I had to replace my gear case gasket
and it wasnt much fun. If you find out this is the problem, you'll need to pull off the flywheel, alternator, and all belts, etc from the front of the engine. You will have to remove the hard fuel lines from the fuel pump to the injectors. Make sure you close the raw water intake prior to bleeding and cranking because it can take a while to get the fuel through. Look closely at the rotation of the flywheel prior to pulling it and make sure you dont turn the rod with the case cover off (especially not backwards!!!) as you run the risk of throwing off the timing. It really wasnt too big of a job especially considering that I am no mechanic. If you are going to do it, spring for the expensive shop manual and follow the instructions as best you can. I would replace every gasket in between while you have it apart.