The 'timing' really didn't become mechanically variable until 1999+ engines. What happens with your engine is the ECU responds to throttle and vacuum pressure. The more throttle, the less vacuum pressure. Vacuum pressure also regulates the fuel pressure, less vacuum = more fuel pressure. Eventually, the additional fuel will foul the plugs so much that the car will stall and refuse to start until the gasoline in the cylinders dries.

What you want to do is follow every vacuum line that enters/exits from the intake manifold. Now, there is one that always cracks with age: It is under the #1 cylinder runner, close to the PS pump. It comes out of the manifold pointed at the fender and then makes a 180 to go under the runners to the PCV/flame trap. It is also the hardest to replace, which is why people replace this pipe when replacing the oil separator and other lines.

To check for vacuum, warm up the engine, turn it off. Remove the oil filler cap and wrap a latex around the opening, I use a rubberband. Then start the engine and watch the glove, if it inflates you have a problem.

The vacuum line for the 1998 engine's fuel pressure regulator is strange. There appears to be 2 fuel lines coming over the right side of the engine, one is the fuel line, the other is a vacuum line attached to the front of the manifold. Follow this line down the back side of the engine and you will see it enters the FPR which is attached to the sub frame. If the FPR fails, cold starts will take a long time to crank and there will be a puff of black smoke on engine start.

There are other vacuum lines that go to the air cleaner. Check the elbows.

If the latex glove expands, start getting ready to replace the oil separator and all of the lines. And then start using synthetic engine oil.
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