Bleeding Coolant - 2001 C5

[carpy]

In more ways than one. Been having a problem with coolant exploding out the expansion tank cap. Today I made an additional header tank out of an old pop bottle, fitted it and filled it up with 1 1/2 litres of water. I then opened up the 3 bleed screws until coolant flowed out. I then started the engine and waited. The coolant level would very slowly increase then suddenly go back down again as if flushing a toilet. I left the engine running and the coolant got super hot (pipes from thermostat and top of rad too hot to touch for more than a couple of seconds). The temp gauge in the car got to the middle but never higher and the rad fan never kicked in. If I squeezed either one of the pipes from the thermostat the water in the additional header tank would initially bubble like heck (and I'd burn my fingers).
Anyone any thoughts as I'm flummoxed.
Cheers
Carpy

[mlkey]

Stating the obvious I know, but it does sound like a head gasket is starting to go. I have one go on mine old HDI at 118k miles. It was not bad, still driveable, but would pressurise the coolant to the point of blowing it out of the expansion cap. Suggest you do the checks for a head gasket.

[carpy]

I half expected it might be something like that. Being a complete noob how do you check for head gasket failure, it was easy on my old rover, the water turned to mayonnaise
Thanks
Carpy

[wotwot]

I think the mayonnaise colour looks so much nicer than black oil lol.
It could be gone between piston and waterway causing pressure build up.
Might try starting it from cold with the water cap off blip the throttle a few times from the engine bay "if you can" and see if the water rises and falls or just comes gushing out the top.

[mlkey]

Several ways. Run the engine and look for bubbles coming up in the expansion bottle, this method did not work for me.. The other option is to get it tested for gases in the coolant. They are prone to going on the rear left corner as you look at them from the front. I would recommend changing the water pump at the same time, and cam belt if it is anywhere near due.

[KevMayer]

Going back a few steps. When was the coolant exploding from the header tank cap?

You may just have a poor seal on your cap. I had this on my Audi and a new cap with new rubber O ring fixed it.

[myglaren]

Only bled mine once when it blew off one of the bleed caps.

I found it extraorinarily easy.

Overdue for a coolant change so will probably have to eat my words soon

[carpy]

Thanks for the thoughts. I did replace the header tank cap. Tonight I attached my additional header tank (pop bottle), half filled it and bled the system via the bleed screws again. I then started the car and let it tick over and not a lot happened but as soon as I blipped the throttle there were a lot of bubbles in the pop bottle. I let it tick over again and not a lot happened and then I blipped the throttle again and guess what, a lot of bubbles in the pop bottle. This happened time and time again. My heart is sinking as I ask the inevitable 'is this the head gasket going', I suspect yes.

[KevMayer]

I'm not sure but it sounds like you may not be bleeding the system properly.

You can't bleed it fully until the thermostat has opened which means running the engine until it's warm.

The last time I blead my coolant and refilled I attached a 'pop' botttle as you. This raises the head of new coolant above the highest bleed screw.

So, with pop bottle in place, run the engine until the top hose begins to get warm. This means the thermostat has opened.

Then, starting at the lowest bleed screw (by height), open it until water comes out. Do all bleed screws in turn.

Keep the coolant level high in the pop bottle all the time to provide the head of fluid. When all bleed screws are shut you can stop the engine and remove the pop bottle. This obviously means you need to catch the excess fluid to stop it going everywhere.

Bubbles in the header tank when you blip the throttle will be because the water pump speed increases and will push through some of the air still in the system.
There are some signs for a blown head gasket like, continuous bubbles in the header tank all the time the engine is running, creamy emulsified oil around the oil filler cap when it's removed, signs of water on the dip stick. You could get a garage to sniff the coolant for signs of combustion products but, this isn't always reliable.