Porsche 924 Scheiße - it's an ignition problem, innit?

[Zelandeth]

Keep it simple.

You need a few things for spark.

You need a solid power supply to the positive side of the coil.  Easy enough to check.

You need a switched ground on the ground side of the coil.  This is probably the most likely to be causing you headaches.  The ignition amp is what takes the tiny signal from the pickup in the distributor and turns it into the switching of the coil's ground.  

You need timing.  If the distributor is rigidly fixed, and the rotor is properly keyed onto its shaft, that's not likely to have changed dramatically.  

However...

The signals going into the ignition amplifier ALSO can impact the timing, as it also takes over from the old fashioned mechanical advance in the distributor.  

So I'd be concentrating on making sure the wiring to the ignition amp is sound (there are half a dozen wires, probably three or four actual signals) don't forget to verify the ground (or groundS), and seeing if you can get hold of a spare - not bad thing to have a spare of anyway.

The pickup itself is worth checking - Saab used both a hall effect or an optical chopper depending on the year - if these get full of crap (especially when the camshaft seal fails and the distributor fills up with oil) it can cause issues.

The fact that it sounds like the timing is screwy in the video makes me look even more suspiciously at either the ignition amp or it's pickup.

[Talbot]

12 hours ago, N Dentressangle said:

When it cut out, it did so suddenly.

For the next day, it got harder to start but would stay running ok and rev for up to 10min before eventually stumbling and dying. It wouldn't idle.

It will now barely start as you can see in that latest video.

I think the timing is OK, but part of the ignition system - either the electronic ignition in the distributor, or the ignition module I cleaned up earlier - is dying / dead. It's not the coil as it (a) tests OK and (b) has been swapped with a different one to no effect. Same goes for the HT leads, cap and rotor arm, and the plugs are brand new correctly gapped NGKs.

Are they genuine plugs and not some of the fake crap that seems to have infiltrated the market.  Both @juular and I have had experiences with fake spark plugs that break down under load so badly that an engine simply won't run.

If you take all the plugs out of the engine, ground them all and spin over the engine, do they all spark strongly?  If so, put them back in the engine, and use each lead as a trigger for a timing light.  When a plug breaks down under load, most timing lights fail to be triggered.

[N Dentressangle]

Yes, genuine plugs, all gapped before fitting.

They worked fine before, so they're not my prime suspects at the moment. I'm looking at:

- the ignition module

- the hall sensor in the dizzy

Testing either of these definitively is not so straightforward as far as I can see. I'm tempted to try an ignition module first as it seems more likely that its complexity and function would make a slower, intermittent failure more likely than the basic, pretty binary operation of the hall sensor, which tend to either work or not IME.

[Nibbler]

Listened to your video. I had a Capri Mk.2 that sounded just as bad as your 924 and was really hard to start. The cambelt had skipped two teeth. Just a thought but maybe worth checking out.

[N Dentressangle]

Yes, I had thought of that but it did run smoothly again after the initial cut out and the belt looks ok.

[juular]

My 240 (also Kjet and Bosch electronic ignition) has been doing the same instant death with no warning.  It's had new leads, cap, rotor and coil seems OK. 

I've got a pump hotwire switch to bypass the relay so I know it's not that. 

It has to be the ignition module or the hall sensor.

I think the hall sensor needs an oscilloscope probed in to test it properly. IIRC it's an 0-5v signal.

Worth mentioning when I removed my ignition module it was really damp, so might be worth checking the condition of yours.

[mercedade]

Recency bias and my total lack of actual knowledge says it's the rotor arm:

 

 

[Christine]

I haven't watched the video , but rotor arm ..is it a rev limiter one ?  My old Scirocco  died once , when giving it the beans .. flames and popping and banging ..to a standstill . I though the cambelt had jumped Turned out to be the spring loaded bit in the rotor arm had jammed , making the spark plugs  fire at the wrong time..  cheap fix!  

[N Dentressangle]

I've swapped out the rotor arm (boring solid one) for known good, so confident it's not that.

Currently trying to sort a used ignition module to swap in.

[N Dentressangle]

10 hours ago, juular said:

My 240 (also Kjet and Bosch electronic ignition) has been doing the same instant death with no warning.  It's had new leads, cap, rotor and coil seems OK. 

I've got a pump hotwire switch to bypass the relay so I know it's not that. 

It has to be the ignition module or the hall sensor.

I think the hall sensor needs an oscilloscope probed in to test it properly. IIRC it's an 0-5v signal.

Worth mentioning when I removed my ignition module it was really damp, so might be worth checking the condition of yours.

I'll hopefully have time to take the ignition module apart tomorrow morning and see if there's anything obvious wrong. I've ordered a good used one which should arrive on Thursday, so we'll see what that does.

Glad you agree on my two suspects. If it doesn't run properly for a few seconds on rocket fuel, it HAS to be ignition or timing, and we've already ruled out timing because it ran OK after the first cut-out.