Stan944

I think I'll get a new coil.
You convinced me that the voltage is too low, although I'm not sure if this is THE problem. Why? The fuel consumption gauge reads very high most of the time. As mentioned earlier (in another thread) the reading of this gauge is related to the injector inhibit time. So the signal on the injectors (or rather one of the sensors that indirectly controls it through the DME) is very high on my suspect list. If anybody wanted to comment on this?
I'm hoping to use the noid light soon...

Tom Carson

so with air/fuel present and compressed 10 X atmosphere (ATM), the conditions are worse for a spark?...please explain why…and since when is the spark plug gap anywhere near half an inch at 1 ATM, 10 ATM or whatever ATM...I don't understand

cranking the engine over...trying to start it, and getting a spark one half an inch...and you think coil is bad??

Chris_924s

A noid light will only indicate if voltage pulse is present to indicate injector trigger- or will it do more than that?

Stan944

I know the answer to this one:
The dielectric strength of air at 10 atm is higher than at 1atm, so you need more voltage at 10 atm, or smaller gap. Here is a graph to support this claim:

from:
http://www.isi-seal.com/searchs/doc/...icStrength.htm
FYI: 1 atm = 760 torr, so the region of interest here is almost outside the range plotted. It's interesting that the plot has a minimum, so it's not a trivial behavior, but I don't know the explanation.
Also this graph is for pure air, and is probably much different for air/fuel mixture.

Tom Carson

that's my question...how much less than one half an inch spark plug gap is necessary?

Jfrahm

Depends on the motor and the ignition system. Stock gap, IIRC is .8mm for the 944 and .6mm for the 944 Turbo (more pressure in the combustion chamber requires a smaller gap for relaible ignition.) If you put in a more powerful coil you can generally open up your spark plug gap, and if you add nitrous or a blower you generally close it up a bit.

-Joel.

Stan944

Another symptom I noticed:
The car runs quite rough now, i.e. feels like power output fluctuates. Does this help to pinpoint the problem?

Tom Carson

this is beginning to sound like a problem I once had. I replaced the positive battery post/cable clamps, removed & cleaned (w/dielectric tune up grease) ALL the connections to the battery and starter, replaced a ground strap from the negative battery terminal post to the firewall and another from there (firewall) to the engine block...more tune up grease...and the engine started everytime, ran smoother and faster.

Stan944

sounds interesting. maybe I should do the same? I wish there was an easy method to confirm if this was MY problem.

Stan944

An update:
Last time it stalled (while driving), I managed to get some results of my test circuits suggested by Danno:
1) injector LED blinked OK
2) LED monitoring engaging the DME relay OK
3) voltage on air-flow meter non-zero
4) the engine kept running roughly for about 15 seconds (lost most of the power), and it stalled completely when I let off the gas.
5) Managed to do an "partially conclusive" spark test with a spark tester on cylinder #2, with the gap of 8mm on the tester: had a guy crank the engine, while I watched for a spark. Initially no spark or every 4-5 seconds, and later, when the engine picked up, the spark showed up. This would be conclusive except for the ground cable of the spark tester got disconnected accidentally (it was dark), and am not sure if I should trust this result. I actually felt an electric shock while holding the tester...

Conclusion: I think it's the coil. Still could be a fuel problem, but less likely. All sensors feeding the DME should be OK since I had the injector LED blinking.
I ordered the coil early last week, should get it soon.

Stan944

For the sake of those using the search engine, I'm posting an update:
The problem (of a stalling engine) was with a partially broken soldering point in the DME, coming from pin #13, i.e. the block temperature sensor. This caused intermittent lack of the temperature signal.
It's been a few weeks since I fixed it, and no (stalling) problems so far. Of course, there were other problems, but that's a different story.
Thanks for helpful comments, especially to Danno.

Danno

Hey, glad you found the problem! Good job!

Jeff F

I just bought a coil from Lindsey to solve my high boost ignition problem. An MSD High Vibration Blaster Coil, PN 8222 showed up, and it was cheap. It fits in stock location with some sleeving or a rubber drink coozy type arrangement. I consider coils a maint. issue, and the upgrade is nice. This is one instance where replacing parts is a good idea.

On a side note, I hope others show up at Texas Mile in July, and take the "Fastest Porsche in Texas" title away from me. www. TexasMile.com. It is a kick in the pants good time. While it is hot in Goliad, TX in July, the girls will likely be dressed appropriately for the heat.

Jeff
'86 951 2.8. Fastest Porsche in Texas (at least for now).
'87 928. Murf. supercharged.
''92 968 Cab. Mostly stock, and staying that way.

SoCal Driver

Seen pictures of the rotor blown to bits because a hotter coil was installed. Keep and eye on it.