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.

 

[Feo23]

Hi all,

There was a lot of good advice on this thread and thanks to all who contributed. I ended the season with pretty much the same overheating problem. Today I took the cover off the HE and shined a light through it - perfectly clean which makes sense as the yard ran barnacle buster through it last fall. I also removed the exhaust elbow which looked pretty good which I expected since the PO replaced it two years ago.

Do HE need to be replaced? What should I be looking for if I ever figure out how to remove it (which I hope not to do). The water pump is the only thing I've not removed - it was replaced 5 years ago.

So to recap
Replaced the impeller (done by the yard)
Replaced the hose from the impeller to the HE
Inspected the HE- clean and both covers looked good
Did a short haul - cleaned the prop and inspected the thru hull
Replaced the thermostat

I plan to replace the hose from the thru hull to the impeller - should I consider others? mWhen the engine over heats and cools down I removed the cap - there is still coolant in there.

Before I move on the the fuel injectors, am I missing anything else?

Thanks

 

[Ralph Johnstone]

Do HE need to be replaced?

If it's clean as you have described, it's the same as a new HE. No moving parts.

When the engine over heats and cools down I removed the cap - there is still coolant in there.

There will always be coolant if the system is 100% full of coolant which means 0% air.

Before I move on the the fuel injectors, am I missing anything else?

Injectors have nothing to do with overheating.

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

Did you reply to this or did I miss it ? Mixing elbow temperatures are just about the best tell-tale you have for overtheating problems.

The top of the mixing elbow not be anything more than warm to the hand With the engine hot and running loaded at 3,000 RPM. The gas inlet to the mixing elbow is bloody hot under all loads.

 

[Davidasailor26]

I agree with all of Ralph’s suggestions. To them I would only add - did you check the amount of exhaust water coming out? If the water flow is not enough that would confirm the problem is with the raw water circuit somewhere. Since the intake hose hasn’t been checked or replaced yet that would be a possibility - occasionally they’ll collapse or delaminate in such a way that they look fine from the outside or at rest, but restrict flow when under suction.

 

[Ralph Johnstone]

To them I would only add - did you check the amount of exhaust water coming out?

I should write out a flow chart on what excessively hot or cold temperatures on the mixing elbow indicate. Trouble is I usually have a seizure after going around the chart 10 - 12 times so I can only answer one question at a time. Got a different temperature, write back again in a week or so.

Assuming you have an underwater exhaust discharge like I do and can't see the volume of water coming out and don't want to tear hoses apart :

- a low raw water flow would cause the water entering the mixing elbow to be hot because it would first be heated by the 180°F AF in the HEX to a high temperature. It would next mix with the 800° F exhaust gasses and get even hotter at the outlet of the elbow :

 

[sailme88]

The easiest and quickest test for failure is to remove the thermostat and run the engine to the point that either over heats or doesn't. If it doesn't overheat, you pretty much know what the problem was. If it does then obviously the issues is elsewhere. In either scenario replace the thermostat prior to the reinstall.
Overheats are also caused by air in the system.. Water heaters are notorious for being below the engine and developing an air embolism. As mentioned by others, occlusion of the exhaust elbow , water muffler and heat exchanger are all in play. I little know other problem of late is the error quality China made raw water impellers. The bonding between the brass hub and rubber vanes fails TOO often.

 

[Feo23]

I should write out a flow chart on what excessively hot or cold temperatures on the mixing elbow indicate. Trouble is I usually have a seizure after going around the chart 10 - 12 times so I can only answer one question at a time. Got a different temperature, write back again in a week or so.

Assuming you have an underwater exhaust discharge like I do and can't see the volume of water coming out and don't want to tear hoses apart :

- a low raw water flow would cause the water entering the mixing elbow to be hot because it would first be heated by the 180°F AF in the HEX to a high temperature. It would next mix with the 800° F exhaust gasses and get even hotter at the outlet of the elbow :

 

[Feo23]

Thanks all,

I was going to look at the fuel injectors anyway since I noticed som fine soot on the stern and the engine was running a bit rough by the end of the season. Thought perhaps this might be contributing to the overheating.

I can't get to the exhaust discharge so checking the elbow temperature is a great suggestion once I get the boat in the water. In the meantime I will replace the in take hose to eliminate it as a potential cause.

 

[kloudie1]

Quick note: The Yanmar thermostat in the GM motors are kinda strange and if replaced by a "Will Fit" thermostat, there may be a problem. The original thermostat has a bypass block-off function that makes a difference in coolant circulation especially with a fresh water heater that has coolant circulation. The thermostat blocks off the bypass when it opens to the heatex.. Attached diagram shows that..

 

[Straycat22296]

You most likely already checked but I have not seen anyone address checking the water intake filter for debris.
On another note, I once had a similar problem and found a plugged outlet hose fitting just past the heat exchanger that had become plugged with corrosion of an incorrectly used non marine fitting. Good Luck in finding a solution.