Hi,
First, the idea that two cylinders side by side have a misfire code makes it seem more likely to be a gasket. If the vehicle was overheated, I would suspect a possible engine issue.
As far as testing, here is a link that explains how to check for a cylinder head issue. This is while the engine is together.
https://www.2carpros.com/articles/head-gasket-blown-test
Now, since he indicated compressed air entering the cooling system, it could be what he said or a gasket. Here is the tough part. The cylinder head requires over 20 hours to remove and replace which is going to be expensive. Actually, removing and replacing, and basically rebuilding the head calls for 27 hours. See pic below.
At this point, we know there is an issue between cylinders one and two. We know compressed air is entering the cooling system. So, we have one choice. The cylinder head needs removed and inspected along with the gasket. If there is no evidence of a gasket issue, then they have to Magna-flux the engine and cylinder head to see if there are cracks present.
I wish I had a better answer, but that is where we are with this issue.
Let me know if you have questions about whether this helps, or if there is anything I can do to help.
Take care,
Joe
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Tuesday, May 16th, 2023 AT 8:16 PM