Bouncing Temp Guage.

[jimbouy]

Whilst motoring back from Cowes at the weekend, the longest run I've given the engine, I noticed that the temp guage rises upto almost Red and then drops sharply down to cold. It then repeats the cycle.... rising steadily and shooting back down.

I hope this isn't a daft question, but the range the temp was going thru. seemed quiet wide.... certainly much wider than any car I've ever driven.

Would I be right in thinking that the Thermostat is perhaps past it's best? Or is this normal on a big old Volvo MD11c??

I don't want to touch anything that doesn't need it.

Regards

Jim

 

[Anonymous]

Are you raw-water cooled? Is the exhaust water coming out with the exhaust at a constant rate or does it change along with the engine temperature? With a Centaur and MD11C you should be able to hear a change in exhaust tone as the water flow changes.

 

[Paulka]

It just shows that the thermostat is doing its job correcly.
Don't worry.
I have the same MD11C since 25 years.

 

[Anonymous]

[ QUOTE ]
It just shows that the thermostat is doing its job correcly.
Don't worry.
I have the same MD11C since 25 years.

[/ QUOTE ]It might not be the thermostat - mine suddenly started doing just that and I fixed it. It doesn't do an engine any good to be going through extremes of temperature.

 

[Bilgediver]

/forums/images/graemlins/smile.gif Thought it was a Volvo from the title.... Nothing wrong with the thermostat...Its working....but are the cooling passageways all doing their bit??????? and is there a constant flow hole in the thermostsat.

On the front of the manifild there is a thermostat housing where there bye pass water mixes with the water coming out of the engine.

Check that the bye pass passage is less than 1/8 . If this is too large then two much water flows through the by pass giving insuficient flow to the engine.

What can also cause this effect is mud in the liner/block cooling passagways. When did you last open the drain cocks when the engine was runnning.??? Do this before the engine gets too hot and you cause thermal shock. If you put your hand in the water flow you ll feel the "[censored]" being ejected from the galleries.

The liners have a poor cooling circuit and this relies on an annular chamber existing at the base of the block casting which joins up the vertical passages...If there is mud in this chamber the water can t circulate and you get the effect you describe.

The water heats to boiling...the steam formed condenses and draws in cold water and the cycle is repeated when circulation is stopped by a blocked lower gallery.

Feeling the core plugs should tell you which liner/block is bad.

You may clear this by flushing out as the engine is running via the drain cocks..If this dosn t work then the heads have to come off and the block passageways flushed out.


JoHn

 

[Anonymous]

I eventually traced the problem on mine to an excessive gap between the impeller and the plate but not until I and engineers had been through everything else first /forums/images/graemlins/frown.gif

It seems to me that the key question is whether there is a good constant water flow. If not, then check weed filter and impeller/pumping. If there is, then go down the line of investigation you have suggested.

 

[jimbouy]

It's raw water cooled. I can't say I've noticed a change in tone. But I think there's more water ejected with the exhaust when the guage is dropping. Although again I haven't really studied it yet.......... but will now.


Jim

 

[jimbouy]

John

Whoa plenty of tech stuff there.


I'll have to dig out the service book and see if I can make sense of what you say.

In my short period of ownership I've never opened the drains.


Sounds like this may be a good starting point.



Jim

 

[Johnjo]

Jim few years back I had the same engine, had a problem with the thermostat...it disintegrated..

So ordered a new one from volvo and noted it was slightly different to the origional, volvo said ....no worries it will be ok... when i queried it.

Anyway the outcome was exactly the same as the symtoms you discribe....temperature guage rising and falling, when i checked again i saw that the old thermostat had a very small hole drilled in the casing whereas the new one did not.

Drilled a very small hole in the themostat this allowed a small constant flow of water through the engine and evened everything out....

Be aware that if this hole is to big it will allow to much water through and will stop the engine reaching the correct temperature, if i remember it was a 1/8 diameter hole i drilled in mine..

best of luck with it

 

[RivalRedwing]

[ QUOTE ]
It just shows that the thermostat is doing its job correcly.
Don't worry.
I have the same MD11C since 25 years.

[/ QUOTE ]

Thermostats are used to regulate cooling water such that the engine enjoys a steady, near-constant operating temperature. Sharp fluctuations most definetly do not indicate that it is doing its job correctly - they imply that either the thermostat is sticking and should be replaced or, as would appear to be the case in this instance, a modification is required.