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2007 ML 350, 2000 Corvette, 1969 Yenko Camaro
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58 Posts
Discussion Starter #1
Finally got some heat in the garage and started to diagnose the rough idle and misfire issue with my 85 380sl.
Long story short - I started pulling plugs. The first one pulled was #4, it was wet with gas. After checking a few more which were perfect, I cleaned #4 and fired it up again. Still missing, pulled the wire and there was no change in the running. Pulled #4 again and it was wet again.
So it looks like I have no spark to #4. Wires were replaced a few months ago by previous owner. Don't know how to test the wires so I used an ohm meter and it read 1.23. The others read 1.02.

Then I pulled the cap, the contacts were kinda burnt and the rotor was definately burnt with black scorching down on the plastic below the contact area. It also looked as if there was a burr along the top edge of the contact area as if it was wore away. Used Dremel wire wheel and cleaned them up but still no firing in #4.
I pulled #4 wire off the cap while running and the spark arc'd from the post to the sides of the little well it sits down in. Is that normal or is it a carcked cap?
Feedback greatly appreciated
JZ
 

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One of the BW Old Guard/R129, W204 Moderator
1997 SL500- 40th Anniversary
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7,166 Posts
Swap the #4 lead with another of similar length, and see if the problem moves to the swapped cylinder. If it does, replace wires.

If the problem stays on the #4 cyl, change cap and rotor.

Considering the cost is not too steep, swap plugs, rotor, and cap, then you know all is well and are not relying on what the PO said might have been done.

Good luck sorting it out.
 

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Registered
82 380SL 96 SL500 03 SL500
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6,165 Posts
Check the ignition first as that's easiest, not to mention that it sounds past due, but consider that the fuel distributor or fuel injector could be dumping too much fuel to #4 causing it not to fire.
 

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Registered
2007 ML 350, 2000 Corvette, 1969 Yenko Camaro
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58 Posts
Discussion Starter #4
Thanks EARS

I'll go get some more propane tomorrow for my heater and do just that.
It's tooooo dam cold here in central Pa.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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32,362 Posts
I pulled #4 wire off the cap while running and the spark arc'd from the post to the sides of the little well it sits down in. Is that normal or is it a carcked cap?
Feedback greatly appreciated
JZ
No that is not normal. New cap and rotor are in order.
 

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Premium Member
1975 450SL
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2,440 Posts
I would second the new cap / rotor. A small crack could be invisible, a little moist dirt could cause a current path, and the resulting carbon makes an even better path.

There have been reports of people getting a nice fireworks show when they fire up a car with old cap, rotor, plug wires at night and raise the hood.

Good luck,
Scott
 

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Registered
1978 450SL (RIP); 2007 Impala 9C3 (Police); 2010 Ford Edge
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212 Posts
So, dumb question, I'm getting ready to check my plugs (I'm happy to have even a rough idle) and don't know what size socket they take. Anyone?
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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32,362 Posts
Standard large plug socket. I think 13/16th.
 

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Premium Member
2004 CLK 240 Coupe
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11,377 Posts
A tip from the past for anyone searching for an elusive ignition/misfiring problem.

Try running the engine after dark, and raise the hood in the total darkness. You may be surprised to see sparks leaping from plug wires to the chassis, around the distibutor, and dancing around the plug cavities. Equipped with a torch, you might locate the source of your fault. :thumbsup:
 

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1981 380 SL
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40 Posts
The first time I heard someone from the UK say "I think I know what's wrong, hand me the torch" we were getting a Yamaha TD2 ready for VIR. I almost fainted before I realized he was talking about a flashlight! THANKS - Stan
 

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Premium Member
2004 CLK 240 Coupe
Joined
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11,377 Posts
The first time I heard someone from the UK say "I think I know what's wrong, hand me the torch" we were getting a Yamaha TD2 ready for VIR. I almost fainted before I realized he was talking about a flashlight! THANKS - Stan
LOL.. I wasn't aware that was UK-only English...

Several years ago, I was visiting Minneapolis to do some computer integration trials that would last about 2 weeks. My hosts asked me about my program, and I said 'We should be able to get though this in a fortnight'

They were astonished, and said 'Four nights? I said 'No - a fortnight!'

Then one of them remembered hearing the word in a Shakespeare play. It derives from 'fourteen nights', but the word has never tranferred across the pond... :)
 
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