I have a '91 Vanagon that's been sitting idle for probably two years now.  I tried to jump start it with a VW GTI and had no success.

For whatever reason, I decided to try to jump start it with an Audi A6 that has a newer and probably beefier battery and it started up.  I let it run connected to the other car for a few minutes during which time the battery/alt light on the dash of the van was flickering.  It finally went out.

However, as soon as I disconnected the jumper cables, the van stopped running.  I jump started it again and this time, instead of disconnecting the cables, I turned off the source car.  The van kept running.  I disconnected the cables and the van ran for a couple minutes, then stopped running.  I was able to jump start it once again, but disconnecting the cables, caused it to stop running.

Can anyone theorize what the problem might be?  I've never had an instance where a jump started car would die after the cables were removed.  On the other hand, I don't normally keep cars sitting for so long either.

Thanks for any info you could give me,

Doug