Calling Volvo experts - N12 cooling

I know some don't like them but am hoping someone knows a bit... Trying to thinking about where to look for this problem - our Volvo N12 keeps blowing little accessory cooling hoses, but only at the bottom of a long down hill run and it never gets anywhere near hot, and the gauge is working fine. The hoses seem in good nick, but repair one and another goes, happened 3 times in three days now.

I'm struggling to see how the pressure can be too high if it's not hot, can anyone take a stab?

Happening at the bottom of down hill run is when the (multiple) thermostats would be closed so some back pressure might be happening. When was the last time when anyone worked on the cooling system - could something have been reversed? An associate had lots of problems with his F12 cooling system - turned out to be a loose piece of silastic in a vent which blocked in service but not when being tested! I would check the thermostats first to rule that out. When asking for parts you will need to have chassis No and engine type as the N12 went though a progression of developments.

Tried doing the same run without a radiator cap just a thought or cap on loose

Have you tested the coolant for exhaust gas contamination? Not knowing anything about the set up on these truck I'm guessing. What sort of exhaust brake is on them? If it only after an extended period on the exhaust brakes, ie down a long hill there could be somewhere that the exhaust back pressure is getting past a gasket? Is there some sort of silly egr system where a housing for exhaust has coolant near it? Most other faults would be more prevelant under a high load situation you would think.

Happening at the bottom of down hill run is when the (multiple) thermostats would be closed so some back pressure might be happening. When was the last time when anyone worked on the cooling system - could something have been reversed? An associate had lots of problems with his F12 cooling system - turned out to be a loose piece of silastic in a vent which blocked in service but not when being tested! I would check the thermostats first to rule that out. When asking for parts you will need to have chassis No and engine type as the N12 went though a progression of developments.

multiple thermostats, sounds lovely. There's a fair bit to the cooling system so not surprised. Only work that's been done on it recently is I've had the rad out to solder a seem on the side, and again recently. I'm thinking leaked to high pressure again though. I'm thinking I should look for a manual to explain the cooling set up, what are the chances of finding one?

I'll get the driver to try it with the cap off see what happens, he did a few runs yesterday with no dramas though. It's a long slow climb up a real (off road) rough road, takes about 20-25 mins in 2H/2L, then down it's a lot of exhaust brakes.

I sort of wrote off exhaust getting into the coolant as I was thinking head gasket and it's otherwise running perfectly with heaps of grunt. But I'll have a look at the exhaust circuit see it it could be crossing over anyway. I'll send a coolant sample off too, any options other than westrac?

Been awhile since i worked on one of these but would suggest if it is not overheating on that long pull your thermostates appear to be working a should. From memory the thermostats are also 2 different temp settings.
The other thing i could suggest is the injector tubes. Once again trying to stir up the grey matter, the N12 and G12's used brass injector tubes in the heads and on a long down hill run with exhaust brake on for any length of time perhaps the back pressure could get between the head and tube and pressurise the cooling system.
The other thing i have come across is a crack in the cyl liner so small i couldnt see with naked eye and it pressurised cooling system only when the exhaust brake was used for a length of time.

The big v have a seperate head for each cylinder and once built properly are not known to give any trouble. Exhaust back pressure has always been an issue on V's - not enough of it to be effective as engine brake.
have manuals for '88 but 12 litre manuals are rare, and especially for N series.
OK the clue is you had the rad out - and I am guessing 1) that you have an air lock. The heater water valve must be open (by having the cab heater control at warm) when you refill the system. 2) the rad hoses have been switched arse about.
The thermostats are in a row above ther water pump, accessed by removing the top casting. At least one closes off a bypass as it opens. What motor do you have? (first numbers/letters of engine no) & what year
3) the oil cooler (should be called a heat exchanger as it also warms the oil in cold weather) is next to the oil filters on the R side. These are known to get blocked with crud in the water which then stops water flow on that circuit, including the air compressor. On downhill runs I would expect the compressor to be making plenty of heat. I would recommend removing the water hose connections to the oil cooler and flushing it out. Shold be able to take most of flow from a garden hose. If it is too hard to get at those conections try to access other points of that circuit.

Is the Air Compressor worth considering, working hard with constant brake application downhill, getting warm & opening up into the water jacket!!
Gilly

Posted too quick, maybe a pressure system cooling test pump with gauge, connect to the header tank & position so it could be monitored by the driver.

Unusual for a head gasket to "blow" to cooling system, it would be just dammed unlucky,
Later 12 litres 120G (I think)-onwards have a huge lip on liner sitting into head and form a carbon seal between head and liner thereby eliminating compression leaks.

Injector tubes do leak but in 2 ways, usually water out around injector on a cold morning, though can & will also fill a pot overnight.
Air compressors are No1 culprit for head gasket leak into coolant, and water into air tanks too!!
Will also spread radiator seems and blow hoses just like all the things you have said previously.

Most critical thing with Volvos Trucks and Buses whether vertical or horizontal engines is make sure the bleed off lines from the block and radiator going back to the coolant reservoir are getting warm when engine heats up, this indicates the lines are clear and getting rid of all the air locks in the block and radiator. Without that the radiator will get a big back pressure and the coolant will not flow through the radiator and everything gets hot then when you shut down, it will spew all the water out of the reservoir cap.
The lines sometimes get blocked solid with crap and the only sure fix is to replace it. The 12 litre engine should have a line from the top of the thermostat hsg and usually you can put a small drill 1/8'' thru it to open it up again. Have changed a quite a few picked up pistons in the workshop and abroad because of this, mainly on 10litres though pretty much the same between 10 & 12.
But then I could be just talkin out me arse.