Spilling coolant from pressure tank cap

[Ross R75]

Originally Posted by Simon W

I have exactly the same problem as Copilot (post 1) and fitting a new tank cap did not solve it.

If the coolant level is on the MAX mark (measured at cold) it leaks heavily from the tank cap as soon as the engine warms up.

If the coolant level is roughly 500ml below the MIN mark (measured at cold) it doesn't leak at all from the cap.

I have no idea what the cause is.

There's really only two answers to that. Either there's an airlock in the system because it hasn't been bled properly, or the coolant circuit is being pressurised by a blown head gasket, which happens more often on this engine than most here care to believe.

[T-Cut]

Originally Posted by Simon W

I have exactly the same problem as Copilot (post 1) and fitting a new tank cap did not solve it.

If the coolant level is on the MAX mark (measured at cold) it leaks heavily from the tank cap as soon as the engine warms up.

If the coolant level is roughly 500ml below the MIN mark (measured at cold) it doesn't leak at all from the cap.

I have no idea what the cause is.

Have you verified the radiator fan operates correctly?
This is the first thing to check if you don't know. Do the Demist Test which a simple word search should find.

If the fan is operating correctly, check for residual pressure on the cold cooling system. It's a useful daiagnostic in these situations. You should go one a brisk run for 5-10 miles until fully heated up. Then shut down and leave overnight. Remove the cap and listen for any pressure release like opening a pop bottle. There shouldn't be any.#


And there's the bleeding of course.


TC

[Copilot]

Originally Posted by Arctic

Hi Pedro
Have managed to solve the problem now? or is it still leaking from the cap.

Hello Artic

I'm about to buy the Viton O-rings or a coolant cap from DMGRS.
I didn't decide yet.
Would be better to go straight for a new coolant cap?

I'm wondering if a leaking thermostat housing could have some influence in this problem as well?

[Arctic]

Hi Pedro.
Go for a new cap because it may not be the O-rings, also a leaking thermostat would also effect the pressure so look into the V of the car for pink residue signs or even coolant.

As above all ready stated is your fan working as it should on the low speed, with car running press the demist button then look through the front grill the fan should be running continually, if not then low speed as failed.

Have you used the OB temp to check when the fan is kicking in engine running press 19 then 7 I am sure T cut will point you to his excellent guide.

[Simon W]

Originally Posted by T-Cut

Have you verified the radiator fan operates correctly?
This is the first thing to check if you don't know. Do the Demist Test which a simple word search should find.

If the fan is operating correctly, check for residual pressure on the cold cooling system. It's a useful daiagnostic in these situations. You should go one a brisk run for 5-10 miles until fully heated up. Then shut down and leave overnight. Remove the cap and listen for any pressure release like opening a pop bottle. There shouldn't be any.#


And there's the bleeding of course.


TC

Thanks TC, I've now done the tests you suggested.


Demist Test

Aircon off, start engine, press Demist button, set temp control to 16 or LO.
The fan starts as soon as I press the Demist button. It continues to run until I press the button again. There's no pulsing on/off - it just keeps running until told otherwise.

(Note: the Demist button is called 'Defrost' in the handbook)

I can't see any resistor at the 2 o'clock position - just the empty plastic mounting lugs. Presumably this means I've got a 3-speed fan (although Arctic's post 12 here says resistors can also be mounted elsewhere).


Residual Pressure Test

Engine hot after running for 20 minutes or more. Stop engine, leave overnight.
There is no sound of a pressure release when the tank cap is removed next morning.

Btw, the coolant itself looks exactly as it does when new - I can see no contamination at all. This pic shows some that I sucked out of the tank using the battery tester (arrowed).




Copilot's car appears to have the same problem as mine and I suspect fitting a new tank cap will make no difference to his.

Here are my caps - old one on the left (orange insert) and new one on the right (black insert). Coolant overflows with both of them when level is on the MAX line.



I haven't tried bleeding my system yet - Copilot probably hasn't bled his either (?).

NB. My engine (not Pedro's): coolant level when hot is between the MIN and MAX marks on the side of the tank (pic below). When cold it's below the MIN mark.

[COLVERT]

Originally Posted by Ross R75

There's really only two answers to that. Either there's an airlock in the system because it hasn't been bled properly, or the coolant circuit is being pressurised by a blown head gasket, which happens more often on this engine than most here care to believe.

There are other answers though maybe not likely in this case.

At high revs the coolant moves through the system faster. If the main radiator can't process this amount of coolant then the level in the header tank will result in an overflow from the cap.
This used to occur on older cars when there was an accumulation of sludge in the coolant. The rad would partially block causing the overflow.

The coolant out of this engine though seems very clean.




Perhaps the water pump impellor is in poor condition and the circulation is low giving a build up in the heat in the coolant. Vapourisation of the coolant could make it overflow.

[T-Cut]

Originally Posted by Simon W

Thanks TC, I've now done the tests you suggested.

The results show the radiator fan is OK. Perhaps surprising for an old 3-speed.

Residual Pressure TestEngine hot after running for 20 minutes or more. Stop engine, leave overnight. There is no sound of a pressure release when the tank cap is removed next morning.

So far so good, though the test is to drive the car briskly for ten miles or so rather than a 20 minute standing tickover.

the coolant itself looks exactly as it does when new

The coolant looks normal.

Here are my caps - old one on the left (orange insert) and new one on the right (black insert). Coolant overflows with both of them when level is on the MAX line.

The black insert should show '140' to comply with MGR recommendation on running pressure. This means 140kiloPascals (or 22psi) relief pressure.

Clearly there's still an issue. It could be due to air entrapment causing overpressure and coolant release. The cold pressure test is to eliminate the only other possibilty (exhaust gas). This is why you should do a brisk ten mile drive.

I haven't tried bleeding my system yet

You should, to eliminate the most likely cause.

You should also check the true running temperture during the ten mile drive. Use the Trip Screen Diagnostic (section 7)
Diagnostic Mode: https://www.the75andztclub.co.uk/for...ad.php?t=55585

TC

[Copilot]

Originally Posted by Simon W

I haven't tried bleeding my system yet - Copilot probably hasn't bled his either (?).

Yes Simon, I did not try to bleed the system yet.

Originally Posted by COLVERT

There are other answers though maybe not likely in this case.

At high revs the coolant moves through the system faster. If the main radiator can't process this amount of coolant then the level in the header tank will result in an overflow from the cap.
This used to occur on older cars when there was an accumulation of sludge in the coolant. The rad would partially block causing the overflow.

That makes some sense once I believe the leak happened when I revved it up when overtaking another car in the motorway. Although my coolant looks clean, my radiator looks the original or at least quite old.

Originally Posted by T-Cut

The black insert should show '140' to comply with MGR recommendation on running pressure. This means 140kiloPascals (or 22psi) relief pressure.

The black insert on Simon's "new" coolant cap is also mentioning 140.

[Simon W]

Originally Posted by T-Cut

So far so good, though the test is to drive the car briskly for ten miles or so rather than a 20 minute standing tickover.

Originally Posted by T-Cut

Clearly there's still an issue. It could be due to air entrapment causing overpressure and coolant release. The cold pressure test is to eliminate the only other possibilty (exhaust gas). This is why you should do a brisk ten mile drive.

I can't take the car on the road TC - it doesn't have an MOT.

Originally Posted by T-Cut

You should (bleed the system), to eliminate the most likely cause.

You should also check the true running temperture during the ten mile drive. Use the Trip Screen Diagnostic (section 7)
Diagnostic Mode: https://www.the75andztclub.co.uk/for...ad.php?t=55585

I'll have a go at bleeding the system as soon as I've sorted out the leaking roof (best save that for another thread!)

Originally Posted by Copilot

........ once I believe the leak happened when I revved it up when overtaking another car in the motorway.

Pedro - if you put the 400ml of coolant back in so that the level is on the MAX line (measured at cold), then run the engine up to hot and wait (with car standing still), is there a point when it suddenly starts gushing out of the expansion tank? Or does it only do it when you're driving the car out on the road?

On mine, there's a definite point where something suddenly changes and the coolant gushes out of the expansion tank at an alarming rate. It'll flood the complete tank rim very fast - much faster than I can turn the engine off.

Originally Posted by T-Cut

The black insert should show '140' to comply with MGR recommendation on running pressure. This means 140kiloPascals (or 22psi) relief pressure.

Originally Posted by Copilot

The black insert on Simon's "new" coolant cap is also mentioning 140.

Here's the 140 mark on the new cap (the one on the right). Note the flattened rings on the old cap (on the left).

[Simon W]

Other points to Note (not mentioned above).

1. Both Pedro's car and mine have small amounts of coolant in the vee.

2. Lates & Pete couldn't find the source of the 'vee leak' on mine despite testing the system by pumping it up to more than twice operating pressure.

3. Mine has a Kaiser metal thermostat housing - the 'vee leak' was present before it was fitted (and after).

4. Water pump and thermostat on mine have both been changed (although some time ago now).

5. My heater matrix seems ok since the heater blows hot air at exactly the point you'd expect.

6. Both cars (Pedro's & mine): I don't think the expansion tank leak and the 'vee leak' are connected. The tank leak on mine is relatively recent and the 'vee leak' isn't.

Leaks are the sign of the devil - .