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I recently replaced the water pump on my car as part of a restoration project. The old pump is still good and now that I want to conduct a cooling system flush I understand that I should reinstall the old pump to do the flush, is this imperative? There is visible rust at the thermostat and the coolant was in dire need of replacement.

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    Welcome to Motor Vehicle Maintenance & Repair! I don't have a definite answer, but I'd suggest it's not imperative. You should be able to flush it either way. Just getting the block cleaned out is what needs to be done. – Pᴀᴜʟsᴛᴇʀ2 Jul 3 '18 at 21:08
  • My guess is that people suggest flushing with the old water pump on just because it will save you time, since you are going to drain the entire system when you install the new pump. However, I have never heard anyone suggest this. In theory, a new pump will actually provide better flow and do a better job of flushing the system. – Sam Jul 4 '18 at 16:56
  • Flushing with the new water pump will also remove any pieces of water pump gasket material or gasket sealer from the system. – Sam Jul 4 '18 at 17:01
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I say just keep the new pump on and flush the system but i would change out the thermostat also.

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