* Transmission Problems: This is the most likely cause. The shaking is likely felt in the entire car, not just the engine compartment. Problems within the transmission (including torque converter issues, internal problems, or low transmission fluid) can cause vibrations that are especially noticeable when the car is in park. A new engine doesn't automatically fix transmission problems.
* Driveshaft/Axle Issues: A misaligned driveshaft or a problem with the axles (possibly damaged during engine installation) could transmit vibrations to the car body, noticeable even in park.
* Engine Mounting Issues: While the engine is new, the engine mounts (the rubber or hydraulic components that isolate the engine from the chassis) might be incorrectly installed, damaged, or worn. Incorrect installation is quite possible given the engine swap. Worn mounts would allow excessive engine vibration to transmit to the car body.
* Exhaust System Issues: While less likely to cause a significant shake *in park*, a poorly installed or damaged exhaust system could create vibrations felt in the car.
* Electrical Issues: Though less common, electrical problems sometimes manifest as unusual vibrations. This is less likely to be the primary cause but should be considered if other possibilities are ruled out.
* Incorrectly installed accessories: If any accessory driven by the engine (alternator, power steering pump, etc.) is not correctly aligned or installed, it might vibrate excessively.
What to do:
1. Check the transmission fluid: Is it the correct type and level? Low fluid is a major contributor to transmission problems.
2. Listen carefully: Where is the shaking originating from? Is it coming from the engine bay, or is it more of a whole-car shudder? This can help pinpoint the source.
3. Visually inspect: Look for anything obviously misaligned or damaged, particularly around the transmission, driveshaft, and engine mounts.
4. Return to the mechanic: If the engine was installed by a mechanic, it's their responsibility to diagnose and fix the shaking. This is crucial as it may indicate a problem with their installation work. Don't pay until the issue is resolved.
Ignoring the shaking could lead to more serious and expensive damage. Get it checked immediately.
