Overheating Again!!

[Peg leg Pete]

so true

[Vanmanerik]

Well we certainly have come up with some interesting information here, the schematic seems to agree with the thermostat being on the return water pipe, thanks for that 'alphabetter'.

As far as I can see there is no secondary circuit to actuate the thermostat so it must rely on the water temperature from the base of the crankcase to do this. It cannot be the temperature of the water returning from the radiator as this will be much less than the 82 degrees to open it.

The other problem I can see is that both the hot and return pipe runs have quite a long horizontal run so the convection current will not be enough on its own to circulate the coolant so if the water pump is a bit dodgy then this could be the culprit.

It will be worth checking the rubber sections of your pipe runs when the engine is running to see if they have 'collapsed' anywhere and this is preventing the circulation. You cannot see the collapsed pipes if the engine is not running as it is the circulation that causes the collapse in weak pipe walls on the return pipe usually.

We could do with a piece of large diameter see through poly pipe on the return to see if the water is circulating.
Maybe we need a secondary water pump (electric?) in the cooling circuit to help it out untill the thermostat opens .

[dandywarhol]

On the VW/Audis the thermostat wax capsule is towards the radiator and does rely on the temperature of the bottom tank/hose to open it.
82 degrees isn't all that hot, especially with the water pressurised at nearly 1 Bar. That 1 Bar pressure means that the coolant can get to 120 deg C before boiling..............82 deg at the bottom of the tank and bottom hose to the thermostat is the way it's designed to operate.

I've only ever come across a water pump as either working or not....if not then the bearing has collapsed or the impeller has sheared off but never dodgy at a halfway stage. If an electric pump was needed then Mazda's engineering team would have fitted one or did I miss the humour part of that vanman???

Sorry to doubt you again but the hose is unlikely to collapse under pressure either.

The radiator/feed hose is the culprit IMHO

[Vanmanerik]

I never realised the header tank cap had a release pressure of 1 bar (15lbs per sq in) this can raise the boiling temperature by 25 degrees C.

But................... if the thermostat does not open till the water temp from the radiator return is 82 C how does this heated water reach the thermostat if it is closed thus stopping any flow??

If the thermostat was on the outlet to the top of the rad as in a conventional car the heated water from the engine would open the thermostat enabling it to flow to the radiator.

Dam crafty these Japanese motor engineers... too clever for us.

[alphabetter]

But................... if the thermostat does not open till the water temp from the radiator return is 82 C how does this heated water reach the thermostat if it is closed thus stopping any flow??

We are not at the end of understanding this - not by a long way!

I looked this morning and there are certainly bypass circuits that allow water to circulate past the thermostat. Just under the oil filler cap there is a pipe that emerges from the engine which goes though a very short thick hose to just above the thermostat. This outlet is also connected to the air vent hose for the cooling system.

There is another thick hose that comes out from above the thermostat to the interior heating circuit.

Finally there are thinner pipes that come out from both points mentioned to the gubbins on the right (EGR?).

Dam crafty these Japanese motor engineers... too clever for us.

So it would seem - perhaps to crafty for their own good in this case as well!

[alphabetter]

...there are certainly bypass circuits that allow water to circulate past the thermostat.

Which makes me think that in terms of the original problem the cause could be a blockage in one of the bypass circuits - stopping the thermostat getting heat if it isn't opened.

[dandywarhol]

I never realised the header tank cap had a release pressure of 1 bar (15lbs per sq in) this can raise the boiling temperature by 25 degrees C.

But................... if the thermostat does not open till the water temp from the radiator return is 82 C how does this heated water reach the thermostat if it is closed thus stopping any flow??

If the thermostat was on the outlet to the top of the rad as in a conventional car the heated water from the engine would open the thermostat enabling it to flow to the radiator.

Dam crafty these Japanese motor engineers... too clever for us.

The heated coolant circulates as any heated coolant circulates - by convection currents.........how does hot water get to the top of the house from a boiler in the basement????

The only difference in this case (and other clever designers) is that the 'stat is a bit further away from the engine than usual. The engine still heats up through a bypass system, and being a diesel engine, very quickly........... convection currents take the heated coolant through the radiator to the thermostat. A cooler running thermostst will IMO be more reliable to one which is having to cope with 110+ deg C of pressurised coolant.
If the radiator cannot allow the coolant to flow to the 'stat (test by feeling the temperature of the bottom tank when the engine appears hot) then the 'stat WILL NOT open fully.
Drilling holes in it will allow some coolant to circulate, as would leaving the 'stat out altogether but that's just guising a problem and liable to crack the head.

99% of Bongos haven't got an overheating problem so the clever Japanese engineers aren't too crafty - they're making a system work!.......bad maintenance is the real reason for the overheating problems as plenty on here have indicated......Bumbly1 is probably the main advocate for the sludge build up theory beacause the not - so - clever Japanese engineers in the garages or the enthusiastic owners who don't understand modern motor engineering didn't do their job properly.......this particular system needs to have the radiator flow tested - I would guess, and it's only a guess, that it would take around 8/10 seconds to empty a full radiator that size...... done by removing the bottom hose, filling the radiator with a hand over the radiator outlet and timing the radiator to empty....... rant over, climbs of soapbox.......

[Vanmanerik]

I agree with most points you make Dandy but convection does not like long horizontal pipe runs, it works well where the heated water can rise easily.

If the thermostat on the return circuit is closed when the engine is cold then the warm water from the bottom hose of the radiator will never reach it as there is no flow.

Alphabetter has had a closer look at the plumbing around the thermostat area and has spotted a bypass system which will be the one that will open the thermostat and for any persistent overheaters I would imagine this would be a good place to start looking because as Alphabetter states 'the cause could be a blockage in one of the bypass circuits - stopping the thermostat getting heat if it isn't opened.'

I have enjoyed the approach we have taken on this difficult subject as it arrises time and time again and we have never really looked at it 'en masse' but only as an individual if you had the overheating problem and the answers offered were not really that helpful and I imagine a lot of money has been spent on new cylinder heads and the like unecessarily.

What we need now is for an overheating Bongo owner to check out the bypass circuits around the thermostat are and see if they can find any restrictions, if so clean them out and see if this cures the problem.

[alphabetter]

Yes I agree with Vanmanerik. Considering where the thermostat sits in the system it seems that the bypass circuit is needed to get it to open. The main learning point for me from this discussion is the importance of the bypass circuit working properly.

I do think it would be really useful if we could collectively continue this investigation and try and get a proper schematic for the cooling system together.

[bongoing-mad-simon]

hi everyone
i agree that it would be good if collectively fury members could come up with guide to the opperation of the cooling system and most common areas of fault. So as in a way coming up with a check list for overheating problems. This would hopefully greatly reduce the cost and stress of this particular problem,as this seems to be a common bongo issue.

____________

simon

I'm bongoing mad R U?

[grumpo]

Just a few technical points that may add something to the debate.

1:
The maximum coolant temperature should be 90 degrees Celsius. This
temperature is indicated on the gauge when the needle points exactly
towards the base of the flag pole in the icon at the top of the gauge.

A clean coolant system and working pump will maintain this temperature
( +/- 2 degrees ) when the engine is running under a heavy load, and
a high summer ambient temperature.

An engine should not normally experience a differential temperature of
more than 10 degrees Celsius, so an inlet thermostat with an opening
temperature of 82 degrees sounds about right to me.

While 90 degrees is well below boiling point generally, it does not apply
to the blade tips of the circulating pump. Because of the reduced pressure
behind the blades, they will cause the pump to cavitate and boil the
water at a much lower temperature, steam in the pump virtually reduces
the pumps efficiency to zero. Increasing the pump's gauge pressure to
1 bar stops it cavitating. You can't make a decent cup of tea on top of
Mount Everest, the air pressure is so low that water boils at around 90
degrees C.

2:
The thermo-syphon effect depends on the difference in height and
temperature of the source and destination. With the Bongo there is no
difference in the height, so disregarding everything else, there will be
no circulation due to thermo-syphon effects.

3:
A single 3 mm hole in a thermostat will pass about 10 times more water
than you may at first suppose, in the good old days; all thermostats had
a hole in the fixed part around this size specifically to promote sufficient
circulation when the stat was completely closed.

Has anyone got a reliable parachute I can borrow ?.

[mobyfix]

since the engine is cooled sufficiently when the thermostat is removed or has holes drilled in it...... the radiator CANNOT be blocked...

Older systems used to have holes drilled in order to act as a bypass system.. as without flow the stat will never open....as long as the water is moving past or through holes in the stat unless faulty it will open at the set temperature.

The drilling of holes will help eliviate this problem if you don't want to clear the real problem...You just redesign and bypass the bypass

Duncan

[bongoing-mad-simon]

hi mobyfix
my bongo overheated after traveling over 200 miles when the thermostat had already opened so you are saying that it is still the bypass pipes?
this was after i'd had a new thermostat and new head. i'd travelled around all day (christmas shopping) with no coolant loss or loss in power it just suddenly overheated on an open stretch of road for no apparent reason.
what are your thoughts on this?


_________________

simon

I'm bongoing mad R U?

[mobyfix]

Nope

different symptoms from the original complaint....

water pump failure, blockage, head gasget failure, electric cooling fan failure, electric fan thermostat failure ...could be any of these..

Have you had it fixed yet ?

[bongoing-mad-simon]

hi mobyfix
yes thanks seems fixed now (see 1st page of this thread), my symptoms were exactly the same thats why i replied to danny boy in the begining.
mine also ran fine with no thermostat the only thing i changed and he hadn't was the radiator.

________________

simon

I'm bongoing mad R U?