I had that happen on my 80 TD, turned out the fuel tank was nastified and ended up having to pull it out and thoroughly clean it.

The symptom was stalling after freeway driving and the visible change in the engine compartment was lack of fuel in the pre-filter.

I was able to get it restarted, after lots of manual pumping on the priming pump, but it would die shortly thereafter. You could test this theory by hooking up a temporary fuel tank in your engine compartment (ala Diesel Purge style) to bypass your fuel tank completely. If it runs fine like that, then you've found your problem. I wish I thought of that when I went through this.

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-justin

1987 300TD, 1987 300TD
2008 R32, 2000 Passat Wagon