Shouldn't be boiling should it?

[Dannyroger]

G'day

My 98 D1 LPG V8 runs fine (although I reckon a bit down on power since I had it LPG tuned) and reads a constant midway on the OEM temp gauge - except for a recent 90 minute highway run in 40 ambience with AC when it neared the top.

On that day, when I stopped to get fuel, boiling coolant escaped from the pressure cap on the expansion tank. I let it cool down, topped up a litre, and was on my way - no problem.

Today, lower ambient, shorter run - temp gauge normal, no overflow but I could hear the coolant boiling in the top radiator hose.

So I'm just hoping someone can confirm, before I start mucking around and breaking things I should never touch, that it's running too hot and the expansion tank cap is no good.
After that, I may also have to look at instead/as well - thermostat, air in cooling system, dirty or blocked radiator, and engine tune.

Does all this sound right?

many thanks

Tim

[TriPolar]

My guess is that you have a blockage somewhere and the water is not circulating. Thermostat or water pump.

[Dannyroger]

looking at some previous posts sounds like working out a overheating issue can be seriously tricky and time consuming. Thinking a good start might be to run it without the thermostat and see if any difference.
Seeing as the temp gauge reads normal, I guess that must be buggered too ...

[bee utey]

If it's making bubbling noises and pushing out coolant with the temp gauge within reasonable operating limits, chanced are a stuffed head gasket or worse still a stuffed engine block. These things crack easily and combustion gas gets out behind the liners through a crack in the block. Removing the thermostat may keep it alive for a bit longer but you risk freezing up the gas converter on cool mornings without a thermostat. A better mod is to fit an extra tee and brass nipple to allow gases to enter your header tank without pushing out the coolant.



Or start saving your pennies for a new motor.

[Dannyroger]

Thanks Bee-Utey
Not the diagnosis I was looking for!
So if this is the case, then most likely the expansion cap, temp gauge and thermostat are all operating as they should.
So how do I confirm that it is a cracked block, slipped liner or head gasket? (I can't see any bubbles in expansion tank when running nor any sign of water in the oil) I don't have facilities but a compression test at the local servo?
And can't quite work out your photo - the T goes into the existing 8mm return line and off the tee a new hose to the new fitting in the top of the expansion tank?
And the biggest question of all - what if I do nothing about it at all?

[bee utey]

Thanks Bee-Utey
Not the diagnosis I was looking for!
So if this is the case, then most likely the expansion cap, temp gauge and thermostat are all operating as they should.
So how do I confirm that it is a cracked block, slipped liner or head gasket? (I can't see any bubbles in expansion tank when running nor any sign of water in the oil) I don't have facilities but a compression test at the local servo?
And can't quite work out your photo - the T goes into the existing 8mm return line and off the tee a new hose to the new fitting in the top of the expansion tank?
And the biggest question of all - what if I do nothing about it at all?

You won't see bubbles in the expansion tank at idle, the problem occurs as combustion pressure builds up under load.

You won't see water in the oil until your problem becomes much bigger, small amounts of water will evaporate from hot oil.

A compression tester won't show a small head gasket leak, only a complete failure or a valve or ring problem.

Your working out of the pic is correct, the tee allows bubbles that normally collect under the radiator filler bung to migrate uphill to the expansion tank and vent without pushing out coolant.

If you do nothing other than checking and filling the coolant before each trip your engine will probably survive until one day you blow out enough coolant to cook the engine and destroy it.

[Dannyroger]

You are THE guru.
Many thanks for all your wisdom.

[gazby]

It may not be your problem, but I have seen a collapsing (inward) bottom radiator hose cause a similar problem in a V8 Ford Fairlane, not familiar with Disco V8 installation but maybe worth a look if the hose is old or oil soaked, looks ok at idle/low RPM but on a rev or prolonged high RPM closes and restricts flow.

[Boony73]

If you do a search of the site the biggest culprits for over heating in a Disco V8 are the viscous coupling, radiator, head gasket and the thermostat. Expansion caps do fail and if you have no idea on how old it is then it can be a cheap quick fix, same for the viscous coupling more expensive but they have a finite service life as well. Just recently I have similar symptoms, a bit more severe and it turned out to be a head gasket and a radiator.

[Dannyroger]

So what I really need to work out is if my Disco IS overheating or whether it's over-pressurising. I think the latter, cos I've not seen steam even when the cap spewed over.