* PCV System Failure: The PCV system is designed to vent crankcase pressure and blow-by gases. These gases contain oil vapor. If the PCV valve is clogged, cracked, or the hoses are damaged or disconnected, the crankcase pressure builds up. This pressure can force oil vapor past seals and gaskets, ultimately leading to oil entering the intake manifold and eventually the throttle body.
* Valve Cover Gasket Leak: A leaky valve cover gasket allows oil to escape from the valve cover. Some of this oil could then migrate into the intake manifold via the PCV system or even directly into the intake depending on the leak's location.
* Oil Separator Issues: Some vehicles have an oil separator within the PCV system to help prevent oil from entering the intake. If this separator is faulty or clogged, more oil vapor will make its way into the intake. The YJ is less likely to have a dedicated oil separator compared to later models, but the principle remains.
* Excessive Crankcase Pressure from other issues: Worn piston rings or other engine wear can cause higher than normal crankcase pressure, exacerbating the effects of a faulty PCV system.
* Rare possibility: Rear Main Seal Leak (Less Likely): While less probable, a rear main seal leak *could* potentially contribute if the oil is finding its way up through the crankcase and into the PCV system in large quantities. This is less likely to be the *primary* cause given the oil's presence in the throttle body.
Troubleshooting Steps:
1. Check the PCV valve: Remove it and inspect it for clogs or damage. A simple test is to shake it; a good valve should rattle.
2. Inspect the PCV hoses: Look for cracks, kinks, or disconnections. Replace any damaged hoses.
3. Check the valve cover gasket: Look for signs of oil leakage around the valve cover. A leaky gasket needs replacement.
4. Inspect the intake manifold and other gasket surfaces: Check for other potential oil leaks.
It's crucial to address the underlying cause, which is almost certainly related to the PCV system or a valve cover gasket leak. Ignoring the problem could lead to engine damage due to oil contamination in the combustion chamber.
