Spark Plug Wire / Coil Issues?
 

Hi guys,

I've got a '76 911 with a '78 911 SC engine. The weather out here has become a bit more wet and humid and the 911 has been experiencing rough running (almost like one cylinder is not firing) upon start up until about 5 minutes later until the engine warms up a bit. This issue is intermittent and happens I'd say ~20% of the time after I start it up. The issue is almost guaranteed when I start the car after a heavy rain. Funny thing is that all cylinders are firing because pulling each spark plug wire alone definitely makes the engine run rougher than it already is.

I've done my searching on the forums and have narrowed it down to electrical. Injectors look good and fuel pressure reads normal all the way around. Airbox is stock and air flow seems normal. Again, car runs smooth after about 5-10 minutes of running.

My next test was to wait till dark and check for sparking in the engine compartment as to which there was quite a bit of. Here's what I gathered:

1. Upon start up (engine cold) - LOTS of sparking from hot + ground wires on the coil and a little bit coming from where the coil wire is plugged into the coil. After running the car from 15 mins the sparks die down and there is only a SMALL amount of sparking occurring from the hot + ground terminals. Coil has not been replaced in my 10 years of ownership.

2. Intermittent sparking (almost like a glow) seen from around mid-length of several plug wires. It doesn't seem like there is as much sparking after the engine is warm but it is definitely still there. Plug wires were replaced 10 years ago (45K miles ago) with stock Bosch wires.

3. Small amount of spark glow seen under the distributor cap when engine was cold - did not see any once engine was warmed up.

Theoretical question - is ANY of this sparking normal? The last thing I want to do is replace plug wires, a distributor cap, and potentially a coil if this in fact normal.

Thoughts?

I believe any sparking is bad... and potentially dangerous if you had a fuel leak occur.


81' Widebody SC

You have sparking through the coil tower insulation to the +/- terminal because that path represents the least resistance to the HV arcing.

Why/how can that be....?

Once the HV arc has repetitively penetrated (***) the coil tower a carbon path will form and the coil must be replaced. The carbon path alone may not always result in misfiring, moisture might aid in the penetration

HV wire from coil tower to distributor center is open of not plugged in firmly enough.

Wrong rotor, to short to be near enough to the plug post to bridge the gap.

Wrong rotor, not enough degrees of arc at the tip to fire with all modes of advance.

Wrong rotor, has the embedded resistor that has now burned open.

*** HV plug wire left disconnected for a brief period.

More appropriate transformer type ignition coil for your CD ignition.

http://forums.pelicanparts.com/uploa...1418788151.jpg

change the wires, cap & rotor - once you see St. Elmo's Fire no amount of "magic dryer elixers" will fix it

X2 on the carbon tracks comment - 40 yrs ago our shop had a Nelson Tester that would show them

search "misfire after storm" for similar experiences & advice

i have a near-new set of magnecor kv-85 if you want 'em at half-of-new-price

Thanks for all the info on this. Yeah it looks like changing the cap, rotor, and wires is the way to go. I am going to change out the coil to be safe as well being that this is a daily driver.

Looking at the reviews, it sounds like the Bosch coil is poor quality. I'm thinking about getting the MSD high vibration coil for a fraction of the price. I'm guessing it's just "plug n' play" (just bolts in without any modification needed)?

wwest - is that an all-in-one CD / Coil unit? Never seen those before. Looked at the website although there's not too much instruction on how to install one.

Larrym - let me PM you about those Magencors.

The '78 has a CD ignition.

Any visible spark is bad. Fix it all. Dielectric grease is your friend.