Coolant temperature at idle rpm on Yanmar 4JH5E

[hellosailor]

Even with a safety cap you can get nasty steam burns. One reason why mechanics always grab the cap with a handful of rags just in case.

If the cap spring is weak, it won't hold pressure, that can lead to overheats. And if the rubber seal around the edge of the cap isn't clean, it won't seal, and it will leak under pressure. If the coolant in one of the anti-leak types (with sodium silicate in it) that can leave little bits of glassy coating on that seal, actually causing a pressure leak, unless the seal and seat are both clean and dry before refitting the cap. Antileak coolant is a mixed blessing.

[Compass790]

What A64 pilot says. Possible head gasket leak. Start it cold with coolant cap off & check if it's bubbling.If it keeps bubbling you have a problem. Coolant will expand & overflow if you run too long so put cap back on within a minute or 2. Or you will find out the way A64 pilot said.

[a64pilot]

Reread his first post.
Has anyone ever heard of a Diesel overhearing at idle, but won’t underway?
Maybe a large amount of air in the system that vapor locks the water pump, meaning it’s just spinning in a bubble?
I have never heard of a Diesel that overheats at idle, but not under a load.

You real sure it’s an overheat alarms and not low oil pressure?

[Sailing Drummer]

About the coolant running out when you remove the cap (if I understand you correctly). On my setup, same engine, the coolant reservoir is higher than the cap on the engine. Removing the cap on the engine will let coolant in the reservoir run out through that removed cap.
Did I understand what you said correctly?

[jjorio]

Hi again,
I couldn't do any testing today, as I was out of the boat. Will do more tomorrow.

Here are a few comments :
@a64pilot
I'm sure it's the overheat alarm, as I got mistaken the other way round and opened a thread regarding oil alarm at idle speed, before realizing I read the alarm wrong (t was actually overheating)
BUT, I think it is possible that when overheating, the oil alarm might sound as the oil can get too fluid to be pumped by the oil pump (just guessing here)

@hellosailor
I had lots of rags in my hand to bury the cap before opening, also waited the engine to cool to a reasonable temperature.
No antileak coolant, and the cap is new (replaced in May 17), and clean.

@Compass790 and Sailing Drummer
I will do the first 2 minutes test asap.
I will have to disconnect the coolant subtank as it will run out in the opened tank.

@Wotname
Thanks for the comment on the impeller, makes sens.

@a64pilot
As I said earlier, each time i checked for coolant, it seem to me that the tank was full, because of the cap not letting the coolant from the subtank flow into the engine and compensate for any loss of coolant.
So, since I changed the cap, May 2017, I didn't topped with a single drop of coolant into the engine, and made a rough 80 hours of engine use
But the coolant level is not supposed to change anyway.
Where did the coolant go ...

@motretu
No strange noises, and v-belt tension fine


The overheating alarm doesn't sound anymore, but I will do some more testing tomorrow.
- Leave the new cap and monitor the level in the subtank when engine is warming up. Should go up when warming up, and down after shutting down. Then to initial level with no loss
- Do the same with the old cap, that was working well, but a bit rusty
- Do the bubbling test with the cap removed, for the 2 first minutes the engine is started
- Check to water in the oil (but there was none in my last oil change 6 hours engine running ago

Will let you know !
Cheers

[Compass790]

Originally Posted by Sailing Drummer

About the coolant running out when you remove the cap (if I understand you correctly). On my setup, same engine, the coolant reservoir is higher than the cap on the engine. Removing the cap on the engine will let coolant in the reservoir run out through that removed cap.
Did I understand what you said correctly?

Whoops, sorry didnt realize coolant reservoir is higher than cap, my comment was a generic one relating to testing for a head gasket leak. Yes you would need to block off reservoir. I dont know this particular engine

[AlanR]

It is possible that your thermostat is not opening at tickover.

My Volvo thermostat has a one-eigth inch hole through it, as a bypass, to allow warm water to reach it. I once installed a thermostat that did not have this hole, and the engine would get very hot indeed before it opened, and then the temperature would drop to normal. I drilled a hole, and it worked as it should..

If the thermostat does not open, the circulating water bypasses most of the engine, and it will overheat. As a test, you cold remove the thermostat, and run at tickover. If, after half an hour, the engine is not overheating, the thermostat is probably the cause, so try a new one.

Good luck.

Alan

[Compass790]

Originally Posted by a64pilot

Reread his first post.
Has anyone ever heard of a Diesel overhearing at idle, but won’t underway?
Maybe a large amount of air in the system that vapor locks the water pump, meaning it’s just spinning in a bubble?
I have never heard of a Diesel that overheats at idle, but not under a load.

You real sure it’s an overheat alarms and not low oil pressure?

Yes I too thought it was weird overheating at idle. Our raw water-cooled motor simply will not get hot @ idle with minimal water flow. We have common sound alarm for water temp & oil pressure but seperate lights. Maybe Alan R is on to something, our YSM8 yammer has the little hole in the thermostat too. Made me suspicious that water not going thru engine when he said water coming out exhaust is cold. That suggests it's bypassing engine cooling & going straight to exhaust injection. That's certainly possible on our model Yammer but don't know OP's version

[a64pilot]

Not all radiator caps have the little valve on the bottom that allows water back in once the system cools and forms a slight vacuum.
It should look like a little brass disc, about dime sized and be loose fitting. Pressure blows it shut against the rubber seal, gravity lets is drop when there is no pressure and a slight vacuum will draw liquid in from the overflow tank.
Check to see if your new cap has this disk.
It’s listed here as a vacuum valve.

[keith001]

When a head gasket leaks or there is a head or block crack, burned gases from the cylinder can get into the closed cooling system. At low rpm, the gasses will blanket the internal water pump, and no circulation. As the rpm increases the impeller will finally push the bubble out to the overflow tank. This only works when the leak is very small. Symptoms are high coolant temp at low rpm, a black film in the accumulator (overflow) tank, loss of coolant, a gas pressure in the cooling loop. Something to consider. If it is a leak(head gasket, warped head, or crack) it won’t get better.

[Capt Montana]

Just had a similar problem on my Isuzu 4BDI with an intercooler system. I thought the raw water side had not enough flow, but the engine temps stayed at 180 f so I never was concerned. This was a 1980 boat I bought as a project and just getting her in the water. Yesterday, the temp went to 250 and I shut it down. After investigation, the engine was almost out of coolant, no leaks in the hoses and when I removed the end cap from the intercooler, I could taste glycol, (anti freeze) in the raw water. So the intercooler was suspect, when removing the intercooler, found the raw water inlet side completely filled with impeller parts. Took the intercooler to the radiatior shop and they found two tubes leaking. They soldered closed the tubes, pressure checked to 100 psi and I am re-installing. Interesting that I had replaced the pressure cap with new 15 lb cap about 2 hours before this failure so I suspect the old cap was not holding pressure and thus when I installed a new cap, the higher pressure stressed the system and blew out weak tubes.

[a64pilot]

I think your correct about the cap, and why I have a tendency to run low pressure caps, as in 4 PSI, although they can be hard to find.
Even at just atmospheric pressure a 50/50 mix doesn’t boil into about 220F or so, and I don’t let one get anywhere near that hot.
High pressure is I think hard on hoses and O-rings etc.

[Idiens]

A few things to check, from my bitter experience with a 4JH4E.
Check the seal on the raw water filter, if it sucks in air, there may not be enough water.
Double check any anti-syphoning system. If it blocks then the engine could fill with sea water, very nasty. If it is leaking when the engine is running, there's less raw water to cool.

The heat exchanger is a devil, apart from blocking with crud and broken impeller blades, it can develop a short cut for the raw water below the rear end raw water entry and exit cover. Careful of those two retaining studs, mild steel into an alu casting .... ooh that sicken click of a sheared stud!

Having fitted an exhaust temperature gauge, I discovered the engine exhaust runs hottest at tick-over. Check the impeller again. Sometimes, if it is put in twisted the wrong way round, it can lose a blade/vane/paddle easily.
One last, but not so related nasty, is the gearbox oil cooler, which on the 4JH4E/ZF25 has no anode, so when it corrodes (hidden beneath the rubber end caps) it results in raw water entering the gearbox (milky oil), plus, on temperature cycling, oil escapes into the raw water. Spot it in time.