What is causing my MSD coil to short thorugh its case?

[Jacquiles]

944 ignition coil shorting

My 944 is a 1986 NA

I previously made a post trying to diagnose a strange misfire. My motor would create pops and bangs like crazy, my tachometer was bouncing up and down very erratically, and acceleration was very slow and extremely jerky.



I found this to be caused by the ignition coil shorting to ground through the case of the coil. The ignition coil being used was a MSD high vibration blaster coil, part number: MSD-PN8222. My temporary solution to this was wrapping the coil’s case in two layers of super88 electrical tape. I originally bought this coil from Lindsey racing through their “Tune up package” which came with the coil, Magwire sparkplug wires, rotor and cap, and new Bosch WR7DC spark plugs. The reason I bought this kit is because towards the end of January of 2024, my 944 would no longer start and all signs pointed to a lack of spark. While this was the case, it was not caused by any of the ignition components, but rather the factory security system. The ignition kill was stuck activated due to corrosion, causing no spark and thus the car would not start. After installing the coil and the new ignition parts and removing and bypassing the security module, everything was fine until the middle of May that same year, only 3 ish months later. This issue of the coil grounding out began immediately after reassembling the lifter tower on my motor. I had some loud hydraulic lifters that no longer held pressure so I replaced them. Upon starting the car after reassembly, the “misfire” began. I first swapped out the Magwire spark plug wires with the original ones to eliminate those. That did not help. After that I bought 2 new reference sensors and replaced the ones on the car. This also made no difference. Eventually I found it to be the coil, as I mention at the start of this post, and wrapped it in tape. I left the original spark plug wires because the Magwires were difficult to tell if they were seated properly. Fast forward to two weeks ago (last half of October 2024), I began doing research on detonation and what it is and what causes it, simply for my own knowledge. I then realized that my car seemed to be experiencing detonation. When I blipped the throttle or floored it with the engine at idle rpm in second gear, I could hear the same detonation sound. I happened to have an empty tank of 87 octane and filled it up with 93 octane to see if any difference was made. The sound seemed to go away. In the first throttle blip video you can hear a loud metallic sound occur right as i blip the throttle, its easiest to hear it at the very end on the last two blips. That metalic sound can also be heard in the first driving test but its a little more difficult due to wind noise. That metallic sound goes away after filling up with 93 but seems to be replaced by a more hollow sound that you can hear three times in the second blip video, the two most obvious are towards the beginning. I have a slight exhaust leak so that may just be the sound of fuel igniting in the exhaust. (Side note: at this time, the detonation issue is side lined until I run out of gas and am able to fill up with 87 again to see if the detonation remains). My initial thought was that maybe the cause of my coil shorting to ground was also causing incorrect ignition firing and that I simply had a bad coil. I swapped the coil out with a brand new identical one, but that coil also shorted to ground through the case immediately and caused the exact same “misfire” issue as I had at the end of January 2024.

At this point I am confident the original MSD coil is perfectly fine. So my next thought must be that I have too much resistance at some point in the ignition system, and thus not providing an adequate path to pass through the spark plug wires and the spark plugs themselves. So, I took all my spark plug wires out and measured resistance on them, which is something I should have done before I even ordered the “tune up kit” from Lindsey racing. I found three of my four spark plug wires AND the coil output wire to measure wayyy way way too high. I read that they should measure 3k Ohm and only one of them measured correctly. One of the bad ones measured correctly if held at the right position, and the other two and the coil output wire measured in the 10 to 100,000k Ohms and occasionally in the Mega Ohms! So naturally I replaced them with Beru spark plug wires from 944online, which were $184 before tax and shipping. Crazy I know, but I wanted to make sure that I was not going to have that as a possibility of failure, and these seated very well unlike the Magwires. At this point, like I said in my side note, until I run out of 93 and put in 87, I will not know if my original sparkplug wires were causing an issue like cross firing or some other issue that in turn caused my detonation sound I was hearing. But part of me wonders if switching to 93 would even remove detonation if it was actually cross firing.

And now I’m back to having no idea at all what is causing my ignition coil to want to short to ground and I don’t actually know what was causing the detonation sound I was hearing.

To recap, I have replaced the coil twice, all spark plug wires, the spark plugs, both reference sensors, and the distributor rotor and cap. I have also double checked my battery connections and the grounds on the engine block. And just for your information, I have the Lindsey racing MAF conversion kit installed. I also run the car with the o2 sensor disconnected, because to my knowledge it shouldn’t cause any issues. I originally disconnected it because I wasn’t sure if it was in anyway related to my original problem in May of the coil shorting to ground. I then read from Clarks Garage:

“For 944s which are equipped with oxygen sensors (not all are), the O2 sensor measures the oxygen content in the exhaust and provides an output feedback signal so the DME will know if the mixture is too rich or too lean. The DME then adjusts the amount of fuel to correct the mixture. The O2 sensor is also known as a Lambda sensor.
When the DME is operating based on the feedback signal from the O2 sensor, this is referred to as Closed Loop operation. If the O2 sensor is faulty or is disconnected, the DME computer operates in Open Loop mode. In this case, the DME simply proves the fuel by determining the amount of air flow into the engine and selecting the appropriate amount of fuel from a set of preset fuel maps. On 944s which do not have O2 sensors (many ROW cars), the DME operates in Open Loop mode from preset fuel maps.
For O2 sensors to operate correctly, they must be heated up to normal exhaust system temperature. There were two types of O2 sensors used on 944s. Early 944s used a single-wire O2 sensor which is simply heated by the engines exhaust up to normal operating temperature. The single wire provides output signal to the DME. Since it takes some time for the O2 sensor to get heated up, the DME operates in Open Loop until it reaches normal temperature”.

I don’t really have any reason to have the o2 sensor disconnected anymore I suppose.

I realize I have only performed true diagnostics on my sparkplug wires, identifying them as a possible cause. Even then I only diagnosed them on the second round of replacing the spark plug wires. Everything else I replaced was just throwing parts at it that I thought were related to the issue. My forte is in electrical diagnostics, but not when it comes to ignition systems. I understand the fundamentals like how an ignition coil works, but all of my electrical diagnostic experience is in the interior of cars such as all interior wiring and interior electrical systems, such as diagnosing parasitic current draw and other such issues. I have access to lots of tools such as a Fluke oscope. I am sharing this info only so that whoever reads this knows to what extent I’m able to perform further diagnostics on my 944 per their instruction. Listed below will be four links to imgur videos, two are me blipping the throttle and my driving test before 93, and the other two are the same but after filling up with 93. All input is greatly appreciated. Thank you for taking time to read this lengthy post.

Blip test before 93


Driving test before 93


Blip test after 93


Driving test after 93

[barnwerks]

I recommend first installing the correct Bosch coil.

[Jacquiles]

I recommend first installing the correct Bosch coil.

This is definitely something i’ve considered, but
shouldn’t it work with this coil? I’ve seen lots of other people use this one so to me, putting in the bosch coil doesn’t seem like the right fix since it should matter if i’m using the msd or not. I will try it but i’d like to figure out why the msd one is doing this when it’s worked for so many other people.

[Tom]

Of all that, what problem still exists with that car?

Pinging/detonation is difficult and often impossible to hear on a 944. I don't hear any in any of those clips, at least on my iPad.

Why do you think the coil is shorting through its case?

[Jacquiles]

Of all that, what problem still exists with that car?

Pinging/detonation is difficult and often impossible to hear on a 944. I don't hear any in any of those clips, at least on my iPad.

Why do you think the coil is shorting through its case?

Really the only problem now is that i was worried i was experiencing detonation, and that if not insulated, my coil will short to ground.

If i hold the coil close to a metal surface i can see it arc from the side of the case to the metal body, or to the cam tower. It’s a very short arc, at most like 1mm but it’s blue and i can set it pretty clearly. And as soon as it begins arcing, or just touching the metal directly, the misfire issue begins and it backfires and does all the things you see in the very first video

[Tom]

I would revert to an original Bosch coil. The MSD 8222 is a downgrade in my opinion. It also has an internal ballast resistor if memory serves, which can tax the ignition driver in the DME. Magnecor wires are also problematic (don't seat right) but sounds like you replaced those already. My strong suspicious is that MSD coil is shorting to ground due to its own design short-comings. Coils shouldn't arc from their case to the chassis.

[Jacquiles]

I would revert to an original Bosch coil. The MSD 8222 is a downgrade in my opinion. It also has an internal ballast resistor if memory serves, which can tax the ignition driver in the DME. Magnecor wires are also problematic (don't seat right) but sounds like you replaced those already. My strong suspicious is that MSD coil is shorting to ground due to its own design short-comings. Coils shouldn't arc from their case to the chassis.

Ahh ok gotcha. That’s what Barnwerks suggested, i just didn’t realize that about the MSD coil. I figured it would have been equal or better I’ll order a bosch coil today and see what happens

[Tom]

I would revert to an original Bosch coil. The MSD 8222 is a downgrade in my opinion. It also has an internal ballast resistor if memory serves, which can tax the ignition driver in the DME. Magnecor wires are also problematic (don't seat right) but sounds like you replaced those already. My strong suspicious is that MSD coil is shorting to ground due to its own design short-comings. Coils shouldn't arc from their case to the chassis.

Ahh ok gotcha. That’s what Barnwerks suggested, i just didn’t realize that about the MSD coil. I figured it would have been equal or better I’ll order a bosch coil today and see what happens

Well, it may produce a hotter spark, or keep the spark hotter at higher RPMs, but for a naturally aspirated car, the Bosch coil is more than enough and is designed to work with the DME (and not over-tax it). Kind of like upgrading to a super long drill bit, even though a normal length bit is plenty long enough to drill through the wall. More importantly, Bosch quality is much better than MSD in my experience (so hopefully no arcing).

[whalenlg]

I made the same assumption and put in the MSD coil in 2020 based on it being on Lindseys product list. Haven't seen the Arcing issue, but Given the DME electrical load issue, I'm going to grab a Bosch and see how it compares.
Note - I bought one on Amazon (Bosch Part Number 0221118322) where it was about 1/3 the price of the usual places with 944.x part numbers.

[Tom]

I made the same assumption and put in the MSD coil in 2020 based on it being on Lindseys product list. Haven't seen the Arcing issue, but Given the DME electrical load issue, I'm going to grab a Bosch and see how it compares.
Note - I bought one on Amazon (Bosch Part Number 0221118322) where it was about 1/3 the price of the usual places with 944.x part numbers.

Porsche is very skilled at taking Bosch parts, putting them in Porsche boxes, and charging 3 times as much. That said, I just poked around and it looks like the factory coil is actually cheaper than the seemingly identical Bosch? Pelican for example sell the factory one for $210 and the Bosch one for $246.

Rogue Tuning did some o-scope tests a while back on these coils and concluded they drew more current than the factory coil, which can lead to an over-heated ignition driver transistor and resulting transistor and/or solder joint failures in the DME. He was able to alter the dwell (charge-up) time in the DME maps to reduce the draw, but for most of us, those coils just add stress to the DME without any appreciable benefit. On a turbo, like yours, a stronger ignition might be warranted if you are pushing the boost, etc., but if so you are better off getting an ignition box like the MSD 6A and matching MSD coil. I did that for years. That will take the stress off the DME and make sure you are charging the coil as intended. For an N/A 944, I think aftermarket coils are just a solution in search of a problem.