Get one of these to test with:
If the test shows combustion in ur radiator then do a compression test to see which cylinder is bad. If all cylinders show good do a leak down test.
If the radiator combustion test does not show combustion gases then get a good radiator flush/degreaser

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If the test shows combustion in ur radiator then do a compression test to see which cylinder is bad. If all cylinders show good do a leak down test.
If the radiator combustion test does not show combustion gases then get a good radiator flush/degreaser