from the center of the cap to the coil. I'm 99.99% sure it's an electronic issue as it seems to drive one day, misfire the next, run on 5 cylinders the next, and then run perfectly healthy in the weekend. From what I've noticed, and since I've only had it run healthy 2-3 times in this ordeal, it'll run fine on dry days and it'll misfire when it's humid. The fact I can go from it working one day and to it misfirng the next makes me feel like there's a wire or something somewhere not sending current. This entire thing happened after my mechanic changed the plugs to W7DC0, which I within weeks changed to NGK BP6ES to determine where the misfire comes from (this fixed it for a day, then it misfired again just far from as bad).

Can I drive the car to the mechanic as is? It does not hesitate to accelerate, it idles fine (you cannot even hear the misfire at idle) and it'll hold a speed. It just coughs a bit while doing all of that, as if it's burning half the petrol in one or more of the cylinders at random intervals.

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Are you using original style copper wires on the HT? this was just on another thread. http://www.benzworld.org/forums/vint...l#post12047961
Here's a picture I took of the parts number for it. I tried checking it out myself, but I cannot for the life of me figure out if they're copper or not. Asked my Mercedes dealership to find the W108 specific ones and they seem correct from what I've been able to find:

Last edited by Jackpody; 05-18-2015 at 01:41 PM.
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post #5 of 38 (permalink) Old 05-18-2015, 01:58 PM
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I remember when I was 16 I had a car that ran like crap when humid. It was the distributor cap.

You should be able to test the coil resistance and report them. There's only three possible measurements. I can't recall the numbers, but it's something like 1 ohm and 9k ohms you are seeking.

Do you have a multimeter?

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post #6 of 38 (permalink) Old 05-18-2015, 02:01 PM Thread Starter
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The car was originally going to MB on Wednesday to get a rear brake issue fixed (loads of squeaking after changing caliper, I haven't been able to source it myself) so if I haven't fixed the rough running by then, I'll drive it there and ask him if he can take a look at it as well. Would driving it pose a risk?
I changed the distributor cap (and the rotor) and it misses just the same as before. I'll resistance-test the coil if I get time before Wednesday morning, however I guess checking the grounds for the coil wouldn't hurt as well.
I should mention that it has always misfired on cold, it just hasn't been misfiring when hot as well. The misfire on cold always goes away after 3 minutes time when the needle moves off the temperature-gauge. The misfire on hot is nothing compared to the one when cold.

Another reason I think it's probably an electronic issue is the fact that when I started it after fitting the new plug wires, it ran on 3-4 cylinders until I removed cylinder #1 's spark plug boot and put it back on. Then it ran on all 6 but with the misfire when driving. I'm honestly lost for words, but I think it's time to replace the center lead from the distributor to the coil as well.

Last edited by Jackpody; 05-18-2015 at 03:03 PM.
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post #7 of 38 (permalink) Old 05-18-2015, 04:22 PM
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If you got MB wires, they are good! The cold misfire could be fuel related.
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post #8 of 38 (permalink) Old 05-18-2015, 06:10 PM
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Yes replace the center lead.
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post #9 of 38 (permalink) Old 05-18-2015, 06:32 PM
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If you have electronic ignition, such as a Crane or Pertronix setup, they generate a ton more current than what came stock. Mercedes approved wires are copper, leading me to suspect you might be arcing to the nearest ground point. I discovered this the hard way with my first pertronix. These systems cite specific plug wires with extra shielding, and they cite a specific coil. If your using a stock coil, you aren't getting the full bang for the buck.

In lieu of taking me at my word, call Summit Racing ( they sell pertronix and other systems) or the manufacturer themselves. Their customer service numbers can be found online
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post #10 of 38 (permalink) Old 05-19-2015, 01:33 PM
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Right now I have two cars with Pertronix and Mercedes wires, and have installed Pertronix in at least a dozen more cars. I haven't seen the problems with arcing. Bosch blue coils work very well with electronic ignition, and the +15 terminal on the coil should get 12V from the ignition, bypassing the ballast resistor(s).

Agree with Will to replace the coil wire. You may have a high resistance wire designed for higher voltage systems. The PN is 130 150 09 18.

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