1. Rear Main Seal: This is a prime suspect for leaks appearing near the starter. The rear main seal seals the crankshaft where it exits the engine block. A leak here will often run down the back of the engine, making it appear to originate from behind the starter or even drip from the head cover gasket area as the oil migrates downward. This is a significant repair.
2. Valve Cover Gasket (Head Cover Gasket): While you mention it, a leaking valve cover gasket will primarily leak oil onto the valve cover itself and potentially drip down the sides of the engine. If the leak is significant, it could run back towards the starter area. This is a relatively easier repair.
3. Oil Pan Gasket: A leaking oil pan gasket is less likely to directly manifest *behind* the starter, but if the oil pan gasket is significantly damaged, oil could travel down the engine block and appear to leak from that area.
4. Front/Rear Timing Cover Seal (Less Likely): While less likely to directly cause a leak behind the starter, a failing seal in these areas *could* cause oil to seep and migrate to lower areas, ultimately appearing near the starter.
5. Oil Pressure Switch/Sender: While less common, a leak from a faulty oil pressure switch or sender could potentially contribute to the observed leakage.
6. Combination of Issues: It's entirely possible that you have more than one of the above problems. For example, a leaking rear main seal and a slightly leaking valve cover gasket could create a substantial amount of oil in that area.
Troubleshooting Steps:
1. Thorough Cleaning: Clean the engine thoroughly with degreaser and a pressure washer (carefully!). This will allow you to pinpoint the *exact* source of the leak after a short driving period. Don't forget to protect electrical components.
2. Visual Inspection (After Cleaning): Once clean, carefully examine the engine after running it briefly. Look closely at the rear main seal area (requires some dismantling), the valve cover gasket, and the oil pan gasket.
3. Dye Test (Optional): A dye test can help trace the leak if visual inspection isn't conclusive. This involves adding a fluorescent dye to the oil and then using a UV light to find the leak source.
In short: The leak behind the starter strongly suggests a rear main seal issue, but you should thoroughly clean the engine and inspect all potential leak points to get a definitive answer before undertaking any repairs. The repair difficulty and cost vary significantly depending on the cause – a valve cover gasket is much simpler than a rear main seal replacement. It's advisable to consult a mechanic for a proper diagnosis and repair quote.
