What would make a diesel engines temperature cycle?

[dpb]

I have noticed that one of my AD41s temperature cycles when operating at approaching 3000 rpm. It gets up to about 95 to 100C then after a few minutes at this temp drops to high 70s for a minute or so then goes back up to 95 and so on.
At lower revs it settles at a more constant temp in the 80s.
Other engine fine.
Thought it likely to be the thermostats but having checked them in a pot of water they seem to operate more or less to spec and more or less in sync.
Apart from the possibility of an air lock somewhere I can’t think of what would make this happen.
Any ideas?
(There was a drip leak on the system on the hose between the thermostat and exchanger so the water level was a little low (bottom of filler pipe), this wouldn’t have caused it would it?)

 

[kashurst]

it may not be the engine itself - have you checked out the gauges and senders (swop over if possible). Also have a look at the raw water circulation pump - the seals may be starting to fail and its leaking at higher speeds.

 

[harstonwood]

Thermostat on the calorifier? if its the engine that heats that, it can cycle

 

[Questor]

It gets up to about 95 to 100C

Does the alarm sound? I thought they are set to trigger somewhere between 92 - 94 degrees.

Could be a clue?

 

[Grunter]

Dont know your setup but know that if you have a leak then you are not holding pressure, that will allow the water to boil at the temps you mention, in effect creating pockets of gas in the cooling system which can give the symptoms you mention. It also causes localised 'hot spots' which most engines dont like very much..
Also dont know what type of water pump you have, if that was a car engine I would say the water pump was slipping at higher load speeds.
can you isolate the calorifier to eliminate that first?

 

[dpb]

Thanks all,

Kashurst, I will check the sender and gauge, but doesn't the fact that they read steady at lower revs suggest they are OK?

Harstonwood, no calorifier fitted.

Questor, no alarm sounding, suppose I should check to see if it has one!

Grunter, I did wonder about the leak being the cause, regarding the pump, wouldn't it just over heat, not cycle if it was faulty?

Guess I'll have to fix leak, service impeller etc and see what happens next season!

 

[Grunter]

I dont know what type is fitted to your engine but most automotive water pumps these days are made on the cheap with the impeller just press fitted to the shaft, these can and frequently do slip under load. under normal operation, more rpm = more heat, but pump is spinning faster so more coolant flow to deal with it. if your pump is slipping and not giving greater coolant flow at higher rpm then the coolant is getting too hot, you then get binding on the pump and it throws a slug of coolant around, coolant that has been sat in the exchanger for longer than normal so when that reaches your temp sender it reads lower than normal running temp. Pump has now cooled and starts to slip again. and so on..

As I say, I dont know your setup and its been awhile since I worked on a boat engine, it may be a completly different pump arrangement, but, the symptoms match. although it could equally be cylinder gasses escaping into the cooling system causing air locks etc..

But I wouldnt want to be a merchant of doom and gloom!! so keeping it simple I would fix your leak, top up and try again before anything else.

 

[Assassin]

It could be something much simpler, the engines temperature rises and the thermostat opens very quickly and lets a lot of cooling water through, this is quickly cooled and returned back to the engine which cools, then builds heat back up.

Seen it many times.

 

[macnorton]

It is also worth checking the voltage of the supply to the instrument is constant, as this can alter the reading.