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Sudden loss of power when hot

 
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jacobroufa  



Joined: 18 Nov 2016
Posts: 294
Location: Rockford, IL

PostPosted: Mon May 25, 2020 9:29 am    Post subject: Sudden loss of power when hot Reply with quote

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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ideola  



Joined: 01 Oct 2004
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PostPosted: Mon May 25, 2020 2:17 pm    Post subject: Reply with quote

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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924RACR  



Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA

PostPosted: Mon May 25, 2020 10:18 pm    Post subject: Reply with quote

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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jacobroufa  



Joined: 18 Nov 2016
Posts: 294
Location: Rockford, IL

PostPosted: Tue May 26, 2020 12:38 am    Post subject: Reply with quote

ideola wrote:
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


924RACR wrote:
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.


Awesome! Thanks guys! I read that link last night and was wondering exactly what heat sink paste so I'm glad you both chimed in. I think I have some in a drawer somewhere actually, so I might give this a whirl today. Glad to read too that it's a more commodity part from the VW bin! After all the specialty parts on the 931 some of which are completely NLA, it's nice to catch a break on a shared-use part!
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jacobroufa  



Joined: 18 Nov 2016
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PostPosted: Tue May 26, 2020 1:03 am    Post subject: Reply with quote

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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CÚdric  



Joined: 27 Aug 2004
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PostPosted: Tue May 26, 2020 1:30 am    Post subject: Reply with quote

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



Joined: 18 Nov 2016
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Location: Rockford, IL

PostPosted: Tue May 26, 2020 2:51 am    Post subject: Reply with quote

CÚdric wrote:
Could be good to swap it out regardless, I've had issues with old ones and new ones costs peanuts


I might grab a spare, yeah. I think it is working fine; will give it a hot start/drive test here in a few.

Found some AS5 in a drawer from when I used to do more stuff with computers. This stuff is fairly aged at this point; maybe about 15 years old?? It is thicker than I remember, but it still spread well enough.



Lapped the heat sink and back of the ignition module with 800 -> 2000 -> 3000 dry, just for good measure before I put it back together.



So I put it all back in place and... no start. GRUMBLE

Pop the hood, wiggle the wires, BOOM fires right up. Everything seems good to go. I rev it hard for grins to the end of the driveway, and it STOPS AGAIN.

Pull the connector, pop it back into place with more assertion, and we're good to roll again. No problems.

I just took it for a good 20 minute drive, really working it. I can't tell if I'm fetching for things to be different or not, but it *feels* smoother throughout the rev range and there's less stumbling in general. I feel like this has been one of the intractable problems with this car -- I remember my mechanic last year talking about trying to get the CIS tuned up right and smoothed out but it never quite went how he wanted. Maybe this was part of the issue all along?

I am going to pick up a spare anyhow, to be safe. I would hate to get stranded for something silly like a $20 part..

Time to go for another drive; hopefully being warmed up and it being warm outside too, I will have the same conditions as yesterday when this problem cropped up. Fingers crossed everything works well!
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jacobroufa  



Joined: 18 Nov 2016
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PostPosted: Tue May 26, 2020 3:31 am    Post subject: Reply with quote

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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jacobroufa  



Joined: 18 Nov 2016
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Location: Rockford, IL

PostPosted: Tue May 26, 2020 3:42 am    Post subject: Reply with quote

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:

Quote:
These are usually quite reliable, however they can be easily damaged by voltage spikes caused by improper type spark plugs, very worn spark plugs, or a faulty alternator. If this unit is faulty you will either have a no-start or a random cutting off condition.


Any chance this is part of the culprit? This module is harder to find however and more expensive. Just curious for folks' thoughts here, as it's part of the same related electrical system.
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jacobroufa  



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PostPosted: Tue May 26, 2020 7:41 am    Post subject: Reply with quote

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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Fasteddie313  



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PostPosted: Tue May 26, 2020 1:14 pm    Post subject: Reply with quote

Could be in the middle of the wire somewhere?
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!tom  



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PostPosted: Tue May 26, 2020 1:38 pm    Post subject: Reply with quote

It's possible for internal connections to fail so that they are intermittent as you describe.
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jacobroufa  



Joined: 18 Nov 2016
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Location: Rockford, IL

PostPosted: Wed May 27, 2020 9:21 am    Post subject: Reply with quote

Fasteddie313 wrote:
Could be in the middle of the wire somewhere?

!tom wrote:
It's possible for internal connections to fail so that they are intermittent as you describe.


Definitely. This was my thought as well; I need to get out with a multimeter and check continuity. I am going to order a spare ignition module as well -- seems cheap insurance against a "stuck on the side of the road" ordeal.

Anyone know about that EIS though?

This thread is interesting... http://www.924board.org/viewtopic.php?t=29280

From the last post:
gohim wrote:
EIS causes problems on a 924 engine when the adjustments are not correct, or there are uncorrected vacuum leaks. EIS on a poorly running 924 engine causes flat spots in the acceleration curve, hard starting, poor engine performance, and sometimes no start on a cold OR hot engine (one or the other, but not both at the same time).


That sounds like my car! (flat spots in the acceleration curve) Interesting...
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ideola  



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PostPosted: Wed May 27, 2020 9:31 am    Post subject: Reply with quote

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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jacobroufa  



Joined: 18 Nov 2016
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PostPosted: Wed May 27, 2020 10:10 am    Post subject: Reply with quote

ideola wrote:
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.


I did read that; didn't realize it was part of the idle/timing procedure! I'll definitely give that a whirl. A proper behind the wheel test is way more fun than hunting with a multimeter.
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