A mechanic did a routine exhaust manifold replacement on passenger side. On the first startup after this replacement he called me saying it made a loud clattering noise, and now it is misfiring badly on cylinder #4 which is the same passenger side. He had to drill two studs out that broke off to extract the bolts. Mechanic tried fixing issue by changing out parts to fix misfire but then did a test that diagnosed issue as valve seat failure.
Is it possible that debris entering the motor accidentally into an open exhaust port during the repair could set off a chain of events to cause a valve seat issue?
My car has 115000 miles.
My car has an automatic transmission.
In most cases, when valve seat’s "drop" it’s caused by a reduction in lubrication or excessive heat built-up inside the cylinder heads. However, it’s possible for foreign debris to get stuck between the valve and the guides; which can lead to damage. However, without inspecting the cylinder head personally, it would be very irresponsible for us to determine whether or not the mechanics previous work caused this damage to occur. It’s more than likely that the valve seat was damaged prior.
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