Overheating/overcooling Perkins 4.236

[Guest]

My engine (Perkins 4.236 lowline 1977 with about 1800 hours on the clock) shows tendencies to overheat, slowly, when under high load and run too cold when under very light load.
Running on cruise RPM´s, appr. 1600, shows normal 90 degrees on the gauge. 1800 RPM gives close to 100 and 2000 + there is fast rise up 120 and I don´t dare to continue.
On the other hand, when running at slower speeds the temperature falls, at 1000 rpm shows about 70 degrees and when idling even less.

The engine has its original cooling system and the yacht is a Moody 42 ketch with prop and gearbox as specified, when running flat out the engine easely reaches its max allowed RPM.

Compression test shows absolutely no problems.
I have dismantled every single part in the whole cooling system, cleaned all tube stacks to a shiny metal suface in all heat exchangers, checked the exhaust manifold for cracks.
Thermostate is checked in boiling water, and I did also put a new one in, just to be on the sure side.
Of course I have also checked the seawater side for blocking, put a new impeller in and the flow seems to be normal.
The exhaust fumes look good, apart from a tendency to be somewhat "whitish" when the engine runs hot.

This fault puzzles me, too hot when running hard and too cold when running light!

I have owned the yacht for 2 years, and this phenomen has been there all the time. The former owner says he hadn´t noticed anything out of normal.

I would appreciate any ideas and hints and wish you all a good sailing season!

 

[Guest]

I have exactly the same problem, and I have stripped down the cooling system and changed the thermostat too!

Spooky!

I was advised by local engineer to check the Gearbox oil cooler on the seawater end by peeling back the rubber end to check for any bits of seaweed etc. which I have done with no joy.

I thought the new thermostat would bring up the temperature at lower speeds as the other engine I have always sits at 90 degrees regardless of speed.

I am as puzzled as you.

Anyone got any other ideas?

 

[Guest]

At least I´m not alone. Feels a bit reassuring.
I´ve been thinking a lot of this problem and I have one idea, but probably a quite far-fetched one. But let´s hear what you think;
To my common sense the only part in the cooling system which can cause BOTH overcooling and overheating must be the thermostate.
If it´s not opening enough and not closing properly either it might cause these symtoms.
I remember when I removed the thermostate for the first time on this engine, the surface on it was browninsh from rust deposits in the cooling water. But on the inside where the stationary part of the thermostate is close the moving part there was shiny marks, almost like shallow scratches down to bare metal.
I tried the thermostat in boiling water, it opened fully and closed properly. Just to be sure, I put a new one in, which I also tried first. As I said, with no difference in behaviour. Since then I have´nt touched the thermostate.
But, what about the thermostate housing? Lets pretend that the cover, or gasket, or some mysterious misalignment puts some force on the thermostate so it goes slightly distorted. This distorsion could then prevent it from moving freely. Thus restricting it from closing fully and also open fully as well.

Give it a thought, and best whishes from a rainy Stockholm!

 

[Guest]

Just a thought, but if overheating causes no apparent problem could the temperature gauge be telling lies.

I don't know that particular engine but is it possible to get at the sensor and test it?

No further ideas, sounds very frustrating problem good luck with it

 

[Guest]

Temperature sender/gauge

Thanks for you hint, but you are unfortunately wrong. I have tested the sender in heated water and it confirms with the resistance values given by VDO. In my particular installation there are two steering positions, thus the instrumentation is doubled. Both gauges shows the same values, so I make the conclusion that the gauges cannot be faulty.