ZT T 120 Coolant Loss & Repetitive Air Lock

[P.happy.T]

Hi I've put my ZT-T 1.8 back on the road & it's running fine for 1200 miles except for a frustrating problem with cooling system. I fill the coolant to the correct (low) level in the header tank top hose is full, Drive it say over 30 / 40 miles let the car cool down open the cap there's a woosh of excess pressure top hose full of air & maybe a pint of water sometimes needed to get back to the level with top hose full again (by squeezing the air out). Headgasket was changed approx 20 k ago intake gaskets changed 2.5k ago.
What's the likely cause, water pump or thermostat or ???.
I hope somebody on here has experienced & solved this problem.
Many thanks !

[Matt_75]

Hi Paul, when filling the cooling system, did you undo the bleed screw and allow some coolant to flow out?

[SD1too]

Originally Posted by P.happy.T

I fill the coolant to the correct (low) level in the header tank ...

Hello Paul,

I think it's worth checking that you are filling the expansion tank only up to the tab marked MAX which is visible by looking down through the open filler neck. There are some historic references on the forum to "just below the fins" which is incorrect.

Also, if you haven't done so already, I'd recommend following the MGR workshop manual procedure precisely. Here it is for your 1.8 engine:

1. Remove expansion tank cap.
2. Release heater return hose from rear of cylinder block.
3. Release bottom radiator hose and allow system to drain.
4. Reconnect both the above hoses.
5. Disconnect hose from bleed valve. Blow through valve to ensure that valve pin is not sticking.
6. Reconnect hose to bleed valve.
7. Remove bleed screw from coolant pipe.
8. Fill cooling system, maintaining level at neck of expansion tank, until a steady stream of coolant is emitted from the bleed screw hole. Refit bleed screw.
9. Continue filling until coolant reaches expansion tank neck and remains static.
10. With expansion tank cap still removed, start and run the engine at 1,500-2,500 rpm whilst maintaining coolant level just below expansion tank neck. When the level starts to rise up the neck, fit expansion tank cap.
11. Continue running the engine until the cooling fan starts. * In practice this is unlikely to happen. An acceptable substitute is to run the engine until normal coolant temperature is indicated or take the car for a short drive.
12. Switch off engine and allow to cool for 30 minutes. * In practice I recommend several hours.
13. If necessary, top-up expansion tank level to MAX marker but no higher. * In practice this procedure may need to be repeated two or three times on successive days.

I have highlighted the most important point which is that the bleed screw should be refitted before starting the engine and it should remain in place thereafter.

Air locks can result remarkably easily from incorrect filling and bleeding procedure.

Good luck Paul!

Simon

[P.happy.T]

Hi Matt & Simon,
Thanks for your speedy replies, I admit that I haven't followed the procedures you suggest. I did try to release the breather bolt but thought it pretty inaccessable for my large hands so thought my squeezing the top hose method would be sufficient.
I'll have a go at the official procedure, I assume the bleed valve with pin & hose is at the back of the head.
Fingers crossed !
Paul

[SD1too]

Originally Posted by P.happy.T

... I admit that I haven't followed the procedures you suggest.

Not to worry Paul, you're not the first!

Originally Posted by P.happy.T

I assume the bleed valve with pin & hose is at the back of the head.

According to Haynes (worth buying from eBay) it's "... in the coolant hose which can be seen between the engine and the air cleaner." RAVE shows it at the true back of the engine (i.e. the gearbox end). It's a hexagon head, not what you might be expecting!

Simon

[P.happy.T]

Hi, I’ve bled the cooling system using the bleed screw, flushing the bleed valve & running with the radiator header tank full & topped up whilst bubbles were coming through until operating temperature is reached as recommended by yourself Simon. I did this 3 times & since completing this procedure I’ve done approx 100 miles & unfortunately lost a pint a pint of coolant again !
What do you recommend now? I’m considering a sniffer test for combustion gasses in the coolant or a cold pressure test to 16 psi…
As always any comments will be appreciated.
Thanks, Paul

[SD1too]

Thanks for the update Paul but this evidence is worrying:

Originally Posted by P.happy.T

... & topped up whilst bubbles were coming through ...

I don't get bubbles when following the same procedure on my KV6 and I wouldn't expect it on a 1.8 either. The air should have been expelled from the bleed screw during the initial filling process. Once the engine is started, the topping-up is just to maintain the correct quantity of coolant in the system.

Reviewing your first post, you say that there is a "whoosh of excess pressure" when you remove the expansion tank cap on a cold engine. Are you still getting that now? To avoid misdiagnosis, make sure that the engine really is stone cold, preferably by leaving it overnight.

Simon

[drewbie]

https://the75andztclub.co.uk/forum/a...8&d=1452982525

Try this method, always works for me

[Tetleysmooth]

Originally Posted by drewbie

https://the75andztclub.co.uk/forum/a...8&d=1452982525

Try this method, always works for me

Good call that. I tried it yesterday to good effect.