* Front Transmission Seal (most likely): This is the most common culprit. The seal wears down over time, and heat from the engine expands the transmission case, causing the seal to leak when under pressure. It won't leak when cold because the expansion hasn't occurred.
* Rear Main Seal (less likely but possible): While usually associated with the rear of the crankshaft, a particularly bad rear main seal *could* seep oil forward enough to appear as a transmission leak, especially if the engine is tilted slightly. Heat and pressure from running would exacerbate this.
* Improper Torque on Transmission Bolts: If the bolts weren't torqued to the manufacturer's specifications, the mating surfaces might not be perfectly sealed. Heat expansion could then open up microscopic gaps. This is less likely to cause a leak *only* after running, but it's possible.
* Damaged or warped Bellhousing Surface: If the bellhousing or engine block surface is warped or damaged, a proper seal may not be achievable. Heat expansion could worsen this issue.
* Cracked Bellhousing (least likely): Although less probable, a small crack in the bellhousing could develop and leak only when the oil pressure and heat increase.
* Transmission Case Crack (rare): A very unlikely possibility is a crack in the transmission case near the mating surface.
Why it leaks only after running: Heat is the key. The engine and transmission heat up, causing expansion of the metal parts. This expansion can open up tiny gaps or deform seals, leading to leakage. The pressure of the oil system also increases as the engine operates.
Troubleshooting Steps:
1. Clean the area thoroughly: Get everything spotless around the suspected leak. Then run the engine for a short period, then carefully inspect where the oil is coming from. This is critical for accurate diagnosis.
2. Inspect the front transmission seal: This requires removing some parts (at least the starter). Close inspection will reveal if it's damaged or improperly installed.
3. Check the rear main seal: While less likely, a visual inspection might reveal evidence of leakage here, especially if oil trails forward.
4. Verify transmission bolt torque: Check the torque specification in your engine's service manual and verify that all bolts are properly tightened.
5. Check the mating surfaces: Once the transmission is removed (if necessary), check the surfaces of the transmission and the engine block for warping, damage, or gasket material stuck in the wrong place.
Unless you're experienced with engine repair, it's advisable to have a mechanic diagnose and fix this leak. A misdiagnosis can lead to more extensive (and expensive) repairs.
