Slight overpressure in expansion tank when car is cold

[hungryhound69]

Hello,

I need a bit of help.

I have a Citroen C3 (FC, FN) 2008. I have been noticing that there is a slight overpressure in the expansion tank when I check it in the morning after the car been sitting all night without driving. That is at least 10 hours sometimes longer. I just did a block test until the temperature reached 72C and the liquid didn’t change colour. I tested the reactivity of the test liquid by corking a glass with bicarbonate of soda and vinegar with the tester and the test liquid turned green as it suppled to. So I am not sure what is causing the overpressure. It is not a lot but it is definitely there.

So a bit of background. I recently changed all the belts and water pump as the pump was leaking. So I thought that maybe there are air bubbles still left in the system. But if it’s air bubbles and the system is sealed then once the car cools there shouldn’t be overpressure but rather slightly negative pressure or same as ambient. I also have put a new expansion tank cap just to be sure.

The car drives fine and doesn’t overheat as I have been tracking its temperature while driving with a Bluetooth OBD dongle. At most it would get to 102C but that is hard uphill driving. Ordinarily the temperture sits between 80C and 95C. Therefore, am not sure if there is a problem like, for example, initial stages of head gasket failure. That is my main concern whether the head gasket is about to fail so I need to change it. But I don’t want to do all that work if it’s not the head gasket.

So I am a bit lost so would be grateful for any help with this.

Thank you in advance

[RichardW]

Welcome along!

Pressure remaining in the header tank when the engine is cold is a sure sign of headgasket troubles....

[mickthemaverick]

I'd suggest you run a head gasket test as I did last year. Not expensive and will show if you have a problem: viewtopic.php?p=810944#p810944

[Paul-R]

I think that's what "I just did a block test until the temperature reached 72C and the liquid didn’t change colour" means.

[PaulC5]

Does the engine fan come on, we had a 1.4i C3 and it would start at 96°C ? Whilst this would not help with the pressure, if it is not working then it could be a cause of the head gasket failing.

[ozvtr]

To your knowledge, has the head gasket ever been changed? If it has never been changed it will need changing...one day. But I'm NOT saying it needs to be changed today!!!
The OEM head gasket for the TU3 engines have a poor track record!
Are you loosing any coolant? If you are loosing coolant it's possible the head gasket has gone. It could also mean the cylinder head is damaged or warped. So don't count on just replacing the gasket!
If you are NOT loosing coolant I cant explain it, but it's not going to be your head gasket.
You don't seem to have any symptoms except the extra pressure in the header tank. If you do not get (or have) any other symptoms, then I wouldn't worry about it.

[hungryhound69]

Thank you for your answers. So to reply to all of you at once.

1. I ran the block test several times. In all cases the liquid colour remained blue. I even tested the reactivity of the liquid by stopering a glass with bicarbonate of soda with vinegar and the liquid did turn green

2. The fan does come on around 95C or there about

3. I am not losing coolant

So the only thing is slight pressure in the expansion tank

Yesterday I put a no-spill funnel and let the car run up to 95C letting the coolant rise into the funnel just in case there are air bubble although I don’t see how air bubbles would cause pressure once the car cools. So let’s see if that does anything.

I suspected the head gasket but block test is negative, there is no overheating and the car isn’t loosing coolant. Just some slight pressure.

One thing I was possibly wondering is whether what I think is overpressure is in fact slight negative pressure and I am confusing the sound of air going in with air going out. Since I do not have a pressure gauge with appropriate fitting to monitor the absolute pressure I cannot be certain. But I would expect negative pressure making a pop sound as the cap is removed since it seals with an o-ring.

[PaulC5]

Despite replacing it, does it have the correct expansion tank cap or is yours faulty ? They should let the pressure increase to maybe 1½ barg (only a guess) as the engine heats up the coolant before venting any excess but they also let in air if a vacuum happens.

[hungryhound69]

yes the cap as per spec. It should vent at 1.4 Bar. A google search says that it lets in air at 0.03 to 0.07 bar negative pressure. What it the negatice pressure is less that range. As I said it's very slight but without a gauge I don't know how much and if it's positive or negative. I think it's positive but there is always room for doubt.

[citroenguy]

A slight pressure relief when openeing the expansion tank is normal, even after 10h.
It is a closed system and will keep the pressure as long as it doesn't go over eg 1.4 bar.

If it is not losing coolant, overheataing or mixing oil and coolant i wouldn't be worried

[hungryhound69]

Hello citroenguy,
Thank you for your reply. You are the only one who said that as everyone else thinks that a pressure in the expanssion must mean a head gasket leak. But I see no other signs. Block test negative, no overheating, not loosing coolant, even Lexia doesn't flag up anything. But naturally I was conserned since I am willing to change the head gasket but since it's such a major job I am reluctnat to do it if it's not needed. In any case the lack of other symptoms makes me lean towards what you are saying to be correct. But I am puzzled why it would keep slight overpressure even after standing around for a long time (10 hours). Is it becouse there is still some residual heat somewhere?? There cannot be another reason in a closed system. But would that mean that after say a week the pressure should be gone?