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My uncle told me to never trust a 'clean' used oil sample on a new-to-you engine
He said a seller could just change the oil right before the test. Bought a skid steer last month, sample came back perfect, but the coolant had traces of fuel. Turns out the head was cracked. Anyone else run into a sneaky problem like that, where one test looked good but another showed the truth?
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the_harper6mo ago
Ever think to check the old filter? A seller can dump in fresh oil, but they rarely swap the filter. Cutting open the old filter shows the real story, the metal that just got circulated out. That coolant test was your real warning light.
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clairem476mo ago
That coolant test found fuel in the cooling system. I missed that detail completely. My mind just saw "clean oil" and shut off. You're right, that's a huge red flag all by itself, pointing straight to a cracked head or a bad gasket. I was so focused on the metal I wanted the oil sample to show, I ignored the real problem sitting right there. A fresh filter would have hidden nothing if the block is mixing coolant and fuel.
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loganthompson6mo ago
Your uncle has a point, but a clean oil sample is still useful data. It just can't be the only check you do. The coolant test finding fuel is the perfect example. You need to look at the whole picture, not just one report. Relying on a single test is asking for trouble. Always combine oil samples with a pressure check, a visual inspection, and a run-up to operating temp.
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