560SEL Random Intermittent Misfire - Mercedes-Benz Forum
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post #1 of 11 (permalink) Old 07-30-2017, 05:23 PM Thread Starter
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560SEL Random Intermittent Misfire

Hello everyone

Over the years I have lost count the amount of times to have made reference to this forum in search of information on the countless amount of new or old Mercs ive owned over the years.

Im currently experiencing an issue with my 560SEL and it doesnt seem I can find anyone else who has experienced a similar problem. I hope reaching out to the community can not only help me to solve this issue but anyone else who may have the same problem as im sure im not the only one to have experienced it.

CAR - 86 560SEL with ECE option (No o2 sensor)

Issue - Intermittent misfire

By misfire I dont mean a the engine has a little stumble every 10 seconds or so, what im talking about here is sometimes the car feels like its running on 5 or 6 cylinders.

This morning for example I started the car it ran very smoothly. I drove it to the shop 3 minutes away, switched off the motor, returned to the car 3-5 minutes later and it was running like only 5 out of 8 pots wanted to work.

After putting in new plugs and checking them after a little rough running I see the plugs are all black, like covered in soot. However after checking them while its running well they are a great color.

My thought are limited I really have no idea where to even start looking. Leads/dist cap/rotor LOOK ok but who knows.... I did notice on weekend if I disconnect a plug lead from distributor while engine running that a spark earths from the electrode to the body of distributor. While running rough and disconnecting leads from distributor it made very little difference to the sound the engine was making.

Also when running smooth if I was to rev it right out it may or may not stutter as revs get higher than 4k.

Also when running smoothly it is down on power. Not heavily but it doesnt quite feel like it is making as much as it should.

Any help appreciated.
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post #2 of 11 (permalink) Old 07-30-2017, 08:25 PM
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This sounds exactly like plug wires. I've had a similar issue.

I would park the car in a very dark place and look closely at the wires while the engine is running. I'll bet you see flashes of light along one or more of the wires or the coil wire.

If you do, I recommend buying new wires from the dealer. Not much more than aftermarket, they fit perfectly, and are warranted for life.
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post #3 of 11 (permalink) Old 07-30-2017, 11:57 PM
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I think that luckymike is right. Ignition wires can be a real pain you know where. There are some conditions which may guide you to correct diagnosis, for example:
1) The rough running/ misfire happens after rain or early in the morning or when the air humidity is high;
2) You can spot blue light flashes under the bonnet when looking at the engine in the dark;
3) While misfire occurs, removing bad ignition wire from plug doesn't make any difference in engine behavior;

If the ignition wires in your car are original, it will be nice if you change them - 30 year old wires, aren't the best conductor for a proper V8 engine spark...
I would suggest You to go for PowerTec ignition cables. I've got a set of those in my car, and they are one of the best if You look at price-to-quality factor.
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post #4 of 11 (permalink) Old 07-31-2017, 12:36 AM
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I have a similar issue that I've never bothered fixing. I always thought it was an issue with the fuel injectors considering those and my plug wires are pretty much the only things I haven't replaced, but the wires being the issue makes more sense.

I'll put some plug wires in my shopping cart.
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post #5 of 11 (permalink) Old 07-31-2017, 01:02 AM
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If anyone is interested, here's the story of my bad igniton cables, maybe it will help You sort out the misfire problems.

https://www.benzworld.org/forums/15918554-post52.html
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post #6 of 11 (permalink) Old 07-31-2017, 01:18 AM Thread Starter
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Thanks for the opinions gentleman
If only this issue was as simple as ignition wires!
When these symptoms first presented about a year ago the ignition wires/plugs/dist cap and rotor were the first things replaced.
When that didnt stop the problem it went into the "do it later" priority list.
Now im back where I started.
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post #7 of 11 (permalink) Old 07-31-2017, 02:56 AM
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Did you change the coil , when a coil starts to fail it does so when they start getting warm .

Also check your fuel pressure when cold and hot to see if proper pressure is being maintained.
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post #8 of 11 (permalink) Old 08-01-2017, 08:08 PM Thread Starter
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Coil seemed like the logical next stage in the ignition supply line
I did swap the coil with my 560sec so although it was not a brand new coil I feel safe eliminating coil as possibility as it worked while in the SEC yet the issue still there in the SEL.
If by chance theres a non member out there reading this and thinking about the time they suffered a similar issue id LOVE to hear about it!
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post #9 of 11 (permalink) Old 08-01-2017, 08:12 PM Thread Starter
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Checking fuel pressure is not on the list of tasks I can perform myself
Finding someone where I live in Melbourne, Australia who:
A) Is experienced with KE Jet / EZL
and
B) Wont help themselves to everything in my wallet
is also something ive not been able to find
Any suggestions from another member down under?
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post #10 of 11 (permalink) Old 08-02-2017, 05:58 AM
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Quote:
Originally Posted by jaydeliverys View Post
Checking fuel pressure is not on the list of tasks I can perform myself
Sure you can, if you have the right gauge set. Here in the States they are readily available and no too expensive.

Re-reading your original post, You're looking either for something tat intermittently affects the entire ignition system, not specific to individual cylinders, or, something that throws the fuel system into super rich mode.

This probably doesn't relate, but... I once worked on an engine that was very difficult to start and always had black, sooty plugs. After many hours and $$$ of fussing with the fuel system. I discovered that the ignition system was only getting about 7 volts due to a high resistance ignition switch. Problem solved...

John
"Real engines don't have spark plugs!"
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