Need advice...

Got 15 plate octavia VRS with the CUPA engine and its done 144k miles, had a coolant problem for a year now and I've replaced EGR cooler, charge air cooler, thermostat, heater matrix, header tank and cap, timing belt and water pump, the car basically loses coolant and holds its pressure even after a couple of days but there's no visual leaks no white smoke and doesnt overheat, sits at 90 comfortably, i did see signs of rad weld in the reservoir when i bought the car however I have done 2 coolant flushes with solutions and done plenty of coolant changes with all of the parts replaced. Done 2 headgasket sniff tests and one was inconclusive as it turned a paler blue and the second test never change colour at all so im stumped, any help would be great.

If its losing a small amount of coolant maybe oil cooler thats leaking slightly. Have you any emulsified oil on cap? Another thought would be small crack in cylinder head thats opening slightly when hot and closing when cold but I would expect to see some white smoke unless its very small. It may have been a head seal rather than rad weld. Its odd that its holding pressure though but if the crack is sealing itself when cooling down might explain it.

Alasdair

37 minutes ago, Alasdair1 said:

If its losing a small amount of coolant maybe oil cooler thats leaking slightly. Have you any emulsified oil on cap? Another thought would be small crack in cylinder head thats opening slightly when hot and closing when cold but I would expect to see some white smoke unless its very small. It may have been a head seal rather than rad weld. Its odd that its holding pressure though but if the crack is sealing itself when cooling down might explain it.

38 minutes ago, Alasdair1 said:

It's not losing loads of coolant and its variable how much it loses, some days ill top it up go to the shops and do some small journeys and it'll have dropped below the min and then other days I could go a whole week or 2 before it drops again but the pressure is more concerning to me as it just seem to hold pressure and for ages. I work on cars for a living and this fault is getting to me hence why im taking it to this form for a fresh set of ideas

Just now, Jb59873 said:

  42 minutes ago, Alasdair1 said:

If its losing a small amount of coolant maybe oil cooler thats leaking slightly. Have you any emulsified oil on cap? Another thought would be small crack in cylinder head thats opening slightly when hot and closing when cold but I would expect to see some white smoke unless its very small. It may have been a head seal rather than rad weld. Its odd that its holding pressure though but if the crack is sealing itself when cooling down might explain it

I am worried that the head could have a small crack in it and its only opening under heat and then closes back up when cold but that wouldn't explain why I lose coolant on short journeys when the car isn't even up to temp, the heater matrix is constantly gurgling behind the dash as if there's air getting in the system somehow, and I always bleed it with my scan tool to allow proper bleeding of the system

Does yours have an auxiliary water pump? Its a long shot but if it has it may be leaking or forcing air into system. I think it comes on when cold to boost heating etc. Have had a couple of waterpumps over the years that allowed air in but werent leaking coolant at the time.

Alasdair

10 hours ago, Alasdair1 said:

Does yours have an auxiliary water pump? Its a long shot but if it has it may be leaking or forcing air into system. I think it comes on when cold to boost heating etc. Have had a couple of waterpumps over the years that allowed air in but werent leaking coolant at the time.

Alasdair

That's not a bad shout, I could be wrong but I believe that the coolant may be getting forced out of the reservoir from excess pressure in the system. As soon as the ignition gets turned on there is a pump that's working as I can hear gurgling behind the dash before I even start the engine

Pretty sure that will be the aux pump. I was told it was to boost temp in cabin while cars warming up which I assume would feed heater matrix. It may be that thats not working properly or there is still air in system/matrix which may cause an airlock somewhere and over pressure? Worth a look.

Alasdair

31 minutes ago, Alasdair1 said:

Pretty sure that will be the aux pump. I was told it was to boost temp in cabin while cars warming up which I assume would feed heater matrix. It may be that thats not working properly or there is still air in system/matrix which may cause an airlock somewhere and over pressure? Worth a look.

Alasdair

Yeah I think I might unplug it and see what difference it makes

Cant see unplugging it causing any harm. Doesnt look to bad a job to replace either.

Alasdair

Aux water pump on this engine is below the oil filter, between sump and rad.

I put a link on a recent thread here about my cracked head, same engine, same symptoms as yours... Small crack in exhaust side to water jacket, closed when cooled down and kept pressure in header tank.

You could at a last resort try some kseal or steel head if you think it may be a cracked head. Kept my old toyota lucida going for another year and a half before it had catastrophic head failure due to cracks between the valves and didnt notice it overheating as sun was really low so couldnt see dash. It was a nightmare to change head as its mid engine under front seats. Not bad once you remove just about everything to get to it. Must have been over 150 nuts and bolts plus auxiliary drive shaft turbo pipes and more.

Alasdair