How to fix a leak in the marine engine cooling system

Stu Davies

Stu Davies advises PBO reader Richard Jales how to solve a leak in his boat's marine engine cooling system

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We have a leak in the marine engine cooling system, evidenced by salts accumulating on top of the engine, mainly but not exclusively around the thermostat housing.

I think there are two issues: the pipe is leaking at its junction with the exhaust riser, and the thermostat housing itself also appears to be leaking.

A badly perished oring

O-ring from feed tube is badly perished

I am planning to replace the seals of the pipe and the thermostat O-ring.

While the whole thing is apart, it’s probably also worth replacing the thermostat.

My plan is to:

  • Remove clamp bolt and clamp
  • Slacken lifting eye bolt
  • Remove two thermostat housing bolts
  • Remove thermostat housing with gentle taps parallel with pipe (not to distort the pipe)
  • Remove pipe
  • Change thermostat to new part
  • Change two pipe seals to new parts
  • Change thermostat housing seal
  • Re-assemble thermostat housing with new bolts (these are different lengths)
  • Reassemble pipe clamp with new bolt

I’m concerned that the bolts in the thermostat housing may shear, as there is corrosion present, and the housing is brass and the bolts are steel.

Galvanic corrosion is possible.

New coat of paint on the thermostat housing and feed pipe to fix the marine engine cooling system

New coat of paint on the thermostat housing and feed pipe

If the bolts shear in the head then it could be a really difficult job to get them out.

My question is are these bolts commonly known to shear and is there a good way to avoid that?

In your experience are they likely to break?

A boat engine with a leaking marine engine cooling system

After getting Stu’s advice, Richard successfully tackled the work required

I’m reasonably competent with things mechanical and have a good set of tools, including a torque wrench and powered impact driver.

Or would I be better off not to risk an attempt at the repair on my boat engine cooling system and get an experienced marine engineer to do it instead?

Richard Jales

Stu Davies replies:

The leaks are coming from the O-rings for sure. Whether there’s also one coming from the thermostat housing, however, I’m not so sure.

So, it’s a good idea to do all of the likely sources of leaks.

New thermostat and O-ring seal in place

New thermostat and O-ring seal in place

Your concerns about the bolts shearing? I don’t think they will, I haven’t had any reports of them doing so.

My advice in this case is to clean off the salts then apply lashings of penetrating oil.

Continues below…

Leave for a day or so then apply moderate pressure to tighten the bolts first, before going for the undo.

You are not trying to turn them when tightening, just to apply pressure to ‘break the seal’.

Your plan of action is sound.

A repainted boat marine engine having repaired a marine engine cooling system

Reassembled engine cleaned up with a fresh coat of paint

Once apart, careful cleaning of the mating surfaces is essential and might also give you an opportunity to apply some primer and green paint to the rest of the engine around that area

Do you need an experienced marine engineer?

I don’t think it’s necessary given your grasp of the task in hand for what should be a relatively simple job.

Enjoyed reading How to fix a leak in the marine engine cooling system?

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