924Board.org

[jacobroufa]

https://photos.app.goo.gl/5sCaCJA1XGWEcQbn8

This video pretty well explains it but I'll try to write it out here too.

So when the Weissach is cool, or on a cool day, I have never experienced this before. I have driven the car today more than in the last year and a half and all seems to be pretty well.. for the most part..

Today the heat index is 90, and I've had the car out a couple times. After sitting for a bit, and I go to start it back up, everything is fine, but when I start to drive, the tach will just suddenly drop to zero and the car bucks, then it comes back and there's sometimes a tiny backfire (fuel still going?) and it's back to normal. This situation goes away after a bit of driving, presumably once we're getting airflow somewhere...? Is there a chance that the coil is overheating or some other thing is going on there? I don't know where to begin diagnosing this; never experienced anything like it before.

Can you tell from the video at all? About 45 seconds in is when it happens. I tried to capture this on camera several times today without luck. Should have let it roll a bit longer as I went out of the neighborhood, as it happened more until I was about a half mile down the road.
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1980 Porsche 931
1981 Porsche 924 Weissach

[ideola]

My first guess is the ignition module on the driver side wheel well. I believe it's the same or similar module as in the 1981-onward 931. They are glued to a heat sink with a special heat sink glue that has a tendency to deteriorate and fail over time. The ignition module will heat up and cause misfires.

See also:
http://www.924board.org/viewtopic.php?p=395844#395844
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75 Fiat X1/9 | 80 US 928 | 80 US 931 '941' | 78 D-Prod Replica | 78 Poli-Form

[924RACR]

When I replaced my ignition module, I used the same heat sink paste (silver based) as used for CPU-motherboard heat sink installations to ensure a good heat transfer to the heat sink. Did also clean off some corrosion on the heat sink.

Would also not hurt to ensure the pins in the connector and the ground are good for that too, while you're in there.
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Vaughan Scott
Webmeister
'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[jacobroufa]

[jacobroufa]

Ignition module.



Pins look good!



And there's no thermal paste/adhesive to speak of!! I will have to remedy that.


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1980 Porsche 931
1981 Porsche 924 Weissach

[Cédric]

Could be good to swap it out regardless, I've had issues with old ones and new ones costs peanuts
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´80 924 Turbo

[jacobroufa]

[jacobroufa]

No problems on a drive just now. It's not as hot today as it was yesterday but I got the car pretty warm earlier, and the higher temp fan switch doesn't kick on until the coolant gauge reads close to 3/4... Got it home and it was definitely there. Let it sit for about 10 minutes to soak in the engine bay, trying to recreate yesterday's conditions. On this last drive though it's still less stumbly than it was yesterday or has been in the very recent past, and there was NO loss of power or hesitation whatsoever.

I think that was it!!

To boot, it has corrected a latent/sporadic complaint of mine, that this 924 is maybe too bouncy in some ways. It has always felt not quite 100% on throttle, always with some amount of hesitation. My mechanic spent a lot of time digging at the CIS last year, tuning it up and making sure everything was correct on the fuel side of things. There were definitely some small issues there he fixed yet things were not 100%. I replaced every single vacuum line last year. It has new plugs and wires and cap and rotor.. Maybe the ignition module heating too much was the cause of a good chunk of these issues!?

Unrelated to the issue of this topic, I still think I need motor mounts though and that will probably correct more of that bouncy feel...

Thanks Dan and Vaughan for pointing me in the right direction here, much appreciated. I really am amazed at how drastically this tiny thing improved the running of the car today.

If anyone has any errant stumbling they can't seem to account for on a naturally aspirated 924, I would 100% recommend pulling this part, cleaning the heck out of the heatsink and back of the module, applying some thermal paste and reassembling. Night and day!
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1980 Porsche 931
1981 Porsche 924 Weissach

[jacobroufa]

Reading some more just to back fill context here...

The ignition module which I replaced the heatsink thermal paste is listed as 191 905 351 A.. The part I have is 211 905 351.. Presumably I can use either as a backup/replacement?

Additionally, the EIS which sits on top of this part on a bracket, 477 906 083... just looking in the PET and looked this part up and one place it's listed for sale (out of stock) https://www.epartsland.com/products/porsche-924-electronic-ignition-stabilization-eis-control-unit-477906083 in this description it says:

[jacobroufa]

Bah. Same issue as yesterday. I am going to have to double check the connections; it's really hot again and just took it on a drive, I hit a bump and lost power... :/ came back.. same behavior. Maybe I get a replacement afterall? Though why would a bump in the road cause it to have an issue unless it is about the connection.
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1980 Porsche 931
1981 Porsche 924 Weissach

[Fasteddie313]

Could be in the middle of the wire somewhere?
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80 Turbo - Slightly Modified

[!tom]

It's possible for internal connections to fail so that they are intermittent as you describe.
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78 924 NA
5-lug

[jacobroufa]

[ideola]

You can bypass EIS by unplugging the leads from the module and connecting them together. In fact, this is required in the procedure for setting idle speed and timing. It would be worth trying to see if it eliminates your problem.
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75 Fiat X1/9 | 80 US 928 | 80 US 931 '941' | 78 D-Prod Replica | 78 Poli-Form

[jacobroufa]