Porsche 924 Scheiße - knackered fuel accumulator?

[N Dentressangle]

All good questions. I've checked the leads to make sure the order's correct, and of course it still ran OK on Sunday - it's deteriorated gradually to how it is now. And I did put tons of brake cleaner in to make sure 😎

Hopefully the new ignition module will arrive tomorrow so we can rule that out. But we'll see. The Rover V8 is notorious for eating ignition modules, and you do still get sparks when they're on their way out - they're just weak and useless ones. But it can fool you into thinking things are OK.

[N Dentressangle]

2 minutes ago, rusty_vw_man said:

Here’s another idea - is the dizzy clamp fixed tight? Could it have moved? Was running, shifts a bit and is an arse to start, shifts a bit more and won’t go at all??

Could go back to basics, find tdc on no1 and check the dizzy marks line up? 

I'll probably do that as a free no-cost check when I have time tomorrow or Friday, but I'm 99.9% sure it's not that.

Mate and I are currently wondering whether we can make a Lucas 25 or 45 do the job. The Bosch dizzies are crazy expensive, and I have nothing personally against points 😉

[N Dentressangle]

OK, had the help of a good mechanic friend today. We replaced the ignition module with a used one and no change.

Hall sensor tested in the distributor, and seems to produce a small voltage as you'd expect when rotated, so not obviously failed. It produces more than a comparison sensor on a working Rover V8 distributor, in any case.

Coil, leads etc all swapped out to no avail. All wiring to the ignition module checked for continuity and tests fine.

The spark produced is tiny and irregular. There is no attempt by the engine to start when brake cleaner is sprayed directly into the manifold whilst cranking.

3hrs of trying everything we could and we're both stumped. Now hoping one of the guys on the 924OC forum will be able to help.

🤷‍♀️

[Westbay]

Have you checked the engine to body earth ?

[N Dentressangle]

27 minutes ago, Westbay said:

Have you checked the engine to body earth ?

Don't think so - will have a look.

All suggestions gratefully appreciated!

[N Dentressangle]

Another earth lead added. No change.

Deep sigh

[Westbay]

1 minute ago, N Dentressangle said:

Another earth lead added. No change.

Deep sigh

Bu**er ...

[N Dentressangle]

I'm borrowing a definitely good ignition module and a pressure tester from a local 924OC guy later, so we'll see what that brings.

Can't remember the last time something resisted diagnosis for this long 😭

[EyesWeldedShut]

On 28/09/2025 at 14:56, N Dentressangle said:

Coil resistances tested and are within range, but swapped out for a spare 6v one anyway:

By 6V do you mean ballasted? It'd be 6V-9V (ish) when engine running. Full-fat 12V when starting though - any chance that the car's not bypassing the ballast resistor so trying to start on the lower voltages? No clue whether there's an external resistor on the 924 or not (it's probably the only bit on ours that never broke) 
 

[N Dentressangle]

10 minutes ago, EyesWeldedShut said:

By 6V do you mean ballasted? It'd be 6V-9V (ish) when engine running. Full-fat 12V when starting though - any chance that the car's not bypassing the ballast resistor so trying to start on the lower voltages? No clue whether there's an external resistor on the 924 or not (it's probably the only bit on ours that never broke) 
 

It's not quite like that, but we don't believe that's the problem. We also fed the coil 12v directly from the battery to rule this out.

[Tenmil Socket]

I can’t view any pics for some reason?
 

[N Dentressangle]

9 minutes ago, Tenmil Socket said:

I can’t view any pics for some reason?
 

It's the tossers at Imgur who've blocked their site from the UK. I have stuck this extension onto Chrome https://veepn.com/vpn-apps/vpn-for-chrome/ which seems to get around the problem for free.

[loserone]

Would not trust. 👀

[N Dentressangle]

9 minutes ago, loserone said:

Would not trust. 👀

Yeah, that's the internet innit.

I am beginning to really hate the internet and technology. It's fucking up the world, fast.

[rusty_vw_man]

So, in summary:

- new leads and plugs, but with old ones put back to check

- changed ignition module

- hall sender is pulsing 

- steady 12v at coil

- good earth  

- piss weak irregular spark at king lead (so not rotor arm/dizzy) and timing is irrelevant. 

if that’s all correct then it’s a genuine mystery! Have you another coil you could use? 

edit to add is the pulse from the hall sender turning up reliably at the ignition module. 

[N Dentressangle]

32 minutes ago, rusty_vw_man said:

So, in summary:

- new leads and plugs, but with old ones put back to check

- changed ignition module

- hall sender is pulsing 

- steady 12v at coil

- good earth  

- piss weak irregular spark at king lead (so not rotor arm/dizzy) and timing is irrelevant. 

if that’s all correct then it’s a genuine mystery! Have you another coil you could use? 

edit to add is the pulse from the hall sender turning up reliably at the ignition module. 

Yes, two different alternative coils tried.

New wires run directly from hall sender to ignition module yesterday - made no difference.

Mystifying, innit?

[juular]

How about eliminating everything except the coil, king lead and a spark plug in a sort of in-situ bench test?

Wire directly from battery+ to coil positive.

Wire from battery- to coil negative.

Jump lead from battery negative to the threads on a spark plug 

King lead from coil to spark plug.

Disconnecting / pulsing the negative to the coil should give you a good solid spark.

If this works you could build on it by including another part of the puzzle at each step, like running the coil with its original positive wire on the car and retesting, then moving to finding a way to include the ignition module and trigger it manually, then add the dizzy into the picture and turn it by hand.

Sometimes testing with a multimeter in a static situation doesn't tell the whole story. You can often get what looks like a healthy 12v or a solid ground from a wire but as soon as you put a load on it, things fall apart due to high resistance somewhere because of a poor crimp or corrosion.

[N Dentressangle]

1 hour ago, juular said:

How about eliminating everything except the coil, king lead and a spark plug in a sort of in-situ bench test?

Wire directly from battery+ to coil positive.

Wire from battery- to coil negative.

Jump lead from battery negative to the threads on a spark plug 

King lead from coil to spark plug.

Disconnecting / pulsing the negative to the coil should give you a good solid spark.

If this works you could build on it by including another part of the puzzle at each step, like running the coil with its original positive wire on the car and retesting, then moving to finding a way to include the ignition module and trigger it manually, then add the dizzy into the picture and turn it by hand.

Sometimes testing with a multimeter in a static situation doesn't tell the whole story. You can often get what looks like a healthy 12v or a solid ground from a wire but as soon as you put a load on it, things fall apart due to high resistance somewhere because of a poor crimp or corrosion.

That could be a very good plan - thanks.

One of the 924OC guys has lent me:

- a known good ignition module. Just tried it and guess what? Makes no difference 🤣 What I expected TBH

- a fuel pressure tester

I'm now going to try the pressure tester to see what's going on with the fuel system, as other than a crap looking spark I've found nothing actually wrong with the ignition system. Back soon with results!

[juular]

Definitely worth testing for fuel. 

Even a simple 5 minute check of pulling the injectors out and putting them in bottles and cranking will let you know if it's getting anything during a start condition.

[juular]

24 minutes ago, N Dentressangle said:

I have now measured the system pressure at 35 psi.

Following the tech sheet, I have tried to check the pressure relief valve, which I presume is this:

I can't see a spring of any kind in there. What am I missing?

35psi, so about 2.3 bar and way too low. You want around 5 - 5.5 bar.

Not aware of a relief valve so it may be a difference in terminology.

Suspect you are looking for the main pressure regulator valve which controls the return of fuel to the tank and does control the system pressure.

It's the biggest bolt on the side of the metering unit and has an Allen recess in it. 

Looks like this.

That's a rather high amount of system pressure loss to be honest, it may well be that the o-rings above have given up and lots of fuel is escaping back down the return line.

[N Dentressangle]

Test One complete - we have a System Pressure of about 68 psi, which is where it should be.

Oddly I only have this if I jumper the FPR to make the pump run continuously. With the FPR installed the pump runs briefly then stops, and the pressure reaches only about 35 psi.

[N Dentressangle]

2 minutes ago, juular said:

35psi, so about 2.3 bar and way too low. You want around 5 - 5.5 bar.

Not aware of a relief valve so it may be a difference in terminology.

Suspect you are looking for the main pressure regulator valve which controls the return of fuel to the tank and does control the system pressure.

It's the biggest bolt on the side of the metering unit and has an Allen recess in it. 

Looks like this.

That's a rather high amount of system pressure loss to be honest, it may well be that the o-rings above have given up and lots of fuel is escaping back down the return line.

Sorry - I checked again with the fuel pump relay jumpered so it ran continously.

[juular]

2 minutes ago, N Dentressangle said:

Test One complete - we have a System Pressure of about 68 psi, which is where it should be.

Oddly I only have this if I jumper the FPR to make the pump run continuously. With the FPR installed the pump runs briefly then stops, and the pressure reaches only about 35 psi.

Ah that's not so bad. A tiny bit low but not enough to make it run badly at all.

The behaviour above is normal.

The fuel pump relay takes pulses from the ignition module. Each pulse runs the pump for a short period, so that if the engine stops in a crash for example, fuel flow stops.

[N Dentressangle]

Test Two - operating pressure.

With the valve open, we are seeing 25 psi in the system. It's about 12 deg C in Stroud currently, so I'm guessing that's a little on the high side.

Worth removing the WUR at this stage to check for blockages to flow? I could also re-fit the standard non-vac WUR I have at the same time.

[juular]

2 minutes ago, N Dentressangle said:

Test Two - operating pressure.

With the valve open, we are seeing 25 psi in the system. It's about 12 deg C in Stroud currently, so I'm guessing that's a little on the high side.

Worth removing the WUR at this stage to check for blockages to flow? I could also re-fit the standard non-vac WUR I have at the same time.

Maybe a touch high but still well within the range that it should run. In fact I don't have the 924 pressure-temperature graph handy so it may well be within spec for that setup. 

If it were me I'd skip touching the WUR for the moment, unless you find a smoking gun that sends you that way.

A blocked WUR would cause control pressure to be nearly at system pressure.

Even in that extreme case it will often still start - eventually.

[juular]

What happens if you jump the fuel pump relay so that the pump is permanently on, and try to start it?

[N Dentressangle]

Just now, juular said:

What happens if you jump the fuel pump relay so that the pump is permanently on, and try to start it?

It still doesn't start 🤣

[MrBig]

Having spent many hours suffering at the wrong end of VAG k-jet and hall effect distributors... I'd be trying to borrow a known good dizzy and seeing what happens,

[juular]

1 minute ago, MrBig said:

Having spent many hours suffering at the wrong end of VAG k-jet and hall effect distributors... I'd be trying to borrow a known good dizzy and seeing what happens,

If that's an option, most definitely a good way forward.

[N Dentressangle]

2 minutes ago, juular said:

If that's an option, most definitely a good way forward.

Yep, I think I agree. But then the spark is shit even directly from the king lead, so I'm not convinced the dizzy is guilty.

Test Three is warm engine "operating pressure", which for obvious reasons I can't do.

Fucksticks