Open Road

While testing the coils on my ‘85 S2, the primary resistance is reading 1.0-1.1ohm on both coils, which is above the WSM range of 0.4 to 0.7ohm.
Secondary resistance readings are both 6.1k-ohms, which is in range.

Both “seem” to work fine, but are of unknown age. Planning on replacing both when they become available again from Porsche (they indicate Q3/2023).

But scratching my head on the above-range primary resistance readings.

Any thoughts?

Thanks in advance.

Josh

What does your meter read when you touch the probes together?

.4-.5

Its been said more than a few times the original coils work the best the new ones not so much.
Are you having running issues or are you looking for something to fix

Maybe both Stan, or neither, lol, kind of pulling my hair out over here.

I’ve had an underlying vibration around the 2,800-3,000Rpm range since I rebuilt the car, both when shifting through the gears, and at speed on the freeway. It’s not really bad, and it hasn’t gotten any worse (or better) in the 10k miles I’ve put on the car since the rebuild. But it bugs me and I’ve recently had some time to try and track it down.

All of my research points to ignition-related items. So I started digging and found the spark plugs were mismatched, WR6DC on the outer cylinders (1,4,5,8) and WR5DC on the inners. That, I thought, was a screw-up, so I ordered a set of WR7s. While waiting, I went through all of the other items I could to check and make sure everything else was in order. Checked the coils, wires, caps, rotors. Also checked that all the hoses and electrical connections were in good order as best I could. Didn’t find any obvious issues. Must be the plugs.

Nope. Installed the WR7s and the car ran like crap.

After talking with Greg, he said he installs plugs like that on purpose, to avoid knocking on those inner cylinders. Not a screw up. And he seems to be right. I’ll be reinstalling the old plugs as he had them.

The coils are old, don’t know how old, so I just wanted to test them to make sure they’re still good and not problematic. They seem to be.

As for the vibration, TBD for now.

What is the condition of the motor mounts?

What injectors? Rebuilt?

Motor mounts are new factory units installed with the engine when it was rebuilt ~10k milesago

Edit: To better answer your question: the motor mounts are in good shape, just looking at them, they don’t look to be squashed or sagging, I know they tend to fail sometimes.

Original injectors, tested and cleaned, also ~10k miles ago

ladybug83 wrote:.4-.5

1 - 1.1 reading, minus .4-.5 error, gives 0.6 actual, so within spec.
If you want to test the function of the coils, I think the spec is that the spark should jump a 13mm gap at standard atmospheric pressure. You can make a home-made rig, or buy a tester to check this. This will tell you if your coils (and amplifiers and distributors and leads) are doing their job properly. Check every plug lead.
Does your vibration increase with increased throttle opening?

It seems to, yes

ladybug83 wrote: ↑Wed Aug 16, 2023 9:35 am It seems to, yes

Seems likely that you have a steady misfire based upon what you describe.

Oscilloscope would be the next tool I would use.

But, you may be able to isolate the cylinder.

Oh…

When did this behavior start? And what “touching” was done to the 928 just prior to the start of the symptoms.

Thanks Dave. Symptom has been persistent since the rebuild, roughly 10k miles ago.

Plugs all look good to me, all tan/gray-colored. None looked burnt or wet. All had some carbon on the base of the threaded part. Nothing (to me) that looks out of the ordinary.

Would a misfire always present itself as a fouled plug? If so, I don’t think it’s a misfire.

With the old plugs back in place, the WR5DCs and WR6DCs, the car runs way better. Still vibration is there but not running like crap as it did with the WR7DCs.

Cyl 1

Cyl 2

Cyl 3

Cyl 4

Cyl 5

Cyl 6

Cyl 7

Cyl 8

ladybug83 wrote: ↑Wed Aug 16, 2023 10:17 am Thanks Dave. Symptom has been persistent since the rebuild, roughly 10k miles ago.

Glad I asked.

What cams are in it?

Does the ‘rebuild parts list’ include:
plug and coil wires
distributor caps
distributors
rebuilt mass air sensor
rebuilt LH 2.3
new oxygen sensor
new cps

How many total miles on the injectors?


Since this is a hybrid 2.3 system… does it use a Hall sensor? Knock sensors? Was the engine bay harness changed out to an ‘87+ or ‘89+?

ladybug83 wrote: ↑Wed Aug 16, 2023 10:17 am Would a misfire always present itself as a fouled plug? If so, I don’t think it’s a misfire.

If one cylinder just “isn’t working at all or mostly not” then one plug will look very different (very fouled) from the others. But, an intermittent or random misfire *may* not present as a visual difference between the plugs even if is isolated to a single cylinder. If all cylinders have intermittent misfires then all the plugs will look the same.

But, you should see if you can isolate which (if any) cylinders are contributing the most to the vibration. The answer is important.

ladybug83 wrote: ↑Wed Aug 16, 2023 10:17 am With the old plugs back in place, the WR5DCs and WR6DCs, the car runs way better. Still vibration is there but not running like crap as it did with the WR7DCs.

That pretty much tells you that you *DO* have an intermittent misfire. But, not exactly why or if it is one cylinder or all.

I miss the good ole days of testing magneto's. "here, hold this for a second." yep.. it works...

worf wrote: ↑Wed Aug 16, 2023 11:02 am

ladybug83 wrote: ↑Wed Aug 16, 2023 10:17 am Thanks Dave. Symptom has been persistent since the rebuild, roughly 10k miles ago.

Glad I asked.

What cams are in it? Euro 211/212, GTS Crankshaft, GTS rods, Carillo pistons

Does the ‘rebuild parts list’ include:
plug and coil wires
distributor caps
distributors
rebuilt mass air sensor
rebuilt LH 2.3
new oxygen sensor
new cps

Yes to all of the above

How many total miles on the injectors?

10k since they were tested and cleaned, unknown total miles, they're the euro-spec ones, I had to find them used and then sent them off to have them serviced. I suspect they could be 38 years old, or more

Since this is a hybrid 2.3 system… does it use a Hall sensor? Knock sensors? Was the engine bay harness changed out to an ‘87+ or ‘89+?
Hall sensor and knock sensors yes. The original harness was modified to capture these sensors and maybe a couple other things that Greg and Kyle would know.

ladybug83 wrote: ↑Wed Aug 16, 2023 10:17 am Would a misfire always present itself as a fouled plug? If so, I don’t think it’s a misfire.

If one cylinder just “isn’t working at all or mostly not” then one plug will look very different (very fouled) from the others. But, an intermittent or random misfire *may* not present as a visual difference between the plugs even if is isolated to a single cylinder. If all cylinders have intermittent misfires then all the plugs will look the same.

But, you should see if you can isolate which (if any) cylinders are contributing the most to the vibration. The answer is important.
How would I accomplish that? I don't have an oscillomascope

ladybug83 wrote: ↑Wed Aug 16, 2023 10:17 am With the old plugs back in place, the WR5DCs and WR6DCs, the car runs way better. Still vibration is there but not running like crap as it did with the WR7DCs.

That pretty much tells you that you *DO* have an intermittent misfire. But, not exactly why or if it is one cylinder or all.

Ive been planning on rebuilding the fuel injection and engine wiring harnesses and upgrading the injectors to modern-style. Not because it's necessarily the culprit to this problem, but just because it could be, and it couldn't hurt. I figure its gotta run smoother with a better/more consistent spray pattern. But that's just my uneducated assumption. Also, that would require a re-tune, and I dont have or know how to use a Sharktuner. I tried to buy one on Ebay this week but the guy removed the listing before I could buy it. Greg doesn't seem too interested in helping me find the solution. He offered to drive it and see whats up, but also said it's normal to have harmonic vibrations. So I'm a little discouraged. I appreciate y'alls input and help.

Folks,
Was able to wrap this up yesterday, I appreciate the help from everyone. Got to spend a couple hours at Greg’s and he immediately was able to find a miss on Cyl 8. Turns out the wire was becoming separated from the connector on the distributor side, under the rubber boot. Just a few wire strands were still hanging on. Replaced the connector with the crimp on wire style with the screw on connector. Immediately smoothed out the running. Got part numbers for the connectors and boots, and I’ll rebuild the rest of the wires at the distributor ends with the screw on type connectors. They do stick on the distributor cap pretty tight, so I guess they’re prone to separate when removing. Juan showed me a nice way of preventing this by using a pick under the boot and lifting (instead of pulling from the top) up so the boot doesn’t stick to the plastic cap too much. The spark plug side of the wires already have the screw on type connectors, so no need to rebuild those ends. Back in business.

In talking with Greg, we both figured that a fuel injection upgrade would smooth out the running, and increase the performance. Gotta be better than the 37 year old injectors. So the next project will be an upgrade to modern injectors and a retune. But first, I’ll rebuild both the fuel injection and engine main harnesses. This to ensure no electrical gremlins moving forward. I hope to start pulling the harnesses after Sharktoberfest, if that happens this year. Sounds like it may happen, the answer seem to depend on whether you ask Greg or Mary. Lol I hope so but it’s a ton of work to put it together I would think.

Well thanks again, sorry for the thread drift away from coils. Haha. Oops

BTW this ignition scanning tool is one that’ll save a lot of time looking for a misfire with the spark plug wire sensor. Christmas is coming.

GTC GTC505 Engine Ignition Analyzer - Spark Diagnostics and Tachometer Readings for Marine, Motorcycle, and Automotive Mechanics, Gen...

https://a.co/d/5tDjOUg

Glad’ya found it.

‘scope would have been my next tool on this.

worf wrote: ↑Sun Aug 20, 2023 1:27 am Glad’ya found it.

‘scope would have been my next tool on this.

and more versatile.