Volvocooling

[dsmolar]

Hello ,

I have an 80?model Volvo in my 9.2s2.I have noticed thatthe temp gauge moves to almost the red then drops back as the thermostat opens,never has run hot or into the red.Have put 2 new thermostats and impeller in.Plenty of water out the exhaust,no steam.The gauge just cycles between 140f and 160f.Since guages should stay in one place,cycleing just buggs me.Rawwater cooled. Is there any problem I should lookfor?Can't see that anything is not functioning correctly except the guage.Oh,the engine is an md7b.


David

[gonzo]

It could be blocked passages in the water jacket. Either debris or corrosion can cause that.

[Frosty]

I have given this some thought and I cant understand why debris or corrosion could cause this,--but I will listen to an explanation.

I would like to know the speed of the cycling ( how many times per minute) and if the guage is mechanical or electrical.

Also how long have you had this problem.

[Syed]

This is what I can think of;
Gush of cold water quickly cools off the thermostatic valve and you have pulsations in the temperature due to repetition of the cycle. I would carefully try to regulate (reduce) the flow rate of the water (Off course within very safe limits). May be the new impellers are very efficient.
Hopefully some body with experience on this particular engine will come up soon.

[dsmolar]

Jack,

Noticed it few months ago.Ran motor for a while and it seemed tocure up,then started again.Put new thermostat in,same thing another new thermostat and impeller,sme thing.Has electrical guage and marked in red and green only.Every thing seems to function correctly,no trash corrision to speak of(fresh water boat)The temp just cycles about every 1/1/2 minutes ;never timed it htough.Check hot temp and that is 160f within range.
Syed seems to have the best idea so far,the water temp is still pretty low,probably about 55Fwhich would be a pretty good differential since it is raw water cooled.

David

[Frosty]

Ok I said I will listen --what causes the gush, what stops the gush and why does the gush stop and start?

[Syed]

Thermostat

[FAST FRED]

Many boats have a thermostat bypass , that allows water to citculate while warming up.

There is usually a restrictor in the bypass hose.

Sounds like the hose was changed , and the restrictor lost.

That gives the thermostat bounce you seem to be having , as much of the cooling is done by the bypass , not the thermostat.

Ask your dealer or consult Da Book!

FF

[Syed]

Themostat by-pass with restriction is definitely a better option.

[Frosty]

dsmolar,--Do you have the bypass thermostat system?

Thats a thermostat with a valve at the bottom as well as the centre.

Ive never heard of a restrictor in the bypass. The whole point of this system is to warm up the engine (and any heater system) quickly.

As it will then be enclosed, the speed of the coolant water circulating will not change the speed of the warm up. It will be desired as temperature distribution around the engine will be even.

[dsmolar]

the thermostat has two small holes opposite each other.I did change a hose but it looked as though it went straight to the exhaust discharge.Did not notice any kind of restrictor.

David

[Syed]

Hello,
David,

It should not be difficult to partially block the water inlet openings, the point is to keep the temperature higher than the thermostat's closing temperature.
This may be regarded as a make shift arrangement till you get an expert opinion.

Syed

[dsmolar]

Thanks for the ideas guys. I'll get started expreimenting.


David