The perrential overheating topic

[jviss]

I know this has been discussed a million times, and believe me, I've searched and read all of the stuff here on this topic. But, I think I have a new angle on it.

Cold it be that my engine installation simply doesn't have enough raw water cooling capacity?

System: 1984 C36, Universal M25 (Kubota D850), 4 year old Oberdorfer M200-15 raw water pump, new impeller yesterday; 4 year old 3" (larger) heat exchanger; 4 year old exhaust riser with injection elbow.

What happens: under load (driving boat), temperature regulates at 180 degrees F (thermostat temp) up 'til about 1700 RPM. At higher speeds temp rises and exhaust steams. Even higher speeds, transom soots up (like I'm steam-cleaning my exhaust system).

Analysis: all the books say insufficient raw water flow.

What I did: checked entire raw water circuit. Through hull clear, plenty of water at seacock, strainer clean, new pump impeller; removed and thoroughly cleaned heat exchanger: found zinc was flaking off, and leaving lots of stuff in the exchanger; removed pencil zinc (for now); checked hoses to injection elbow, including vented loop; checked injection elbow - could look in, but ran a wire through, and poured water in to see if it flows.

The heat exchanger looked a little bit blocked, with pieces of zinc and some marine growth that made it past the strainer.

After reassembly, seemed to get better flow, but still the engine warms up past thermostat temp at higher speeds under load.

I really don't know if it always did this, perhaps it did.

I believe I checked the entire raw water circuit. Maybe the elbow is slightly blocked, I don't know. Maybe the exchanger could be cleaner?

Anyone know what the flow should be, in quantitative terms (i.e., GPM at xRPM)?

Any insight, guesses?

Could it be a problem on the engine coolant side?

Thanks.

jv

 

[BobM]

Did you try...

...flushing the system with acid? It is something I have seen listed as commonly done on raw water cooled engines, but perhaps you need to try it on the engine side as your cooling passages may be blocked up. I don't know how it is on a Universal, but my Yanmar has a bunch of exterior cooling plumbing and the hoses can get pretty degraded.

Another thought...what are you running for raw water intake hose. If it isn't re-enforced hard wall tubing made for the application you may be sucking it shut under throttle.

Bob

 

[Maine Sail]

Yes...

The earlier M-25'sd had an undersized HX and the later ones had a larger HX. I can't remember specifics but it was something like the M-25 had a 2"dia. HX and the M-25 XPB series was a 3" diameter.

At cruise speed it would work well if perfectly clean but anything over, or a little bit of crud, and the temp would begin to climb.

If you upgrade the HX you should be fine..

Here's an older M-25 with the upgraded "blue colored" larger HX installed..

 

[BobM]

Have you checked the flow?

You should be able to break a connection somewhere to check the flow through the engine. I'd probably pull the thermostat if I had to in order to do it and then just run the engine very briefly.

I don't think that is the issue though, since you are turning the water in your exhaust into steam. To me that would seem to indicate you have good heat transfer to the exhaust water and that the problem is on the exhaust side.

 

[jviss]

Clarificaitons

It's a fresh-water cooled engine.

I have the larger ("upgraded") 3" heat exchanger.

 

[MoonSailer]

Temperature Sensor???

I have had chronic overheating and bought a optical type heat sensor that says that my engine is not overheating!!!! I shoot the engine with the sensor while the gauge says that it is over 200 degrees the heat gun says that it is 174 degrees!!!!! BUT I don't have black smoke. Black smoke means engine overload. You might need to repitch your prop. A diesel has an optimum load. Too much of too little is harmful especially too much. Black smoke indicated that the govenor is adding too much fuel trying to meet the load but not enough oxygen.

 

[Stu Jackson]

Just put in a 160 F thermostat

nm

 

[jviss]

Hmmm...

Stu,

That's encouraging, thanks! But, the steam in the exhaust concerns me. Peter Compton's excellent book "troubleshooting Marine Diesels" points to insufficient raw water flow as the number one cause of exhaust steam.

By "warms up past thermostat temp" I mean that the engine is generating more heat than the cooling system can absorb. Engine cooling systems like this when properly designed and in good condition, are meant to regulate the engine temperature to the thermostat temperature, i.e., maintain constant temperature regardless of load: hence the term "thermostat." If the engine temperature increases beyond the thermostat temperature, the system has gone out of regulation, and it means the cooling system cannot get rid of the heat energy generated.

My system regulates to the thermostat temperature from idle to load up to about 1700 RPM. Beyond this the temp climbs with the load.

I wouldn't be so concerned but for the steam.

Does yours steam?

 

[jviss]

Point of temperature reference

Moon,

Could well be that your outside metal temp is 174 while the coolant is at 200. The standard engine temp reference is the coolant temp, usually right at the thermostat housing, as is the location of the temperature probe on the M25 (in thermostat housing, on the downstream side of the thermostat). Since there is heat flow towards the ambient air around your engine, its quite possible the paint is at 174 while the coolant is 200; heat flow is analogous to electrical current, where the temperatures at the nodes of interest are the voltages, and the heat transfer coefficient of the material of interest is analogous to resistance. So, you will see a temperature "drop" (like voltage drop) when heat is flowing (current) through the engine block or related part (resistance).

 

[Stu Jackson]

No steam, except for one day last winter

when we determined a blockage in the RW inlet and blew it out with a dinghy foot pump.

U noted: "1700 RPM. Beyond this the temp climbs with the load. I wouldn't be so concerned but for the steam. Does yours steam?"

Aha, maybe I missed that the first time. No, ours does NOT do this, I can run at WOT all day and it stays at 185-190, which si why I put the new HX on the old puppy, as you did.

"Temp climbing with load after 1700 rpm" concenrs me, as it should you. Lemme dig around in our archives for some information, I'll be out most of the day, but between, you, me and the C34 and C36 Associations, we might, just might, know someone who's asked this question before and received a reply.

 

[jviss]

Thanks, Stu

Just another question - what RPM range do you see/use on yours? Max no load? Max in gear?

Thanks,

jv

 

[Don S/V ILLusion]

other options

Didn't see where you stated what temp rises to -

some simple things you can do to isolate the problem -
remove the thermostat and run engine up - this won't hurt anything;
don't know what you did to clean heat exchanger - make sure it's bare metal inside;
replace water pump - may run fine at little load but not at higher rpm (common insidious problem)

also assume you have the pre-existing prop and that it is clean?

 

[jviss]

ques

"other options

Didn't see where you stated what temp rises to -

some simple things you can do to isolate the problem -
remove the thermostat and run engine up - this won't hurt anything;
don't know what you did to clean heat exchanger - make sure it's bare metal inside;
replace water pump - may run fine at little load but not at higher rpm (common insidious problem)

also assume you have the pre-existing prop and that it is clean?"

Don, heat exchanger - it wasn't bright copper inside when I got done, it was kind of grey - not totally clean, I guess. I had a bottle of white vinegar on board that I would have used to really clean it, but the bottle had broken!

Which water pump are you talking about? I installed a new coolant circulating pump 2 years ago. The raw water pump is about 4 years old.

 

[RichH]

Simple logic pathway.

FLOW
1. Look at the engine specs for volumetric output flow vs. rpm. Then run the engine at the spec rpm and measure the flow at the exhaust outlet with a bucket and a stopwatch
a. less than spec flow means worn pump (slipping), worn vanes, restriction
aa. common restriction possibilities: accumulated calcium carbonate precipitates (fouling) in exhaust manifold or 'slab rust' formation in exhaust manifold (common in universal cast iron manifolds)

Heat Transfer
2. Get an optical pyrometer (infrared hand held thermometer). Find the 'hot spot' or the zone at which the temperature is/becomes HOT on the engine while 'following along the pathway of the cooling circuit with the pyrometer)
3. Measure the thermal differences of the inlet/outlet of the Hx. The outlet on the raw side should be 15 degrees or less HOTTER than the inlet (or see engine specs for diff. values.) -- this is known as the 'approach temperature' and is spec'ed for the design. If not in your manual call the engine builder for the value. Validate that the 'thermostat' is operating at spec open and close .... just put the t'stat in a pot on your kitchen stove and measure the temp of the water, etc. If you are running hotter (and if the volumetric flow is ok and not restricted) then descale the engine AND the exhaust manifold with a commercial boiler descaler.

For cooling you only need two things ... an unfouled engine/Hx/exhaust manifold AND you need the proper volumetric flow through the system. If fouled or restricted, then 'problems'.
What Maine describes (being undersized) can be verified with an optical pyrometer ... thermally profiling the engine/cooling system (+ checking the temp of the water outlet). Check all the surfaces at low, normal and high rpm (all under load). All these values should be 'linear' .... if not then the Hx is undersized .... but can be made more efficient with HIGHER volumetric flow (change the pulleys on the pumps to run at a moderately 'higher' rpm). Heat transfer is simple - flow/mass rate of the water, surface area of the exchanger .... but all has to be unfouled and unrestricted. Do it by the 'numbers' and develop an 'operating history' of the temps and flow rates.

 

[Don S/V ILLusion]

if all else fails

after checking the simple and obvious, I'd verify the water pump is working properly - not the raw water pump but the block circulator pump. Even new ones can be defective so I'd ignore age as an idicator of fault potential. Measure it's output against it's specs.

Also, if you remove thermostat and the engine overheats, the problem, and therefore the solution, becomes more apparent.

 

[MoonSailer]

Rich You Gave More Detail

Jviss I actually did what Rich suggests. I checked all over the engine with what he calls an optical pyrometer. The raw water was hotter coming out of the exchanger as was the antifreeze that is recirculated. I checked where the thermostat housing is located. No where could I find a place hotter that about 180 even though the temperature gauge indicated about 205. Actually the water coming out of the heat exchanger was only about 135 degrees or so.

Steam happens naturally!!!!!! In your exhaust elbow you inject water into the hot exhaust. It is only physics that some of the water will vaporise and the exhaust become very saturated with water vapor(steam). On a hot dry day it is hardly noticable but on a cool moring you can see some of this water vapor condensing as it hits the cooler air. Actually you cannot see steam!!! You only see droplets of water as the steam cools and condenses.

Actually the thermo sensor is a simple temperature sensitive resister. A diesel mechanic suggested the optical pyrometer as he said that it was not uncommon for temperature sensors to fail. Of course he said this after I checked all of the normal overheating problems.

A thermostat's purpose is to keep the engine from running too cold!!!! It should be open under normal operation. You need to look up your boats normal operating temperature.

 

[jviss]

OK

Thanks, Don, but I can't imagine a pump being defective to the extent that it pumps less. But, you gave me an idea. Maybe when I'm running at high RPM, my 125A alternator is causing the belt to slip on the crank pulley, which would slow down the circulating pump. I should try this with the alternator switched off.

RichH, great reply. I can measure these things. I got the spec for the Oberdorfer pump, I need only to know the crank to pump drive ratio, and the head or PSI to get check the pump; I guess I can check at zero head.

Anyone know the circulating pump spec?

It also occurs to me I might have a restriction on the coolant side, since the coolant runs to the water heat, and there's a bypass circuit, since the water heater is apparently too much of a restriction. I can remove the water heater from the circuit and try it.

 

[RichH]

Universal exhaust manifolds ....

and most other cast iron 'wet' manifolds (gas on one side/cooling water o n the other eventually fall apart due to 'slab rust'.
When cast iron cools in its mold it forms 'stratifications' like an onion. Once (after time) the inner surface layer gets penetrated by rust huge 'plates' of rust begin to come off the casting and can block or restrict the water passages. If you have lots of steam or extreme engine temps ... check the flow through the exhaust manifold .... remove it to check it. Many times such 'problems' are intermittant as the slabs will temporarily block the passaages held in place by the pump pressure; stop the engine and the platelette falls to the bottom of the manifold.
Soluition ... NEVER long term 'store' and engine 'dry' ... meaning dont drain the engine but rather fill it with antifreeze that contains a rust inhibitor. Engines that are run hard and long will develop a layer of (black/blue) ferrous rust which is 'protective' against the destructive (red) ferric rust. Most navies of the world will hardly ever entirely shut down their engines ... so that slab rust doesnt form in the cast iron components.

If youve been 'draining' the engine for long term (winter) storage and you now have a 'temperature problem' ... its probably the exhaust manifold thats beginning to internally fall apart. If so, remove the manifold, push in a stiff wire to break up all the 'slabs' and shake them out, reassemble then *run the hell out of the engine to re-deposit a good layer of BLACK rust* on the 'inside'. Dont drain marine engines for long term storage, keep them 'wet' (with antifreeze with a rust inhibiter).

 

[jviss]

Wet it is.

Only "stored" the engine dry once, when it was home for the winter for a refurb.

 

[Stu Jackson]

Back

180 thermostat, runs 185 -190

I think your Oberdorfer is an M202-M15.
You can get the specs at Depco.com.

Alternator off is a probable, remember our discussion last week, I think I sent you a link to our alternator discussion, if not here it is again. http://c34.org/bbs/index.php?topic=4454.0 It's long, but I finally get to it! If your batteries are charged when this happens, then it's not the alternator load 'cuz the alternator shouldn't be working.

We run 2,000 to 2,500 cruising speed, 3,000 WOT out of gear, 2,700 to a tad less than 3,000 in gear, but that's all relative (as we've discussed before - pretty close since it's off the alternator but I've never measured). Our Blue Circle is a match on poles with the OEM Motorola, so it should be fairly close.

I'm still lookin' around for you.