Yanmar 3ym30ae - loss of power

[Tactical Tony]

Wow what a great forum to be a part of and thanks so much to all for replying with your expert advice.
Certainly by the majority of the comments I should be looking firstly at the exhaust elbow and manifold inlet. Combined with this is checking the air filter, fuel filters and then see how it performs then.
I’ll get on to these suggestions as of Tuesday but unfortunately, as the boat is now on the hardstand for about 6 months I won’t really know the results until an in water test. I guess though, once I remove the exhaust elbow I’ll quickly realise if that is the culprit or not.
Again, thanks to everyone and once I do the cleaning I’ll let you know the results.
Cheers, Tony

[sinnerman]

Quick check would be to remove the three injectors and take them to a diesel shop/injector shop.

Should be easy to find one in Thailand, since diesels exist everywhere.

Note which injector is from which cylinder, so yo can replace it in the same one when you return. Not a big deal , but a good practice.

Have them bench check the injectors for leakage and spray pattern.

[Tactical Tony]

Well a huge thanks to everyone. Yesterday and today (May 4) I removed both the exhaust elbow and the mixing elbow on both motors.
The result for the loss of power and lower revs on the port engine would certainly seem to be a major blockage in the mixing elbow. This contained a major blockage versus the starboard engine.
I cannot confirm categorically the problem is fixed as the boat is on the hardstand but from the majority of comments pointing to this being the first place to look I’d have to say I’m pretty confident.
As a further bit of info for you all, I overdid the solution I used to clear the blockage. I used hydrochloric acid in a 50:50 ratio which resulted in eating through one of the mixing elbows. I left them in for 24hrs. It had no effect on the exhaust elbow of both. So I’ve ordered new ones for both engines.

Again thanks - great forum.

[Scubaseas]

Having found the effect now look for the cause. Does the engine come up to temp? Open thermostat can cause clogging. Maybe run some diesel purge (Liqui Moly 2005 or similar works very well) through the system or pull the injectors for cleaning. You can also put diesel purge into the dry secondary filter when doing a service to avoid pumping/priming.

In theory if both engine have the same time they should have similar clogging. Also good practice to "Blow out" the soot by revving the engine at WOT for 10-15 seconds before shut down and taking it out for a good run at wide open throttle now and then.

Mixing elbows are really consumables. They don't last forever and often just need to be replaced.

[Tactical Tony]

Thanks for the additional information Scubaseas. I can easily be accused as not running the engines flat out as I had become complacent running them whilst cruising at low revs for economy - realise now it’s false.

This whole exercise and research has opened my eyes to the complete cooling system. Thanks for your help.