Coolant loss through filler cap

[MuttonChops]

Hello folks

I have a Citroen Xsara Picasso with the 2.0 HDi engine. The car is losing coolant through the expansion tank cap. The system pressurises when it's hot, then it fizzes out from under the cap.

My garage has tried a number of tests but we've been unable to identify the cause.

1) Pressure test on the coolant system overnight. He used a device you put on the expansion tank, then pump up the pressure and see if it goes down. There was no pressure loss.

2) Test for exhaust gasses in the coolant. He has a tool to test the coolant for the presence of exhaust gas. This has been tested a number of times and it's never failed.

3) Replaced the expansion tank cap.

Because the above tests showed nothing, we've changed the water pump. This hasn't fixed things.

We're at a loss as to what the cause is. The plan for the time being is to run the car for a week, then go back and test fopr exhaust gasses in the coolant again.

Has anyone else any ideas?

Thanks

Al

[Stickyfinger]

Two things come to mind..

An air-lock or a water jacket blockage.....have you tried one of the more aggressive cleaners (follow instructions and times) and some back flushing.
Localised boiling can pressure the system once its hot...I had No 6 on my Healey do it for a while and we found a blocked channel (casting slag) during one of its rebuilds (I run it on Castor) which we had not seen before.

One of the best mechanics |I know found the problem as he could hear the bubbles "cracking" via a stethoscope !

[wurlycorner]

How hot does the guage say it is getting when this happens? Is the thermostat and cooling fan working correctly?

[darbuck]

Double check the stat it could be slow to open or not opening at all

[Gibbo2286]

'tis almost certain to be the head gasket I'm afraid.

[Peter.N.]

I would second that, unusual for Hdi's they are much better than the XUD but occasionally the do go, I know of two. Mine have done 234k and 200k and are still good as are the DMF's and clutches but there is always an exception.

Peter

[Zelandeth]

Definitely sounds head gasket like. Causing the coolant to be pressurised and forcing it out.

With the engine cold and running can you see (or hear) bubbling in the expansion bottle with the cap off, especially if the engine is revved? You shouldn't be able to detect any real pressure in the system until the temperature starts to rise...if it's pressuring the system within a few minutes of idling from cold there is definitely some form of gas getting into the system from elsewhere, and that basically means head or head gasket.

Other thing to check is to give the coolant a sniff. If it smells of fuel that is another dead giveaway. The tests done to detect combustion traces are not 100% reliable - my Skoda still passed them despite the header tank bubbling away merrily and steaming like a steam train for the first few minutes. Seems to be very hit or miss.

The only other things I can think of would be a dodgy thermostat causing lack of circulation (it would overheat before boiling though), or another physical blockage in the engine. Would have to be something pretty extreme I would think though, especially if it just started doing it rather than being a fault that has always there that would suggest a manufacturing defect.

[MuttonChops]

Thanks folks

I'm going to try Stickyfinger's suggestions this weekend.

[darbuck]

just a thought but it could be a failing water pump not circulating properly causing the pressure usually one or two vanes broken can cause it .

[myglaren]

just a thought but it could be a failing water pump not circulating properly causing the pressure usually one or two vanes broken can cause it .

Water pump was changed.

[Stickyfinger]

When Back Flushing after the time for the cleaner has worked (Remember to have the heating on FULL with low fan to get the matrix flooded with cleaner.).
Do the radiator first, bottom to top then the other way.
Once the Radiator is done, reconnect the hoses to the block (remove the T-stat) and then back flush the block with the rad still off.


TAKE CARE with HOT WATER...IT IS UNDER PRESSURE and will come out fast. Depressurise by taking the cap off SLOWLY covered with a towel before disconnection the hoses.

NOTE: It has been known that cleaning can expose leaks in the system, Double check after you have refilled with water....if all is OK when fully hot, remove the water again via the lower hose, only then refill with Anti-Freeze and Distilled water (its cheap in truck stops)

[wurlycorner]

So how hot does the gauge say it's getting???

[darbuck]

just a thought but it could be a failing water pump not circulating properly causing the pressure usually one or two vanes broken can cause it .

Water pump was changed.

missed the point about the water pump apoligies but failing that either sticky stat or head gasket territory.

[Stickyfinger]

I would of thought the garage would of checked the stat during the "tests" before they changed the water pump.....if they did not, return to zero/do not pass GO, find another garage.

I would bet on the HG myself as well, but a cleaner/flush is cheap even if a long shot and as you should remove the stat for the flush....worth the punt

[RichardW]

My money is on the HG. Very simple check.... leave it to stand overnight, remove the cap - does it hiss? Refit the cap, start the engine run for a minute or so. Shut down, remove the cap, does it hiss? If answer to either is yes, HG is toast. If you can look into the header tank (can't remember on a Picasso HDi) then you might be able to see a stream of bubbles rising into the tank.

My BX TD was like that for 70k before I finally twigged the HG was away and changed it....