A100 Only runs when cold

My 1988 Audi 100 Avant (post-revision with RT 5 cyl. engine, non-cat) broke down at the weekend, an hour from home :-( I was going about 20mph when it suddenly splutted and cut out
altogether. Tried to restart it but although I could sometimes get it running it would only last a few seconds.
Anyway, have been checking things out today and struggling to determine the problem. It will start from cold [1] but after about 5-10mins it just cuts out. There is no misfiring, but the idle speed gets into a rhythm hunting between 900 & 1300rpm before it packs up.
I've tested the cold start valve and warm up valve as per Haynes manual and they are ok. Throttle body switch appears to work ok. I've bypassed the Fuel pump relay and it makes no difference. I can see the idle stabliser valve moves when the ignition is switched on and it looks clean inside. All vacuum hoses appear to be in good condition and not blocked. The air sensor plate is positioned ok. The diaphragm device near the coil is open circuit. Coil resistance is ok. I get a spark - all plugs, leads & distributor cap are fairly new).
What other tests can I carry out? There seems to be several things that would prevent starting but what would make it cut out? The Haynes talks about a fuel cut-out on the air-box but I can see no connections on it. Haven't checked the voltages at the Ignition control unit as I can't find that either [2]. There are 2 connectors on the bell housing (near driveshaft) - one is speedo I believe, what is the other?
[1] after about 10 seconds cranking but this has always been the case so may be a red herring. [2] It's near the heater blower below the windsceen on my other A100 but not here! Can't see it under the front passenger carpet either.
Thanks for any help anyone can offer.
Gary
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Things that come to mind: - Clogged check valve in the fuel line by the tank?
- Clogged intake screen on the in tank fuel pump from the coating of the tank flaking off. (Look for dark crud in the fuel filter)
- Clogged fuel filter
The above could be checked with a fuel pressure test.
- Some 5Ks have a fuel pump fuse in addition to relay but that would not be a likely cause.
- Fuel pressure differential sensor on front of fuel distributor. (check archives)
- Clogged CAT

One is the engine speed sensor and the other is the TDC sensor for the ECU. These can go bad sue to 'heat soaking'. These are a real possibility for your problem.
Good luck.
You might also want to check the archives at Audiworld: http://forums.audiworld.com/v8 /
Tony '91 100Q 5spd
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Thanks for your response Tony.
I've now eliminated the ignition system so need to find something in the fuel system that will fail after 10-15 mins use! Would you expect a blockage to fail in this way so consistently? (there was plenty of fuel coming out of the (newish) filter.)
I don't think I have an ECU/fuel pressure differential/CAT.
Not sure how to test the bell housing sensors. The pic on sjmautotechnik looks quite different.
Gary
TonyJ wrote:

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Unplug the O2 sensor, and see if it misbehaves. It's a longshot, but I have seen several cheap O2 sensors bleed heater voltage onto the signal wire. Nothing like 12 volts on a signal wire to lean it out.
On Mon, 13 Oct 2003 22:07:14 +0000 (UTC), Gaz B

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The crank position and engine speed sensors on the bell housing could be but probably aren't the problem. I don't remember which but one (may be the position sensor) tells the ECU/ICU when to start firing the spark and is only used during starting. The other is used continually by the ECU / ICU but if it looses the signal the ECU/ICU will use a default algorithm and keep the engine running, although not too well.
BTW, The model that we both have has both an ECU and an ICU (ignition control module) the ICU is located in the driver's side kick panel. Some people have had a problem with these when water has gotten in to the connector and caused corrosion. Cleaning usually fixes this. I had suspected this on mine and took it apart. It was clean as when the car was built. turned ou the performance problem that I had was due to a bad fuel filter and very much due to an old air filter.
The SJM pics are of a turbo which tend to have more problems with these sensors because the turbo makes a lot of heat which causes a condition called 'heat soak', basically the sensors loose their magnetic properties until the cool down again.
Could be a bad hall effect module in the distributor. Possibly the coil is going out and the output breaks down.
Tony
Gaz B wrote:

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TonyJ wrote:

Thanks Tony. I'm not familiar with term kick panel - is this below the pedals, or on the side with the bonnet release? When you say driver side are you talking LHD or RHD? :-)

I wondered this but I tested it and all seems well.
G.
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Gaz B wrote:

Given that your car has a bonnet rather than a hood like we call in there in the US I guess we are talking my LHD has it in the 'kick panel on the left. the Kick panel I refer to is on the side by the ??? release but I do not know what the RHD configuration would have done with it. I guess it is sort of like an easter egg hunt : )
Language barriers are difficult : )

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Thanks again Tony, quite amusing really! I should have mentioned this is a UK car. Still can't find it though - on either side, but never mind, I'm 99% certain it is fuel related - I always get a spark. I'm no longer sure it a temp related issue though. It still packs up after about 10-15mins but starting it from cold takes ages now. I will keep testing...
G.
TonyJ wrote:

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