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[Doctor Gollum]

I hope that I'm wrong, but could it possibly be head gasket? That would over-pressurise the coling system and cause the coolant to appear low, only to return to normal level when the pressure is relaeased. 

Ron.

More likely to be a failed cap in that scenario 

[LC0112G]

"Although hot and pressurised slowly unscrewed cap not fully just enough to let it breathe and all of a sudden tank filled up with coolant?.. "

That doesn't sound like a leak to me, if tank really filled up to it's original level?

Could it just be that due to overheating, coolant liquid boils causing tank to appear empty, and when opening the cap pressure releases and allows coolant to fill tank up again?

Could it be ceased up thermostat or old coolant liquid causing the overheating, or even defected water pump?

The "all of a sudden" bit is because under pressure, an antifreeze/water mix will boil at 120+ degrees C. If/when you release the pressure, the boiling point drops to around 100-110 Deg C (depends on you water/antifreeze mix).

So what is happening is that when you release the pressure, it's allowing the water to boil in the hottest part of the engine (block or head), expanding, and forcing (non boiled) water down the pipes into the expansion tank. This leaves steam in the hot part of the engine, which will eventually condense once things cool down, but in the mean time isn't doing much cooling. Generally you should avoid taking the expansion cap off on a non running hot engine for the above reason.

[LC0112G]

Today omega overheated and saw 100 on degrees on needle... never seen it that high normally always half way or just below not matter on condition...  heater got turned on full and still didn't drop so pulled over turned car off.. checked header tank and was empty which is strange as was full yesterday ...

Although hot and pressurised slowly unscrewed cap not fully just enough to let it breathe and all of a sudden tank filled up with coolant?..

Anyone any ideas once coolant in tank I left cap loose and drove home heater on full whack window open and managed to stay about 95 on gauge got home no coolant again ...

Any help be appreciated car running fine in it self

Check the fans are working. Had similar symptoms last summer, and turned out to be a duff thermostat switch. Even on a cool day and at speed the fans need to run to draw enough air through the radiator.

And I'm not surprised the expansion tank was empty if you'd driven with the cap loose/off - Water will boil in the block if you do this, and the resultant steam will force water out. The system is designed to be run under pressure to stop the water boiling.

[Diamond Black Geezer]

This 


Been there, done that. Result is left hand side of engine bay tastefully painted in rusty water 


How's things OP? Let us know your findings 

[laney101]

I just filled header tank up again and ran car.. Noticed a slow drip under car rufly under where hbv is.. So could be that or associated hoses... Guessing once stinking hot will get worse due to expansion of hoses etc...

Ordered lower temp thermostat as run to hot normal anyways...

Got a new hbv on order and temp switch going to do all three as cheap enough hbv 20 quid.. Thermostat 25quid.. And switch was a tenner.... Shall fit all check hoses if any split cut split out and replace will aluminium joint as no parts numbers or time to go miles to a particular dealership that takes tradeclub discount..

[Diamond Black Geezer]

Sounds like you're in similar territory to me a few months back...

OK, to expand a little on just part of the story... discovered the leak dripping form the rear of engine, which of 'of course' was the HBV, which was replaced, only to discover the leak reappeared. A tiny split (we're talking two or three mm) was found in one pipe, which of course only appeared once the coolant got under pressure. Replaced, all seemed cured. Fluid level dropped a few days later, bah! Re-investigated and found the other pipe was split, also. 

[laney101]

any have part numbers for pipes incase ?

[Diamond Black Geezer]

I don't have the part numbers sadly, no. I got mine from a breaker up the road. Most likely you won't be able to get them from off the Dealer shelf anyway these days.


The pipes I had to remove are the two ones in the centre of the image below (not the one top left) though that's a potential culprit also.



Image above robbed off something called the Omega Owners Forum 


PS the 'coolant bridge' which literally spans the two cylinder banks into which the large hose goes into - on my engine was corroded, and had been letting water by at some point also. (furry 'crystalised' coolant buildup) so I changed this whilst down there.

[laney101]

all sorted and runs much better now..

new housing and pipe.. new lower temp stat new temp sensor new clip on all hoses and all hoses checked while there i replaced all gasket on plenums for new ones and cleaned throttle body out.. runs well coolant level stayed up and get to 90 on gauge and come down to low 80s quick runs well very happy

[Diamond Black Geezer]

Happy days! Good stuff, very glad you're sorted. 

My only observation is low 80s seems rather cool for a V6 I thought they generally sat at 92 bob on. I know the gauge is very very approximate, but something to ponder on, perhaps?

[laney101]

car all settled now no coolant loss.. get to 90 on gauge and then drop a little and doesnt go past 90 so im happy running a lot better..

courtney sport air filter also fitted along with their thermostat..


and looking to have remapped in next few weeks

[Diamond Black Geezer]

Ahh, a cool running thermostat, I assume, then? Do let us know how you get on with the remap. 

[laney101]

Shall just unsure where to go for definite haha

[Andy A]

car all settled now no coolant loss.. get to 90 on gauge and then drop a little and doesnt go past 90 so im happy running a lot better..

courtney sport air filter also fitted along with their thermostat..


and looking to have remapped in next few weeks

Glad got her sorted, Which air filter did you go for?

[laney101]

Courtney sport own airfilter