• On September 1st, Maine Sail suffered a major hemorrhagic stroke. One of the most generous members of our sailing community, he has helped thousands. Now it's our turn. Click here to learn more

Heat exchanger temperature reduction amount

Oct 9, 2008
1,733
Bristol 29.9 Dana Point
—Update 10/24 posted below—

How much should a HX cool the coolant?
In this case, 2” diameter HX in Universal M18. Thermostat 165. Motor has run at 165 forever. Now recently on a couple long motoring back to harbor trips, trending up over about 2 hours to 185 before I stop the boat and let it drop to 165 in about 3 minutes at idle.

Sea temp 68.
Coolant level is correct.
Omitting details - Assume all additional aspects of the cooling systems are tip-top. Bottom and prop are clean etc.
Now analyzing if the HX has reduced cooling power due to maybe internal buildup.

When measured with infrared gun at one point at cruising speed, when at 180 at the thermostat housing and elsewhere, coolant pump intake was 145. So the HX is reducing coolant temp by 35 degrees.
Is this normal or is the HX underperforming?
 

Attachments

Last edited:
  • Like
Likes: jssailem
Jan 11, 2014
7,855
Sabre 362 113 Fair Haven, NY
Have you checked the impeller? The change in temperature is a clue, what's making the engine run at 180 when it always ran at 165? Look for inadequate or restricted flow or a bad thermostat. If the thermostat is hanging up, it could allow the coolant to overheat, once it opened and you let it idle it would cool down quickly. I'd pull the thermostat and check or just replace it. See what happens.
 
  • Helpful
Likes: jssailem
Dec 28, 2015
1,357
Laser, Hunter H30 Cherubini Tacoma
Consider continuity of the temp sending unit lead and the unit itself.
 
Oct 9, 2008
1,733
Bristol 29.9 Dana Point
Have you checked the impeller? The change in temperature is a clue, what's making the engine run at 180 when it always ran at 165? Look for inadequate or restricted flow or a bad thermostat. If the thermostat is hanging up, it could allow the coolant to overheat, once it opened and you let it idle it would cool down quickly. I'd pull the thermostat and check or just replace it. See what happens.
The raw water flow is fine. Brand new raw pump. Water discharging good. Also the elbow is clear and good. New coolant pump too. Hoses etc all good. I had the end cap off the HX and it looks good although can’t see way inside. Exhaust Manifold was cleaned couple years ago. New radiator cap on that too.
I’m wondering based on the temperature numbers provided, is the HX cooling the coolant at a normal rate?
If yes then the thermostat is a good next inspection task :)
Although, the temp goes from cold start to 165 then stays there for 20 minutes or so, so I’m thinking the thermostat is opening. Then after 20 min it starts to trend higher. maybe the water heater heat exchanger is keeping it in line while the heater tank heats up. This sequence only happens after cold start with cold heater. If the heater is hot, the engine starts trending back up immediately when I get under way again.
I’d think it might overheat if I let it go another hour after it hits 185.
—So it could be the thermostat I guess. I replaced it 10 years ago with Kubota part.
 
Jun 11, 2004
1,221
Oday 31 Redondo Beach
How much should a HX cool the coolant?
In this case, 2” diameter HX in Universal M18. Thermostat 165. Motor has run at 165 forever. Now recently on a couple long motoring back to harbor trips, trending up over about 2 hours to 185 before I stop the boat and let it drop to 165 in about 3 minutes at idle.

Sea temp 68.
Coolant level is correct.
Omitting details - Assume all additional aspects of the cooling systems are tip-top. Bottom and prop are clean etc.
Now analyzing if the HX has reduced cooling power due to maybe internal buildup.

When measured with infrared gun at one point at cruising speed, when at 180 at the thermostat housing and elsewhere, coolant pump intake was 145. So the HX is reducing coolant temp by 35 degrees.
Is this normal or is the HX underperforming?

The raw water flow is fine. Brand new raw pump. Water discharging good. Also the elbow is clear and good. New coolant pump too. Hoses etc all good. I had the end cap off the HX and it looks good although can’t see way inside. Exhaust Manifold was cleaned couple years ago. New radiator cap on that too.
I’m wondering based on the temperature numbers provided, is the HX cooling the coolant at a normal rate?
If yes then the thermostat is a good next inspection task :)
Although, the temp goes from cold start to 165 then stays there for 20 minutes or so, so I’m thinking the thermostat is opening. Then after 20 min it starts to trend higher. maybe the water heater heat exchanger is keeping it in line while the heater tank heats up. This sequence only happens after cold start with cold heater. If the heater is hot, the engine starts trending back up immediately when I get under way again.
I’d think it might overheat if I let it go another hour after it hits 185.
—So it could be the thermostat I guess. I replaced it 10 years ago with Kubota part.
I don't know what the temperature difference between the HX coolant intake vs outlet should be but I had an almost identical problem with my M18 a couple of years ago. I removed the HX and cleaned out the tubes by soaking the HX in diluted muriatic (pool) acid, rinsing it out and then running a properly sized wooden dowell gently through all the tubes to remove most of the remaining softened up deposits. Rinsed that out, painted and reinstalled and the problem went away.
 
May 29, 2018
300
Canel 25 foot Shiogama, japan
The raw water flow is fine. Brand new raw pump. Water discharging good.

Efficient heat transfer relies on a few factors some of the being,

1. Temperature difference (TD) between mediums. Simply, if the seawater is warmer the TD is lower.
2. Amount of coolant. Your coolant flow seems to be good.
3. Heat transfer efficiency. This is the surface contact area and efficiency. If the inner surfaces of your heat exchanger are coated with calcium, they are basically insulated and the amount of heat that can be transferred is limited.

As 1 and 2 have been eliminated I would suggest following Richard 19608's advice.

" I removed the HX and cleaned out the tubes by soaking the HX in diluted muriatic (pool) acid, rinsing it out and then running a properly sized wooden dowell gently through all the tubes to remove most of the remaining softened up deposits. Rinsed that out, painted and reinstalled and the problem went away."


There are specialized (over priced ) products, but vinegar will do the job, too.

gary



 

Ward H

.
Nov 7, 2011
3,129
Catalina 30 Mk II Barnegat, NJ
I had the end cap off the HX and it looks good although can’t see way inside.
i don’t think you can tell if the HX tubes are clogged by taking only one end cap off. You need both caps off to look through It. Actually just put a flashlight at one end and see if the light shows at the other end. At least that’s how I check mine.
 
Oct 9, 2008
1,733
Bristol 29.9 Dana Point
Thank you all for insight and similar history. :)

I’ll remove HX and have it cleared out at a radiator shop. Then I will know it’s done right. And they can tell me condition and if it’s still fit for service. Also I’ll change out the thermostat just in case. Coolant will be out anyway (which has to be done for thermostat removal in my case because the water heater circuit expansion tank is higher than the engine) so it’s an easy task and I already have a spare.
 
  • Like
Likes: Ward H
Apr 22, 2011
735
Hunter 27 Pecan Grove, Oriental, NC
You said the exhaust elbow was clear and good. Did you check the nipple where the raw water enters the elbow? Water restriction in the nipple caused my GM to overheat at high rpm.
 
Oct 9, 2008
1,733
Bristol 29.9 Dana Point
You said the exhaust elbow was clear and good. Did you check the nipple where the raw water enters the elbow? Water restriction in the nipple caused my GM to overheat at high rpm.
Yes the whole elbow setup is fabulous.
 
Oct 24, 2010
2,399
Hunter 30 Everett, WA
I once had a restriction inside a hose. A flap of rubber came down at high RPM and stayed there until I shut down. Then it cleared until I got to high RPM again. After checking the heat exchanger I'd consider looking for another obstruction. Ours was really hard to find, and I found it by accident.
 
  • Like
Likes: jssailem
Oct 9, 2008
1,733
Bristol 29.9 Dana Point
Update: I replaced the heat exchanger.
It was $100 to acid wash the old one and repair a couple of the hose pipes which were oblong from the factory (?), which restricted flow and caused salt built up there which I had to clear out every 3 years or so.

For $275, I obtained one from Mr. Cool.

There’s a lot going on there:
1. It has end caps on both ends. The old one only had one end cap, with the end containing the zinc pencil enclosed. This resulted in broken zincs piling up in that end, blocking flow. Had to remove the HX every few years to shake it and clean them out by poking with a screwdriver.
2. The zinc is bigger, which not only will last longer but might reduce the chance of their breaking off.
3. The hose pipes are round :).
4. It’s new, so clear of calcium and any coolant clogs.
5. I think the zinc mount threads on the old one were getting stripped. It was blooming salt.
6. The new zinc location is slightly farther from the hose pipe on that end, making zinc removal drastically easier.
7. The new one looks much better.

So the extra money was worth it.

Pics of old and new attached.
 

Attachments

Last edited:

dLj

.
Mar 23, 2017
1,833
Belliure 41 Now on the Chesapeake
I kept waiting for the punch line - did this fix the problem? I'm assuming it did, but I hate to assume... You know the old saying, when you ASSUME. it makes an ASS of U and ME....

dj
 
Oct 9, 2008
1,733
Bristol 29.9 Dana Point
I kept waiting for the punch line - did this fix the problem? I'm assuming it did, but I hate to assume... You know the old saying, when you ASSUME. it makes an ASS of U and ME....

dj
All the symptoms indicated HX buildup. Including outside the forum.

Then upon removal, the HX showed about a mil of calcium buildup internally, which had been overlooked in previous inspections.

The combination of symptoms and results of examination point to reduced HX performance due to buildup.

This is not an assumption. It’s the primary result of investigation. Which this forum was part of, obviously.

I won’t be able to immediately do a long test under load, to confirm the primary result.

Anyway I certainly can use a new HX, for the additional reasons posted, so in the unlikely event the problem turns out to be a cause previously eliminated from the suspect list, not a big deal.
 
Oct 9, 2008
1,733
Bristol 29.9 Dana Point
We have confirmation of a cure. :clap:
After 1 hour running, all is normal.
The problem was the HX, due to calcium buildup.
See pics.
140 at the gauge equals 165 at the thermostat. The gauge has always been off, 10 years of my owning it, but it has always been perfectly reliable. So I know that if it goes over “140”, the motor is operating out of character.
16DD3E14-6C45-467B-88A7-95E97FF1504D.jpeg
C2F86A3A-E494-424D-94D8-6F7C3AF902FF.jpeg
 
  • Like
Likes: jssailem