Check the intake manifold and front and rear plate gaskets on the manifold for vacuum leaks. This is very common. The rear plate is a PCV diaphragm and also can cause crankcase pressure buildup. If you have to do any gaskets I would recommend replacing the rear PCV plate whether it has failed or not because of high failure rate. Also with the intake manifold off check for coolant leaks from the belly pan as i mentioned in your other post on coolant seepage.
If you dont have any vacuum leaks then monitor the pre-cat 02 sensors. On 98 model year especially they tend to roll slowly causing problems and driving up hydrocarbons.