Engine Overheating

[Feo23]

I’ve got what I think is an overheating problem. 1994 Hunter 29.5, 18hp Yanmar with ~800 hours. It will run all day at 2000 RPM and about 3 1/2 kn. If I go to 3400, the alarm goes off after 10 minutes and I get smoke in the cabin. I assume it is a high temp alert and not low oil pressure. The oil and coolant are all at their proper levels. I’ll drift around a bit and at some point I’m able to start the engine up and continue on at 2000 RPM. Beyond doing routine maintenance I don’t know a lot about diesels. Any suggestions where to look?

thanks

 

[Don S/V ILLusion]

Could be any number of reasons:
clogged exhaust elbow
faulty injectors
faulty water pump/ bad impeller
faulty thermostat
clogged intake thruhull


probably more but that’s the first which come to my mind.

What is the service history on the engine?

 

[Davidasailor26]

Don’s given a good list, but easy things first - how much water comes out the exhaust, and have you checked whether the impeller has all its blades? After that I’d go to the heat exchanger, and exhaust elbow. I’m assuming the “smoke” in the cabin is steam from overheated coolant. If it’s actual smoke there could be other issues and I’d lean more toward exhaust elbow first.

 

[Feo23]

Yes, could have phrased that better, steam from the over heated coolant plus some smoke from overheated oily residue on the engine.

It is a new boat for me. I ran it on the hard in April and had it surveyed early May - the usual engine stress test while tied to the dock. Ran it pretty hard for 6 hours from the Cape through Buzzards Bay so I think this might be a recent issue. The boat's been in the water a bit over a month.

In deciphering the bad handwriting on the service invoices, beyond the usual maintenance I found;
Water pump replaced 11/20
Impeller replaced 1/23
Replaced 1/2 " thru hull with 3/4" 1/23
Exhaust Elbow replaced 9/23

I will check the easy stuff first like the intake thru hull, impeller and thermostat. The exhaust water output looked a little anemic but this is my first inboard so I don't have anything to compare it to.

Thank you both for your suggestions, I'll let you know how I make out.

 

[Ralph Johnstone]

The exhaust water output looked a little anemic

Check the water temp. in the exhaust gas discharge when the engine is hot. It should be about skin temperature.

 

[jssailem]

In addition, due to your statement about "oily residue," perhaps it is a coolant that is the residue. Check the coolant pump to see if there is a discharge at the weephole. This would indicate there is a leaking seal in the pump.

 

[SV Watercolor]

On the older Yanmars I find there are typically a combination of factors and not always a silver bullet solution. Since you only really have noticeable symptoms at higher RPMs, it sounds like you are getting "some" flow of raw water but not "full" flow. Assuming you have a 2GM with 18HP, and have checked the "easy stuff" from raw water intake to impeller, consider cleaning the heat exchanger. You can find some YouTube videos of people doing this but the basics are:
1) Drain raw water from heat exchanger with small 1/4" tube (there should be petcocks for this) 2) Connect ~ 3/4" tubing to raw water input and raw water output to a bucket 3) Run descaler (Rydlyme or other) from the bucket through exchanger using a small bilge pump supply to the front/intake of exchanger and return to the bucket from the back of the exchanger. 4) Remove descaler tubing and restore normal hose config (unless doing optional step).

5) Optional - Drain coolant from Heat Exchanger via petcock and small 1/4" tube into clean container. Drain off the residual descaler via raw water petcock. Remove the front exchanger cover and you can gently access most (not all) of the tubes to visually inspect and if needed, run a small implement (~5mm) down the length of the tube (BE VERY GENTILE - DO NOT FORCE ANYTHING). In all likelihood you do not need to do this but you can confirm a nice clean exchanger. Consider having a replacement gasket for the from exchanger cover in case it needs replacement. Put bolts back in their original positions.

After the heat exchanger, and the other "easy" stuff, I'd also make sure the belts are properly tensioned an not slipping at high rpm. Last thought - the impeller being replaced recently . . . maybe verify it is the right impeller and installed correctly. Replacement with one that was maybe a spare on the shelf for 6 years that was already brittle might now be degraded after use.

 

[ice breaker]

Everyone has given great advice. I would only add that back flushing the heat exchanger might help blow out any old impeller pieces that are down stream of the pump. I back flushed my fisher panda gen set and got out almost an entire impellers worth of pieces out.

 

[Feo23]

That might be my next step, just waiting on the HE gaskets to arrive. The impeller and thermostat looked perfect - although I've not yet done the hot water test. That make sense as they were replaced before last season and the previous owner put on very few hours.

What disturbs me is when I went to drain the coolant from the two drain points, I only had a few ounces drain out - I did remember to open the cap. The plastic reservoir is filled to the proper level. On to the HE's tomorrow.

Thanks

 

[jssailem]

when I went to drain the coolant from the two drain points, I only had a few ounces drain out

That is not right. You might want to research this issue. You should have 3-4 quarts of coolant in the engine.

 

[SV Watercolor]

Hi All - the 2GM has ~.75 gal of coolant total. I think the HE itself will hold much less - maybe a quart-ish. With a cool engine, my HE is full when I open the HE cap (not the reservoir cap). You might consider running the engine to operating temp so the thermostat opens _~180F), let it run a few mins at normal temp, then shut down. Let things cool off a bit. You can either CAREFULLY open the HE cap (especially if things are still warm it could be under pressure), or drain the HE coolant by the petcock to see if any coolant made it into the HE.
If you still have no coolant to speak of, that sounds like a water pump or thermostat not getting coolant to the HE. Thermostats are relatively inexpensive so you could by a new one or test the one you have. The operating temp should be stamped on it. You might also check to make sure the reservoir tubing isn't clogged while your at it.