A27FC Overheating not cooling system related?

[rnummi]

OK, so I've perused the threads...I'm stumped. Here's the symptoms. 4D61 Lehman Engine runs like a watch under load all the way to 2700 RPM. Boat is plugging along at 4.5 knots. I would say 75% of throttle. Any push beyond that and I get a slow temp rise till the alarm buzzes. I have only gone to buzz twice to see what the heck is up. At 75% throttle I get a steady 175 degree temp. I can cruise for hours without a hiccup. When I push it a bit, temp starts climbing. When I back off, temp goes down. Here's where it gets weird. With no load (i.e. neutral) the engine can rev up to max throttle (4000 rpm) without overheating... water is pouring nicely out exhaust, volume appropriately varies with throttle speed. Boat returns to normal operating temp within 5 mins of reducing throttle.

All the usual suspects have been addressed:

Raw Water impeller has been changed.
Fresh water pump has been changed.
Heat exchanger removed, boiled and gaskets replaced.
All water hoses changed.
All belts changed.
Raw water strainer emptied and cleaned.
Thermostat replaced with new.
Run with and without thermostat in place... results same.
Oil changed.
Transmission Fluid changed.
No smoke or oil residue in exhaust stream.
Oil remains clear after operating (no milky white etc.)


I am now officially stumped. Why am I overheating? What am I missing?

[Vic K]

Water intake is disrupted

[kerrye]

Does the engine have an air filter and is it clean? The reason I ask is that as the fuel to air ratio is changed as a result of a dirty air filter, EGT's rise which can cause a rise in engine temperatures. I saw this happen on a CAT 3208 in a Wanderlodge bus I once owned. Knowing the EGT's which is probably not possible on your engine would help diagnose the problem since you could see if they were rising with increased throttle.
To help diagnose the problem I would use a non-contact thermometer to determine the temperature of various parts of the block/engine to see perhaps if there is internal blockage in the cooling passages. It might also help confirm the accuracy of the existing temp sensor. It might also give you a rough estimate of EGT's if you measured the exhaust manifold near the head.

[kerrye]

I agree that water flow may be disrupted. Have you checked the volume? Perhaps the increased suction at higher rpm's is collapsing the water intake hose.

[hetek]

I noticed you didn't include the oil cooler in your list. Here's what I found on my 4D61:

Disconnect the water hoses from both ends of the oil cooler.
Put a catch basin under the inlet side of the cooler.
Take a hose and shoot fresh water into the outlet side.
See what comes out of the front.

I did this and a handful of pine needles or similar came gushing out. I had a raw water strainer and I was surprised that that much stuff made it through.

Might help to improve flow if you have a blockage there.

[Vic K]

Before you start tearing everything apart apply KISS. I would bet that you have a clam shell cover over the raw water intake. Might be debris blocking the amount of water taken in at higher rpm under way but not when the boat is out of gear. Or as someone else stated a bad intake hose. Good luck.

Vic

[rnummi]

Ok, oil cooler flushed. Raw water filter (and all other filters) cleaned/changed as appropriate. Clamshell checked ok. Plenty of water flow out of exhaust that increases dramatically with throttle up. Do I need to remove and boil out oil cooler? It gushed appropriately with reverse flush. I was speculating a transmission problem or over prop. PO swears it's original prop though.

Pic is of the prop when I bought her. Since cleaned and tried.

[Jeremyvmd]

Could be a overprop issue. Even if it's original, as boats age and we use them they get heavier what may have been an appropriate prop when new may no longer be...

[JT48348]

Could be a overprop issue. Even if it's original, as boats age and we use them they get heavier what may have been an appropriate prop when new may no longer be...

??
How does that work? How does a larger/heavier prop equate to engine rise in temperature under load? And how large/heavy would a prop even have to be cause a noticeable overheat issue.

[Jeremyvmd]

Could be a overprop issue. Even if it's original, as boats age and we use them they get heavier what may have been an appropriate prop when new may no longer be...

??
How does that work? How does a larger/heavier prop equate to engine rise in temperature under load? And how large/heavy would a prop even have to be cause a noticeable overheat issue.

with the engine in neutral it's under no load so the motor isn't working hard. In an over propped situation you are putting more load on the motor and making it work harder, engine starts struggling and motor overheats. Similar to towing a large trailer. With no load a truck may run fine at highway speeds, however when overloaded (same airflow to rad, cooling system working the same) the truck overheats because of more load on the motor... it wouldn't have to be that oversized. If you were talking about a brand new motor, it would take more to make it overheat, but we are not talking about a brand new motor, little wear and tear, combined with a cooling system that is likely not working 100% etc etc, it might. It might not take much at all to make the boat overheat due to overloading

just as a bit of evidence (and hardly what I'm basing my entire argument on, in fact that whole article on prop selection is a good read) - quote by Peter Compton from boat diesel.com in talking about proper prop selection

"A lot of emphasis is placed of selecting a prop that allows the engine to achieve rated rpm under full load and thereby not overload the engine. Most of us run high performance diesels and that overload often manifests itself with severe overheating that can cause valves, pistons, manifolds and cylinder heads to fail"

edit - link to the article for anyone who is interested
http://boatdiesel.com/Articles/index.cf ... 3&PageID=1

[JT48348]

This reminds me of the great 2015 Diesel Bug Scare. It sounds good and may in some remote situation have a factual basis, but in fact is vox polpuli or pseudoscience. Overproping would affect the engine elsewhere than just above 2300 if that even were possible on an A25/A27.

You didn't answer my question: how big or how much heavier would a standard prop have to become to suddenly overtax an engine?I bet a lot. Not a little. That looks like a fairly standard prop on an A25/A27, not some giant mutant screw. Prop Magic is not what's going on here.

My guess, as already stated, is you have a coolant system problem...somewhere. Keep it Simple. Something is obstructing your coolant system typically under load. That's the clue.

-a physical obstruction that is more prevalent under greater load/pressure, or not noticeable at idle to low speed (filters, grates, thruhulls)
-a pump or pump component that acts up under increased load( heat, pressure, etc)
- a rubber component that collapsed or closes under higher load

[Jeremyvmd]

you don't know that the prop isn't affecting the motor below 2300rpm, it may be affecting the motor at lower RPMS but the maintenance work that has been done has allowed the cooling system to keep up with it. I don't see the harm in looking at all the possible causes rather than getting tunnel vision. Having read the above posts it seems like he has done a pretty good job of checking the cooling system.

On the surface I agree - cooling system is the likely cause, however when all the horses have been eliminated, its time to start thinking about zebras - I run into that in my job on a nearly daily basis, the biggest mistake you can make is not consider all the possibilities.

as for how much oversized a prop would have to be to cause overheating - there are too many factors to take into consideration for me to even hazard a guess. Not to mention I am not a diesel or marine mechanic (nor do I pretend to be, online or in real life) - I just like to read and learn as much about a topic as I can, and give helpful opinions or direct people to information when I can. It would be a question better asked of someone with official training or a degree in related topics. When it comes to someone like Peter Compton, if he believes something to be true I am more likely to take his word on a topic (one which he actually wrote a book on) rather than a garage mechanic...you would be well served reading the article I linked above, he actually provides data, rather than just opinion when it comes to the topic of proper prop selection. If nothing else its an interesting read and opinion on the topic of prop selection

[JT48348]

I don't know, man. When my car overheats I think it must be something wrong with the coolant system. I am however slightly amused when my neighbor tells me it might be "over sized" tires or other fascinating distractions. And then I go back to work checking the coolant system.

Minutiae Obfuscation

[Jeremyvmd]

if you say so...I'm gonna let you have your opinion, right, wrong or indifferent. Not worth my energy since you don't want to listen to a cogent argument, but I would recommend doing some research...
Have a nice day

rnummi - I would recommend doing a once over on the entire cooling system again. if nothing is coming up, take a photo-tach to your motor and check max RPM under load and in neutral...never know, might get lucky. Load can absolutely play a role in engine temps, can't tell you from here if it's contributing to your issues, but I beleive in not leaving stones unturned

[kerrye]

I would do some more diagnosis with a non-contact thermometer to check exact temperatures at various places on the engine and at the components including the oil cooler.