Sudden Coolant Loss

[Ceecee85]

I've had a look round the Wiki and the archive but can't find anything specific on the issue I have.
Basically, when the engine gets up to temp and the needle on the temp gauge point straight up everything's fine for about 10 miles (depending, it seems, on the speed I'm going) then the temp gauge moves all the way over to the right and the red light flashes. I've read all about the coolant levels in the archive and yes, this happens because the coolant level is low and the sender in the expansion tank is out of the water. I've checked water level is fine before starting off- expansion tank full to brimming and the top up tank filled to cold level line- all as it should be. A new expansion tank and cap have been fitted a couple of months ago to try to alleviate the problem- old tank showing signs of being pressurised and had hairline cracks- a common problem apparently.
So, she's losing about a litre of water once it's up to temp and pressure. Where's it going and why?
Yesterday, I ran the engine on the drive until it was warmed up and increased the revs to try to recreate the problem. I watched the pipe between the Dalek cap and the top up tank twitch as the pressure valve opened in the cap- I though it might be coming from a split in the pipe, but no. Then the wife came out to tell me to stop revving the engine as it was getting late and she didn't want to upset the neighbours. I dutifully turned the engine off and scratched my head. Then got back in the cab, turned the engine on and pulled her forward about 6 feet to park her up at the side of the house. I got out and went round the back to put the lid back on the engine and she'd p!ssed a big puddle of water all over the drive. I had a good look from up top and below and could not see anything wet on the engine or coolant pipes. I have no idea where it all leaked from in such a short time with no trace. And I had aspirations of being a forensic scientist! I guess I need another set of eyes watching it next time.
I've tried to be as brief as possible so some of the detail is abridged so that you good people don't get too bored and press the back button before getting to the end so if you're reading this well done. Any help/ tips/ advice warmly received. Thanks in advance.

[Highroller]

Could it possibly be a failed o ring in the temp sender? This happened to me, it only leaked when running.

[SamsBus2012]

Sounds to me like the problem is occurring once the thermostat has openend so maybe leak is forward of the engine? - Suggest you repeat the experiment but his time once thermostat has opened, look under vehicle whilst you have an assistant (perhaps your wife?) continue revving the engine.

[Ceecee85]

Thanks guys.

One of the points I left out before so as not to bore with too many details- I drove it about 25 odd miles and the problem happened (mix of motorway (18 miles), A roads and B roads (the remainder)). Topped it up when cold (about 6 hours later) and drove it home (via the same route but in the other direction obviously)- without incident. Took it out the next day to the local shops and the problem reoccurred.

So where's the thermostat then?

[SamsBus2012]

Sorry, just noticed you have a diesel - so not 100% sure - think the housing is on the main hose just behind the ditributor pump - others will no for sure!

[Ceecee85]

No bother. Thanks. I'll have to have a furkle and see what I can find.

[California Dreamin]

Given that a cooling system in good order 'SHOULD' pressurize, can you tell me whether the hoses were 'hard' at the point you felt the overflow from the main tank/Darlek cap open?
Also...what is happening with the radiator fan ie: does it come on when the gauge goes east?
Is the heater blowing hot at this point?

There are some very POOR quality Darlek caps around, making sure your 'newish' cap is functioning correctly is a first priority.

Places to look for that leak:

Small breather hole underneath the water pump.
Sensors ('O' ring seals)
Plastic coolant housings.
Head gasket.
General hoses and joints.

Martin

[Ceecee85]

Thanks Martin.
I'm going to check the whole thing out tomorrow at the garage so will let you (and everyone else) know.
Didn't feel the hoses at the time the pressure valve in the cap released but will do.
The fan comes on and spins fine as it should and hot air comes out of the dash.
It's a JK expansion tank and Dalek cap. I'd love to think it's ok but I'm not sure how to check it.
Cheers, Steve

[Oldiebut goodie]

Those dreaded words - Just Kampers!

[Ceecee85]

Indeed.

Right, brief update. I feel like a bit of a twonk but the mystery has been figured out (I hope). Drove a couple of miles on the way to the garage this morning, stopped for breakfast, went back to the van and low and behold, big puddle of steaming water but under the front. The pipes leading to and from the rad were leaking. Obviously, on the drive the other day it had leaked from the same spot and I hadn't seen it due to the slope of the drive (I'm sticking with that excuse) and I'd driven the van over it positioning the engine slap bang over the top of the puddle.
The pipes had been rubbed through by the steering column believe it or not. Anyway, new bits going on today so we should be ready for our hols next week.

So the jury's out: twonk or not? Answers on a postcard to the usual address .