Short answer
Cooling down, close the seacock, drain the raw-water side, remove the end caps and withdraw or rod out the tube stack. Clear debris mechanically, then descale with a product approved for the metals involved. Renew the end-cap O-rings, refit, and check for leaks under load.
You will need: Spanners, new O-rings or gaskets, approved descaler, containment for waste
A heat exchanger passes engine coolant through a stack of small tubes surrounded by raw water. Both sides foul: the raw-water side with scale, growth and impeller debris, the coolant side with degraded inhibitor and sediment.
What you are actually removing
Two different things, and they need different treatment. Debris — rubber from a past impeller failure, weed, shell grit — is mechanical and comes out mechanically. Scale is chemical, builds slowly, and needs a descaler matched to the metals in your exchanger. Using the wrong chemistry on the wrong alloy causes damage that is far more expensive than the scale was.
The sequence
- Let the engine cool completely and close the raw-water seacock.
- Drain and capture the raw-water side, and the coolant if the job requires it.
- Remove the end caps carefully; the O-rings or gaskets are usually renewed, not reused.
- Withdraw or rod the tube stack, keeping track of orientation.
- Clear debris mechanically, then descale with an approved product and the specified dwell time.
- Rinse thoroughly, inspect tubes for erosion or perforation, and renew the seals.
- Refit, refill, bleed, and run under load while watching temperature and checking for leaks.
When it belongs to a workshop
Tubes that are eroded, perforated or blocked solid mean a re-core or replacement rather than a clean, and that assessment is a workshop's. Pressure testing after any doubt about the tube stack is worth the money: a pinhole between the coolant and raw-water sides contaminates both.
Waste descaler and coolant are pollutants. Dispose of them through an approved facility — see refuelling and pollution rules.