Is it a group 41 or a group 42 battery? The 42 is the smaller one that came in most gas cars. The group 41 is the bigger battery that the diesels call for and will also fit in the gas cars. I beleive it did come in some of the gas cars though. If well charged, the group 42 will also start a diesel with no problem though.
Have you tested the voltage at the battery while the car is running and the battery light is off?