Calling Volvo experts - N12 cooling

OK minor update - plumbed a pressure gauge into the waterway at the back of the block and it behaves like a tacho, topping out at ~60psi at 2200rpm and back to 15 at idle, following revs perfectly. Had a gauge rigged up to the top of the rad and it never exceeded 7psi. So Obviously the cap is venting the pressure until the Tstat is shut (at the bottom of a hill) and it blows a hose. Exhaust brakes have no affect. Although shouldn't it have a bypass that would vent some of this pressure?

Either way obviously there is a combustion leak into the cooling system. To double check I used a Tee-Key kit. It blew the tester off the tank when the TS opened, but the liquid still changed colour.

Talking to a freelance volvo mechanic, he said warm it up and while running pour a bucket of water all around the heads and I should see some bubbles around the culprit head. That or he's come across a couple of heads that turned porous over time (unusual). So I think it's a head job at least!

60 pounds? Geez, that seems like a lot. That would raise the boiling point of the coolant to over 140 degrees.

What you could try doing is a leak down test on each cylinder. Simple to do, either buy the tool, or just make up a fitting to go in the injector hole, fit a 150psi gauge to it with a pressure regulator and hook it up to an air compressor. Open it to full pressure and listen for bubbles. Then adjust it back to the point where the air going in equals the air leaking out. The cylinder with the lowest pressure, or the ability to hold it, is the culprit. The cylinder being tested needs to be at top dead centre on the compression stroke. If you do it so you test each cylinder with the piston almost at bottom dead centre of the power stroke, you can check for cracked liners (if they're off the wet variety).

A word of warning though, make sure the engine can't start. 100 pounds of air in a cylinder will be enough to swing it into starting.

WOW, never seen anyone put a gauge on a motor before!
Seems really strange the reservoir is not popping the cap sky high and making it a missile then. No wonder its blowing everything else!
There should be two caps on the header tank the green one is the pressure relief the black is just a topup one,

Best bet will be the cylinder leakage test as Dieseldog says, will help see where the combustion is getting in.

Pouring the water over the heads will only show if the head gasket leaks externally not into the coolant. Had one where it was rebuilt and it was overheating ver quickly and bubbling all around the heads, they said all the head gaskets are leaking, ripem out. Yea yea, Bullchips. I investigated further, didn't jump to conclusions. Ended up the new water pump they had supplied, impellor was spinning on the shaft, which they were prone to do too.

Don't know if the trucks have them but buses have a (check, or one way) valve in between the water pump and thermostat housing to assist with startup heating by circulating water without going to radiator. They play up very occasionally and for the life of me can't remember what the sysmptoms are.

Not Volvo, but a cooling system story.
Peugeot 504 Diesel.

Could NOT keep it cool.
Tried everything practical and I became a lay expert on everything to do with Pug cooling.

It drove everyone, including my Guaranteed Genuine Peugeot Expert Mechanic, INSANE.

Two water pumps, an auxiliary electric fan and everything else simple latter. . .

A dud welsh plug in the head behind the water pump.
(just loose enough to cause problem when. . .)

To dear to fix. Dumped the car and it has put me off everything European made ever since.

This story is just to support the frustration you guys are going through with the Volvo.

Maddening in the extreme, you have my total sympathy.

-b


Kuz ya see, it didn't actually leak. Cooling system on both sides of the welsh plug and all pressure tests came up good.
It just broke the cooling systems "flow circuit", to use an electrical analogy.

The pump drew hot coolant from the head, (past the leaking welsh plug) and returned it to the engine cooling circuit
thereby reducing/ polluting the pumps take from the cooled by the radiator source.
And, the hotter it got. . .
Which explains why the radiator never got too hot to touch, but it was properly hot - another red herring.
Whoda thunkit?
SCREEM

How did you eventually find the problem?

Who me? The Peugeot?

Well, not to deflect the Topic but, just briefly;
(and perhaps to provide a bit of comic relief)

This is the Hole into which the welsh plug fits, then the water pump and impeller fit in on top of that.


How did we find the dodgy welsh plug?
See the swirly shiny score marks on the face of the plug?
It had been bouncing and skipping around on the water pump impellor, when we finally took the first water pump off.
Its a steel plug into a alloy head, and so to fix it with an alloy plug and weld it all back in (or some sort of unexplored re-machining process)
would have required removal of the head and some delicate and highly skilled on the bench alloy welding $$$.

Shite or bust we marine grade SS screwed it back in place.


And bogged it. . .

It didn't work.


I loved that car

and like all good love stories, it ended in :'( 's

-b

Thanks for the explanation.

60 sounds high, if the decaying grey matter is working I seem to remember Cats were specced around 30 psi.
One of the reasons you don't chuck out thermostats, they are designed to hold pressure in the block and head. This is to help control cavitation corrosion on liners.
Would have to go digging to find the reference as I no longer have access to Cat manuals
Cheers Steve

Thanks guys. Leak down test seems like a good idea. Might have to make an adapter to fit the air line. Was only going to try the bucket of water test as the volvo mechanic said it's a common place for the heads to go - they leak round the top of the liner into the cooling system and out into the Wide World. So to do the leak down I still have to have an injector puller... might do it second.

Interesting story bobs, will remember it. Have a soft spot for those 504s (no other pugs but).

yeah apart from 60 being high it just follows revs so closely to be very very wrong. Rad cap is .5bar (~7psi?). I was considering ditching the thermostat for a while but maybe not seeing what you wrote

I'm not sure about higher pressures being used to control cavitation. Generally a higher system pressure means a higher boiling point, so an engine can run hotter and therefore more efficient and with cleaner exhaust.

Cavitation is actually caused by vibration, and over many thousands of hours, so pulling a 'stat for a few days testing wont hurt anything.