My advice is that your engine is toast, sorry, I also think if not done at a Toyota dealer you should be able to get a used engine for around $1200 to $2000 and then labor on top of that would run about the same, so anyone quoting you more than say $4500 max is just trying to make money off of your misfortune.

That being said even if you get the engine replaced for a reasonable amount or any amount for that matter, you could still end up with other issues soon after or over time if the job is not done properly, so IMHO I would try and sell your T4R and get another one with a good history.

Also before any blame is placed on your mechanic, I want to try and get you to clarify about the "oil on the spark plug", did the plug have oil on the tip of the plug where the spark occurs or on the threaded portion of the plug as in the following photo??

If the oil was only on the threads as in the photo, then it wasn't due to a "valve seal" but instead it was due to the spark plug tube gasket and not a major issue, the spark plug gasket is a part of the valve cover gasket set, which is a BIG difference than a "valve seal", to replace the spark plug tube gasket which would be done while replacing the valve cover gasket is not a major nor costly job and shouldn't have caused the issue you now have, now a valve seal replacement would be a more complicated and costly job, which would require removal of the engine head to do and if not done properly as mentioned could of caused your issues.

How much did your mechanic charge you to replace the "valve seal" this would be very telling on what he actually did?

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