1995 850 Spark,fuel,compression and no start

[erikv11]

So the intermittent bit was the coil, interesting. Thanks indeed for posting up the answer. A lesson here for me is that the perplexing no-starts are so much easier to solve if you can just swap everything out one by one, glad I have the spare parts to do it. But hope I don't need to!

[abscate]

The ether test is pretty good. Even a weak spark will light up ether.

I missed that.

[erikv11]

But this time it didn't ...

[abscate]

But it pointed to the ignition as the problem, i think?

Do you think this was a case where you could get spark at atmospheric pressure, but at cylinder pressure of 10x, you had no spark?

[erikv11]

OK I see your point, yeah, I totally agree the ether test pointed to ignition, that's what led me to think the CPS was out: presumably it was blue spark when the plugs were pulled, no spark later one when trying to start it. But more likely, it had (or tried to have) weak spark all the time, due to a bad coil.

Maybe the pressure in the cylinders is a factor, I like that. Under compression the air resistance across the plug gap would increase. So if there is a short somewhere inside that faulty coil, it may be revealed under compression as the path of least resistance and the spark takes that path instead. Cool idea.

[abscate]

About four decades ago I did plasma spectroscopy, and the Paschen effect governs the breakdown potential of a gap. For a spark plug, the potential is roughly proportional to the gap distance x pressure. At cylinder pressure, about 10x more voltage is needed to spark.

You could make a test plug with a 0.280 inch gap and try testing the coil with that.

Total sidebar...there is an interesting effect when as the pressure or gap gets low, the potential needed goes back up...something I've come across in my airborne laser life of days past.

Isn't it great when work sometime did in 1889 is still relevant???

[momentumbob]

I did a visual check for spark on all plugs, twice, in daylight. In my experience if you can see a spark in these conditions there is enough to light off combustion but the ether test was the definitive test. It just took me a while to get my head around what I was being told. The car is still starting reliably every day.