Thank you all.
We'll go for it after Easter.

Best greetings for everyone.

On 4TNE92 & 98 the crank case breather is part of the rocker cover and it feeds directly into the inlet manifold.
If you look at the rocker cover at the rear end of the engine ( flywheel end) you will see part of the rocker cover on the inlet manifold side corner that has a slightly different casting, this is it.

crank vent system is located under the cover of four valves per cylinder
Greets Krzysztof

You must have what's call CCV (Closed Crankcase Ventilation) system on the lift. It's a kind of air/oil seperator. They have a filter in them- maybe it's clogged, or there's a 1 way check valve in the filters drain that may be faulty. Just a thought

Maybe not a pipe visible but there is something. Remove the filler cap and you will feel pressure but as you increase the rev's/ speed of the engine the pressure felt at the cap will decrease as the air intake recirculate's the crank case gas's. Oil can be present in the intake manifold on a brand new yanmar. Think it's something from there marine engine background

It is true that the cooling capacity is small.
Check for proper fan rotation.
Identification propeller engine side.
and correct functioning the thermostat.

Thanx guys for the great feedback.
I really appreciate this.

I asked my servicemen to proceed with the coolers renovation.

@grubbymits
We checked very thoroughly, there's no pipe between rocker cover and manifold.
We have several hundreds of trucks under our maintenance, Linde, Still, Toyotaa, Yale and simply each brand and type has the vent for deaeration.
This Hyster with Yanmar is the first truck, where we can't find it.

Recently we found, that similar engine is installed on some Doosan 3,5 tonners and in the catalogue there is something that looks like round cap on the rocker cover. Perhaps this is presumed for installation of the vent.
Unfortunately nothing like that we found on this Hyster.

I am still stupid with this engine.

Regards to all.

Having used plain water for a time may have caused some growth of galvanic corrosion and scale inside the tubes of the radiator which has reduced the flow volume enough that now a point is reached (after 20 minutes) that the radiator can no longer cool the engine.
The presence of any amount of mineral or metallic scale deposits in the tubes reduces the flow below design parameters. Or put another way, reducing the coolant flow through a radiator is like having a smaller radiator.

Crank case pressure is always there on this engine.there is a pipe from rocker cover to inlet manifold

I would agtree that water as the cooling agent would eventually cause problems- such as rust. But rust would form in the engine, not the radiator- the rad is aluminum. A good flush of the cooling system is in order I think.

As far as the engine pressure, if there's no vent, you may have to install one in the valve cover. ( I have done this on a few occasions).

I would suggest taking oil pressure readings & compression readings & checking them against spec. This pressuration issue could be a symptom of a worn out engine.

Hi Bbforks,

Thanx for the answer.

The trucks are from 2008 and they have about 8000 hours.
Yes, the pressure is in the crancase.
My guy has been looking for vent everywhere and found nothing.
No hose coming to intake.
We have taken the trucks under maintenance just recently, from the customer documentation seems they used 15W40 Texaco.
I think it's OK.
What we have learned too is, that for some time they have been using water as a coolant.
I am afraid, this could cause some problems (rust), maybe the vaporisation caused the pressure.

Anyway, we must have a closer look on the cooler.

Any ideas, how to deaerate the system in this type of Yanmar?

Not familiar with the Yanmar engine, but the Hyster's I work on have the GM engine. They have a vent from the valve cover to the intake hose, between the air cleaner & intake.

When you say air pressure, I assume you mean built up pressure in the engine's crankcase? If so, how many hours are on the machines? What grade of engine oil is being used?

Even with built up crankcase pressure I'm not sure this is the cause of your overheating. I've had to install vents in the cover over the radiator to cool off the ones I work on. My units weren't overheating, but the electronics & LPG regulator were constantly failing due to the heat. Now that the vent is in place, the heat related failures have gone away.