Hi, Fabia Mk2 scout 1598 TDI

 

Cooling system leaks, not visible anywhere. Have replaced the expansion tank cap. Occurs mostly after a 40 minute motorway / single carriageway drive. Speeds 60-70 on motorway, 40 -50 elsewhere.

4 times have noticed coolant very low in expansion tank after car has been standing.  any ideas please?

Should have said.    The cap is loose after the leaks despite clicking when tightened. Would I need  new expansion tank also?

That is the correct function of the cap to prevent overtightening and to give a positive feedback that it is correctly tightened.

 

Like so many new posters you have asked a technical question regarding your engine without specifying what engine and fuel it is.

 

If it is an EU5 diesel then I would suspect the EGR cooler.

Edited 6 January, 20251 yr by J.R.

@J.R.   1.6 TDI.

Ah yes but for the cap to untighten itself? The Fabia is a  Scout TDI 1598 cc  CR 105 Engine CAY D43366

You may have head lift, does the tank release any pressure when cooled down?

I knew when I saw the notification that once again I had missed the engine type in the thread title , that cannot be seen when you are reading the posting, I look to the LH side where the engine type is often shown and its blank.

6 minutes ago, Burt9012222 said:

Ah yes but for the cap to untighten itself? The Fabia is a  Scout TDI 1598 cc  CR 105 Engine CAY D43366

 

Loose (normal) or untightened ie unscrewed enough to release the pressure?

 

1 hour ago, J.R. said:

I knew when I saw the notification that once again I had missed the engine type in the thread title , that cannot be seen when you are reading the posting, I look to the LH side where the engine type is often shown and its blank.

The engine type is in the text, not the thread title, but I missed it also.

I shall be going on a drive in the next couple of days at least 45 minutes each way up to about 70mph, which should give me the answers to the questions you have posed . Will check halfway if there is any pressure in the tank. Will leave car for a while on return to see what happens when cooling down, ie if any coolant is vented.  Cap  was certainly just loose last time I looked. Will recheck. Didn't realise it would be that loose after such a click !

Just done a short run which displayed nothing except the stat is letting water into the hose and then into the tank, and the blue light is going out (no temperature gauge) Thanks for all your replies so far.

1 hour ago, sepulchrave said:

You may have head lift, does the tank release any pressure when cooled down?

Also, a quick way to check is look at the oil filler cap inside.  If water is getting into the oil system you will see a buildup of emulsion around the inside of the filler cap - looks a bit like soap.

 

Hi MikeTheThinker,    Thankfully no emulsion grotty type stuff on the oil filler cap, Paul the desperately trying to suss it out boyo

8 hours ago, MikeTheThinker said:

Also, a quick way to check is look at the oil filler cap inside.  If water is getting into the oil system you will see a buildup of emulsion around the inside of the filler cap - looks a bit like soap.

 

Head lift is NOT the same as a failed head gasket, the coolant is pressurised by combustion gases ONLY under full boost conditions, even when the coolant is cool this pressure remains in the cooling system, during the head lift the gas pressure pushes coolant out of the pressure release valve which is in the expansion tank cap, hence the cap becomes loosened by the coolant blowing past it.

The valve releasing to relieve pressure will not unscrew the pressure cap. The coolant does not blow past it but through it past a spring loaded seal through an internal gallery.

 

I suspect the OP has misinterpreted the free movement of the cap when fully tightened or has mis-described it.

 

I don't know whether its possible to cross thread the cap but if it is it would give a situation where it clicks but isn't closed and sealed so would leak coolant and not have any residual pressure

An update after a run.    Did a 30 mile run, average speed 45 ish. Temps outside about freezing or below. Coolant topped to max  beforehand. expansion tank level to halfway between max and min.

Coolant bubbling in from small top hose.    No pressure when I unscrewed cap.

Return journey. Colder still outside. Same result except coolant had not gone down. Still no pressure escape.   Took cover off engine. Very small escape of steam in middle of photo attached. 

Rad top hose warm, bottom hose cold. 

 

Any ideas what equipment is in the middle of the photo please?

No coolant pressure when the engine is still hot after a run?

The coolant is bubbling into the expansion tank from the small hose connected to the main top rad hose. the tank , top hose and engine are warm.  there is no pressure in the tank when the cap is turned,which I did three times yesterday

 

 

 

 

3 hours ago, Burt9012222 said:

The coolant is bubbling into the expansion tank from the small hose connected to the main top rad hose. the tank , top hose and engine are warm.  there is no pressure in the tank when the cap is turned,which I did three times yesterday

 

You'll need to do more than just 'turn' the cap, you'll need keep loosening it until there's a pop of pressure or the cap comes completely off. If you get a pop, immediately screw it back on to avoid the coolant boiling.

Thanks for your reply, sepulchrave.    There is no pressure at all regardless of position of cap.     No pops either.   When the engine is running the coolant flows into the tank..It stops when engine stops.  

I have an expansion tank coming tomorrow hopefully.  

Have you any idea what that item is in the middle of the  photo?   it has a convoluted hose attached to it. 

PCV breather?

 

It's coated with schmoo which is a good indication.

Thanks FREEDOMlite. that hits the spot. Ebay haven't got them at the moment so its main agents on Monday.   Will see what this new expansion tank does in the meanwhile.