Engine overheating alarm

[matan]

Cruisers
Your suggestions are needed. Engine Overheating alarm comes on.
Next in the series of small but debilitating problems. The main engine is overheating. We did the trip to the Abrolhos in May with no overheating problems even though we did some hours of motoring. Since then we have been getting an overheating alarm after about 20 minutes running at slow speed in the marina.
We have flushed all of the external hoses and back flushed the oil cooler. Checked the pump and impeller. Cleaned the raw water intake screen. There seems to be a reasonable amount of water coming out w/ the exhaust. This water is not hot when it comes out.
The marine electrician suggested changing the sender. Did that, but get the overheating at 20 minutes. We also changedthe thermostat.
Motor is 11 years old and has done less than 400 hours. The cooling system is raw water thru the block, the only heat exchanger is an external oil cooler.
Our next strategy is to de-scale the head and block.
Any suggestions on how to do this ?
Or any other suggestions,
Jim
Rabbit Proof Fence
Fremantle , Weatern Australia

[Lepke]

Salt water cooled engines can get a rust and/or salt buildup that insulates the engine from the cooling water. You need to flush the engine water passages with a good marine descaler.
With the engine running hot, even more salt will settle out of the water.

[Compass790]

What Lepke said. There are vids on youtube about it & lots of info in other threads on this forum.
Vinegar didn't seem to work on ours but I just tried it cold.
Try the google custom search & see what pops up.
If you are in a marina you can put a tee in your water intake & fresh water flush it after each outing. We do that & no need to descale in 6 years.

[Wotname]

Matan, you haven't told us which model engine you have so the following is generic information relating to raw water cooled diesel engines.

Remove your temperature sensor and test in a container of hot water using a thermometer to note the water temperature and ensure the engine gauge is accurate. You should be running a RWC engine below say 55C (50C is better) otherwise calcium build up will occur fast.

Check your engine zincs - they must be maintained otherwise rust will build up inside the engine causing over heating.

De-scale regularly otherwise calcium build up occurs which causes overheating.

To de-scale a RWC engine, remove thermostat (ensures de-scaling fluid passes though the head / block).
Remove zincs otherwise they will be eaten away by the de-scaling process.
Block the bypass hose from the RWC pump to the exhaust injection point otherwise the de-scaling fluid simply flows directly past the block rather than though it.

Rig up a container (say 10 litres) and a pump (like a small electric bilge pump) and circulate the de-scaling fluid from the container though the engine and back to the container.

Use a de-scaling fluid of choice. I have used vinegar (good for mild scaling) or hydrochloric acid (good for heavy scaling). Some prefer commercial products and tout various reasons why they are better.

I let the vinegar circulate overnight but as hydrochloric acid is much stronger and faster, maybe a hour or two.

EDIT. I would probably remove the oil cooler while de-scaling unless I knew it was safe from whatever fluid you use.

Compass 790 makes a good point about the temperature of the de-scaling process - warm is better (quicker) than cold. Also flushing with fresh water must be a good thing.

[CassidyNZ]

Start with an infra-red thermometer aimed at the thermostat housing (normally the hottest spot). In other words, understand whether the engine is actually overheating or whether you are getting a false alarm.

If it is really overheating then start on all the onerous tasks of checking circulation, blockage, etc. if it’s not really overheating, then start looking for sensor faults, wiring, etc. At least you’re not wasting time with the wrong tasks.

I’m assuming you have only an alarm, there is no mention of a temp gauge in your post. If you have a temp gauge, what is it telling you?

[matan]

The engine is a bmw d50. It is a three cyl. Hatz diezel that bmw rebranded. The thermostat housing is not hot when the aalarm goes off. You can hold you hand on it for a few seconds before it begins to become. You can touch the engine all over, with the exception of the core plugs which are hot but not terribly so. The engine does not radiate heat. I have had a marine electrician check thewiring and he recommended a new sender. I replacedthe sender and no change. The alarm, there is no temp guage port onthis motor, ssounds after a warm up period of 20 to 30 minutes at a fast idle. Water passed easily through the oul cooler when we back flushed. No material came out. I would like to do a descale without removing the oil cooler because spares are difficult to find for this 35 y.o. Motor. It looks a straight forward job, but because of the placement low on the starboard side it will likely involve draining the sump.
Good tip about removiing the zinc annodes when flushing.
Will flushing open a blocked water port? Thats what i suspect this is all about.

[Wotname]

Matan, as suggested above, check the alarm sensor even though you have replaced it. It is very simple to do, heat it in a container of water until the contact closes when measured with an ohm meter and check the temperature of the water should be between 50 and 60 C.

Flushing with hydrochloric acid will remove most salts pretty quickly that might block a port.

Curious - is the engine 11 years old (first post) or 35 years old (post #6)?
What is the condition of the zincs?

[PLANET EXPRESS]

Check the wiring to the sender good look at places that it rubs against metal really inspect the insulation closely. Seen this happen on a yanmar where the insulation rubbed thin on the sensor wire. It was really hard to spot had only one strand barely out of the insulation on a very small spot.

[Cat36Mahalo]

You might have an airlock. Check hose temp and if low - open hose clamps and let air out.