204ps BiTDi engine coolant under pressure when cold - help required

[c_paul]

Hi All,

I have a weird one that many of you many be able to help with if you have a moment.

Is it normal for the coolant system on a 204ps BiTDi to be under pressure when cold?

If anyone has one, that is cold, and could spare me 2 minutes of your time, could you please take the coolant cap off and see if there is any pressure there?

I have a 2016 and think the head gasket or cylinder head has a small pathway to the coolant circuit but the dealer thinks otherwise. Main symptom is coolant under pressure when cold. Was losing coolant and has had the egr replaced but it is still losing coolant and is still under pressure. Garage claims residual pressure is nothing to worry about.

This is getting very expensive.

Thanks,

Chris

 

[Bundy88]

Hi Chris,
My 2022 204ps is cold and has no noticeable pressure...11k miles on the clock.
Cheers
Al

 

[DXX]

If there is no detection of combustion gases in the coolant system it could be a very small crack in the charge air cooler that seals when the induction system returns to atmospheric pressure when the engine is stopped.

 

[Dellmassive]

The coolant system needs specific procedures to bleed out properly..

Running pumps in specific secuence, from diagnostics kit.

So you may have an airlocked system?

 

[RobertO2023]

“Dealer says residual pressure is nothing to worry about” I heard some cr..p in my life from dealers but that’s rubbish. Get a knowledgeable technician to undertake a sequence of tests to determine why residual pressure exists. You mention still losing coolant which could point towards a head gasket but that would show contamination in the coolant. I would start by releasing the pressure from cold and doing a cold engine reverse pressure air test over 12 hours.

 

[c_paul]

Update Jan 25: in case anyone sees this… so the egr was replaced. I don’t think the coolant is going down any significant amount - I haven’t topped it up for over 5k miles.

I think there is air in the system somewhere and air is getting in. After a long journey I let it go cold and then release the pressure in the system and the level goes up in the coolant tank with bubbles gurgling back up to the tank.

Not happy but not causing me too much trouble.

Other than the inconvenience, only issues are the rear heater doesn’t heat and I am worried the over pressure will cause premature failure elsewhere.

My current theory is still hairline crack in head or head gasket.

 

[RobertO2023]

It does seem to point to head gasket problem or possibly a cracked head. There should be no pressure in the cooling system when the engine is completely cold. If the rear heater has stopped working after a drain down that would suggest it has not been properly bled through and air remains in that part of the system. I hope you get it resolved.

 

[c_paul]

Update Feb 26:

I still haven't resolved this but I got bored and did some experimenting. I stripped out the D pillar trim to get to the rear heater and the pipes to the matrix arm't warming up.

I found a technical paper from VW that states when bleeding the rear heater matrix you have to clamp the pipes to the main heater matrix. It is only one line in a doc I stumbled on., and ... good luck with that. There is only about 1cm of those pipes before they start T'ing off to god alone knows where. But I ran the main bleed problem and the low temperature system bleed program repeatedly whilst systematically trying to clamp different pipes and magic! I have some flow to the rear heater = heat!

Sadly, I drove 150 miles and the output is now negligible. I tried again and the pipes are stone cold. I ran the bleed program again observing the header tank and when it is sitting at 3k revs, there is a really noticeable flow in the header tank with a significant qty of micro bubbles. By the time it has spent a few minutes at 3k revs, the coolant looks like asses milk. Although this clears within a few seconds of idle.

My current thinking is that at high revs, there is exhaust gas entering the coolant system as tiny bubbles that are collecting in the rear heater, forming an air-lock.

I haven't had to top the coolant up since the EGR was replaced, but the coolant system is under pressure when cold after a long journey.

An oil analysis showed coolant in the oil, although the coolant is clear and the oil doesn't look contaminated. The dipstick is clean and the oil filler only has the normal t6 tide mark.

Additional problems over the last year:
- it went into limp mode with a turbo actuator problem. This was cleared and hasn't come back.
- the oil consumption jumped up but settled back down a bit - local garages said they would have to take the engine out to investigate
- there is now an 'electrical fault' with the oil pressure regulating valve logged in VCDS, although this hasn't resulted in any warning lights or limp mode

fun fun fun.

I am, like lots of other people regretting the purchase of a t6 with a CXEB engine.