Overheating? Shall I panic now?

dandywarhol · Post by **dandywarhol** » Mon Mar 17, 2008 7:54 pm

Degassing by what method TGP? I'm not disbelieving but curious..........

lizard · Post by **lizard** » Mon Mar 17, 2008 8:18 pm

Can we have an expert to recap on what we have learnt in the dicsussion, as I am

flippa · Post by **flippa** » Mon Mar 17, 2008 8:33 pm

....like me lizard ....baffled

.....but am waiting in anticipation on some enlightenment

lizard · Post by **lizard** » Mon Mar 17, 2008 8:43 pm

Anyway flippa, your supplied (blue) hoses look nice under the bonnet.

flippa · Post by **flippa** » Mon Mar 17, 2008 8:51 pm

i aim to please lizard and btw im not finished with you yet

i have another 5 hoses in the offing .....

dandywarhol · Post by **dandywarhol** » Mon Mar 17, 2008 9:27 pm

lizard wrote:Can we have an expert to recap on what we have learnt in the dicsussion, as I am

Fair point lizard - Don't claim to be an expert by any manner but I think smac (so far back I've forgotten his name now

) has the start of a choked system.

smac02 · Post by **smac02** » Mon Mar 17, 2008 10:23 pm

Hi all,
I see this subject is fast becoming a full on saga.

Today fitted a new thermostat. Was going to fit a new water pump but saw that one had recently been fitted and as time was against me I'm leaving that one till later.

Result is that the engine is running cooler!! The temp gauge (with Mason Alarm) now reads 1 o'clock up most of the time and a bit higher when I give the engine some gun. So some success.

I tested the two thermostats together and the old one opened up much slower and much later than the new one. It opened up all the way but very late in doing so. It also closed sooner. So, almost certainly the main culprit of my hot running.

I again flushed the whole system through and spent time getting rid of any airlocks (how long does one have spend doing that to get 100% success)?
I will be fitting a replacement rad as soon as it arrives in an effort to ensure cool running even though a rough flow test seems ok.

Just to give an idea of the events today I monitored the warm up period. Top hose nice and hot, bottom hose cold, thermostat cold on bottom hose side. When stat opened, as expected bottom hose gets hot then cools to slightly warm. When engine is revved fans cut in and bottom hose returns to cool / cold.

HOWEVER. My fans don't know when to cut out!! Just as before once they've switched on they stay on till I switch the ignition off and restart the engine. Took the Bongo for a drive and every time I checked the fans were running even when the temp gauge was at normal.

Several conclusions, the fan(s) relay switch is sticking, the sender switch in the head is faulty. I still have a niggling doubt that the head gasket is at fault and sending masses of heat to the rad but guess that's not logical when I've proved that heated water is flowing and that the fans are capable of cooling it.

After reading another older post I have changed over what I believe is the fan relay to see if that changes the way the fans operate. Maybe a knackered sticking relay

Oh what a joy Bongo's are.

Regards Steve Mc

The Great Pretender · Post by **The Great Pretender** » Mon Mar 17, 2008 10:50 pm

dandywarhol wrote:Degassing by what method TGP? I'm not disbelieving but curious..........

Andysuea pointed me in the right direction with the heating system.
The reason the tank forms part of the system and is not just an expansion tank is to supposedly degass or remove air from the system. But it cant because of the design and it looks like only the top of the rad is used most of the time (through the tank). This would explain why the bottom hose is cool, stat closed.
There is more investgation to do but hope this stimulates some little grey cells.

haydn callow · Post by **haydn callow** » Tue Mar 18, 2008 12:23 am

TGP... Don't go along with that point of view.....I belive the bottom hose is cool because the stat is open and the rad is doing it's job very well.....once the system is bled after filling no air should be in the system for the tank to "degass" air is expelled as the engine heat expands the coolant and then upon the engine cooling air is drawn back into the tank via the pressure cap valve. Once bled no air should be present in the circulating coolant unless a fault developes.
The Bongo cooling system is a very interesting and complex system. It would be nice if Mr Takashita would come to the big bash and explain exactly how it works. I think I have got a grip on it's basics but I'm also sure there are plenty of you out there who have equally strong and different points of view..
To get back to the subject of this topic it would seem that Dandy was on the case stright off with the partialy blocked system..i.e. the lazy stat which was to all intents and purposes a semi blockage.

The Great Pretender · Post by **The Great Pretender** » Tue Mar 18, 2008 12:53 am

That is a shame Haydn, just explain to me how a thermostat that opens at 82c will be open for cool water, your reasoning is unsound. Said it before im not here for an argument im looking to resolve the problem. There is no way air built up in the cylinder head will get back to the rad or tank untill there are big problems. Have you tested pressure in the system?

dandywarhol · Post by **dandywarhol** » Tue Mar 18, 2008 12:58 am

Don't completely agree there either Haydn.
There is air in the system - it's above the coolant in the expansion tank. Remove the cap when cold and air is there causing no problem atallatall - it's air caught in pockets in the water jackets/matrices that cause the problems.

Under normal circumstances (everything operating as Mazda designed it) there should be only enough pressure built up in the entire system to raise the boiling point of the coolant to an acceptable limit.

A rough rule of thumb is that 1 psi in pressure lifts the boiling point approx 1.5 deg.C. As the fans come on at 108 deg. to cool the engine where moving airflow through the radiator isn't (normally when stationary or slow moving traffic) we could then assume that 108 deg corresponds to around 6 psi of system pressure.

The cap is rated at 1.1 Bar (approx. 16 psi) so there's a fair bit of leeway (10 psi) before the cap valve "blows off" pressure and the coolant escapes to atmosphere. That leeway allows "slight" overheating when climbing at low speed etc. without causing damage. I seem to remember some folks with a digi temp gauge showing 114 deg. at times. At "blow off" time we're in Danger Time, there's a reason for going that high and it isn't normal........blockage/thermostat/coolant loss/gasket/crack....etc.

If the cap valve has released the pressure then as the system cools down, the system will then be in a vaccum and that's when the second valve opens on the bottom of the cap and allows air INTO the tank to stop the hoses "collapsing". This air won't cause any more problems than the air already there before the coolant was forced out but the cause of the overheating needs to be addressed. 16 psi of pressure in the system will allow the boiling point to go as high as approx 120+ deg.

I feel this thread has the capability of going on and on like last year and (if already not) folks will be slitting their wrists.......

The Great Pretender · Post by **The Great Pretender** » Tue Mar 18, 2008 1:05 am

ok lets get back to basics, the pressure cap is to stop overheating and loosing water when THE CAR IS STATIONARY and latent heat builds up.

The Great Pretender · Post by **The Great Pretender** » Tue Mar 18, 2008 1:11 am

Dandy, exellent description of an expansion tank, allowing exess out as pressure is high. Then sucking it back when less than atmospheric pressure. But that isnot what a Bongo has.

dandywarhol · Post by **dandywarhol** » Tue Mar 18, 2008 1:14 am

The pressure cap is there to allow the pressure to rise and raise the boiling point of the coolant above 100 deg. (sea level) - and that stops the system overheating.
The engine is happiest when the coolant temp. in the head is around 95 deg.C. Allowing the system to pressurise raises the temp to around 108 before the fans are called in..................

dandywarhol · Post by **dandywarhol** » Tue Mar 18, 2008 1:18 am

The Great Pretender wrote:Dandy, exellent description of an expansion tank, allowing exess out as pressure is high. Then sucking it back when less than atmospheric pressure. But that isnot what a Bongo has.

Mine has

It's what the cap does.

Overheating? Shall I panic now?

The Ongoing Saga