Mechanics have run out of ideas on how to fix cooling system

[helen&tony]

Hi
At first, I wondered why Mazda used a quality matrix for the radiator, then whacked on a crappy piece of plastic moulding top and bottom, sealed with a gasket and a sealant compound. The answer's simple, in reality, it's easier to make a suitable standard-width matrix, and you order a radiator by height according to the cooling needs.
Having got the right height rad, you stick on a standard top and bottom, and off you go! There is absolutely no need for a filler on the Bongo radiator, but you merely fill it there because there it is, and it's easy! Mazda have no need to get tooling made to produce a top moulding without a filler-neck hole on it, so they're all made with it. All it takes is for Mazda to order a filler neck to be bonded in as required, and in the case of the Bongo, it doesn't need a side-pipe...So, the cap on the radiator is of no purpose other than to seal it at coolant change...it has no real purpose . Virtually all cars are made with a pressurised system above that pressure naturally generated by expansion, as it raises the B.P. Thus pressure is set by the spring value at a higher force than that caused by natural expansion. The availability of a blank sealing cap for a radiator is probably just restricted to old cars without a pressurised system, most of which are unlikely to be the same pattern as the cap on the Bongo.
Right, as the rad outlet at the top needs sealing, it's a case of any old cap will do....pick a value at random....it WON'T serve any purpose.....the plastic tank regulates the pressure.
Cheers
Helen

[scanner]

Trust me - it is purely to cap off the radiator and has absolutely no other function.

I'm not arguing, just puzzled why some folk seem to think it must be a 0.9 one?

No idea - absolutely NONE. [/quote]

Precisely.

[Diplomat]

it's a case of any old cap will do....pick a value at random....it WON'T serve any purpose.....the plastic tank regulates the pressure.
Cheers
Helen

Trust me - it is purely to cap off the radiator and has absolutely no other function.

I'm not arguing, just puzzled why some folk seem to think it must be a 0.9 one?

No idea - absolutely NONE.

Precisely.[/quote]


Well, it's got to be a pressure rating higher than the one on the header tank unless you trust the outer seal, otherwise hot coolant might spew out of the radiator with unfortunate consequences, probably one of those dreaded air locks.

I've also gone as far a putting a Dymo label near the rad cap saying Do Not Remove

Frank

[helen&tony]

Hi
Frank...
There's absolutely nowhere for it to go, as the cap seals on the actual rim of the filler neck...you could literally chop the spring section off, and the little capsule inside....it seals like a blanking plate....as said before, a cork would do
Cheers
Helen

[scanner]

Well, it's got to be a pressure rating higher than the one on the header tank unless you trust the outer seal, otherwise hot coolant might spew out of the radiator with unfortunate consequences, probably one of those dreaded air locks.

That's what I thought, but apparently I was not being "useful" in suggesting so, because experts have spent a long time learning about the system and it's not so.

[Diplomat]

Fair comment, Helen, but one would have to have blind faith in the outer seal. I think I would stick with belt and braces, minimising the chance of leaks below the uppermost level in the system. However, the designers haven't helped by using that metal to plastic interface.

Frank

[Northern Bongolow]

sorry every one i didnt mean to start this falling out .

i must admit at the start of this i was pretty much sure that a cork would do, BUT after reading franks usual thought provoking comments above i now see things slightly differently .

the sprung/bottom seal COULD act as a first seal if it was a 1.1 bar cap so the whole internal pressure load was not placed upon the top ring seal, so it may act as a 2 stage (safe) seal.

the 0.9 cap may have been used as it happens to fit other cars, so was in the parts bin, they are fitted to early nissan micra.s amongst others.

i wont and dont use a 0.9 just because the fans come on at around 12/ 13 pounds internal pressure, the second stage fans just after, this doesnt leave a lot of room for it getting hotter before a 0.9 cap would blow off.
http://www.evi.com/q/convert_0.9_bar_into_psi.

[Simon Jones]

Anyway, back to the matter in hand . Alvino, are you having problems with the pressure building up and forcing water out the header tank vent? If so, then that could point to head or gasket issues. If it's just getting hot, my first thought would be an issue with circulation such as a partly blocked radiator although you mention this has been changed already. I would also get a seperate temp gauge rigged up so you can see the actual temperature of the coolant or head. It's always possible it's misreading the temperature.

The rough running when starting up coud be a sign of water in the cylinders but it's much much more likely to be dodgy glow plugs.

[alvino]

great to see there is so much contribution to this forum. Its a great resource. Lots of healthy discussion from which we are all learning a bit

Back to my current cooling system problem solving.

In answer to Simon Jones question:
yes. prior to the last action of the mechanic which was to drill a hole in the thermostat, we were having pressure build up and force water out the header tank vent. However this has not been proven to be a head gasket issue as the mechanics have pressurised the system several times to identify leaks and have not been able to show pressure dropping. The latest theory is that the fans are not coming on due to a problem with the Lower Temperature Sender Unit or the fan relay switch.

Is anyone able to give a brief description of whats involved in replacing the Lower Temperature Sender Unit and if necessary the fan relay switch. How long should this typically take a competent mechanic (not me!).

Cheers
Alvino

[jimmo62]

In a previous life (VW Golf!) I had a problem with the fan not coming on. I diagnosed by temporarily fitting a switch to activate the fan manually which proved that if I turned it on the temp would stay under control. Since the senders are quite expensive it might be worth trying this first.

[Simon Jones]

Sometimes a cracked head only becomes apparent when up to temperature so a cold pressure test may not be conclusive. The next thing I would do is switch the A/C on as that will turn the rad fans on so effectively bypass the temp sensor. I've got a feeling it will still overheat and that will point back to head or gasket fault.

[widdowson2008]

............ The latest theory is that the fans are not coming on due to a problem with the Lower Temperature Sender Unit or the fan relay switch.

Is anyone able to give a brief description of whats involved in replacing the Lower Temperature Sender Unit and if necessary the fan relay switch. How long should this typically take a competent mechanic (not me!).

Cheers
Alvino

Not entirely sure what you mean by 'Lower temperature' sender Do you mean the one that controls the dash-board instrument? If so, I wouldn't bother changing that as it has no input to the issue in hand.

The fan relay switch is a relatively easy piece to change. One out-one in. If you are careful/quick when making the change, you shouldn't loose more than a cupful of coolant, hence avoiding having to bleed again. It took me about 5 minutes and I aint a mechanic

However, can I ask why you want to change it? and have you checked the expansion tank cap yet?
The diagram below shows the temperatures relating to the cooling system.
The fans (which are 2 speed) are controlled by the ECU via the fan relay switch


Simon mentioned getting a seperate temp gauge. Thoroughly agree with him.

[alvino]

Hi all. Yes I am planning to get an after market engine temp monitor - probably one of the Haydyn ones. We do not seem to be experiencing overheating at this stage since the hole was drilled in the thermostat. However, I have not taken the van on a big hill yet to make it work hard. I will do that tonight and then see if the fan kicks in by itself or whether I need to put the AC on to make it kick in.

I already have a quote in NZD for a head gasket replacement of around $1500-2000 (currently about 750-1000 pounds). If it turns out to be a cracked head, does anyone know roughly how much more than just the gasket that would be?

re my question about what is involved in replacing the lower temperature sender unit, I was referring to the item in the bongo shop that states it "fits low down on head. Controls the cooling fans. For all petrol & diesel Bongos."


Cheers
Alvin

[widdowson2008]

Hi Alvin
Checked all through your post – one point has been raised several times but there aint a response anywhere. Have you checked/changed the expansion tank cap?
If this is faulty, then you can replace as many heads, head gaskets, switches as you want BUT you will still have the same result – overheating.

Consider the costs
New head? $2000/3000?
New head gasket? $1500/2000
New expansion tank cap? $2

If the cap is faulty, the system WILL overheat - it has no other option.
I would certainly suggest eliminating the cheaper (and easier) options first.

[Northern Bongolow]

re my question about what is involved in replacing the lower temperature sender unit, I was referring to the item in the bongo shop that states it "fits low down on head. Controls the cooling fans. For all petrol & diesel Bongos."


Cheers
Alvin

drain the coolant to the block level. remember youneed to bleed it when refilling later.
flick the wires off just above the starter motor on the drivers side/rear of the engine bay, its 2 thirds of the way back on the head. deep socket and remove, watch for the sealing washer, replace with new , job done. mind you dont cross thread.
bleed up the cooling system.