Fiesta Duratec 1.6 coolant bubbling noises when hot

[EdGasket]

I have the coolant level just above minimum in the expansion tank. When the car gets hot I hear bubbling noises in the cabin, I think its the coolant boiling in the cylinder head and flowing back to the expansion tank. When I turn off and look under the bonnet, coolant continues to flow into the expansion tank for quite some time, the expansion tank looks like it is bubbling/boiling and the level in the tank is very high though not overflowing. I can hear a slight escape of gas from the expansion tank cap. When cool, the level returns to just above minimum. New thermostat, water pump and expansion bottle and cap fitted. The thermostat was tested in a pan of water and opens correctly.  No white smoke or creamy oil, no bubbles in coolant until hot, so presume Head Gasket is OK. The electric fan does turn on /off from time to time. I am using 33% blue coolant mix (yes I know it should be red; long story but I am sure the radiator is not clogged)

For the past four years I have been running with no thermostat in the car as I had overheating issues which I subsequently traced to the electric fan and associated circuitry. With no thermostat, there were no problems with bubbling or anything like that; the only downside being longer time to warm up and poor heater in winter. As I was doing the timing belt and had the engine stripped down, I thought I'd replace the thermostat but it is causing apparent overheating again. What I notice is that because the thermostat is halfway down the side of the engine, it remains closed for a long time while the top of the engine and top hose get quite hot with coolant flowing back into the expansion tank.

So my questions are:

1) Is it usual to hear bubbling inside the car from a duratec engine?

2) Is it OK to run like this or should I remove the thermostat and just run it without one?

3) Would increasing the coolant concentration or switching back to OAT red coolant increase the boiling point and resolve this?

[EdGasket]

Has no one had bubbling?

[unofix]

On 6/1/2023 at 9:55 AM, EdGasket said:

1) Is it usual to hear bubbling inside the car from a duratec engine?

No.

On 6/1/2023 at 9:55 AM, EdGasket said:

2) Is it OK to run like this or should I remove the thermostat and just run it without one?

Replace the thermostat only if you think it is faulty.

On 6/1/2023 at 9:55 AM, EdGasket said:

3) Would increasing the coolant concentration or switching back to OAT red coolant increase the boiling point and resolve this?

You should not use different types of coolant. Drain and flush the system. Refill with Ford specification coolant. Concentration for the UK is 50/50

If all else fails try these....

[TomsFocus]

There's probably an airlock in the heater matrix causing the bubbling behind the dash.  It is quite common, but not 'normal'.  These don't always bleed perfectly just by running with the cap off. 

[EdGasket]

10 hours ago, TomsFocus said:

There's probably an airlock in the heater matrix causing the bubbling behind the dash.  It is quite common, but not 'normal'.  These don't always bleed perfectly just by running with the cap off. 

Yes could be. Today I ran it up on ramps to hopefully get any air out of the heater matrix,and ran the engine until hot; seemed a bit more stable so maybe there was some air trapped in the heater.

I did the sniff test for HG failure and it was negative. Also tweaked the fan to come on a bit earlier.

The only thing that seemed a bit abnormal was the level in the expansion tank rose from just over min to above max quite quickly but drops back when the fan kicks in.

Does not overheat so I'll carry on and see how it goes.

[EdGasket]

I'm getting a lot of fluid pumped back to the expansion tank which then looks like its boiling though car has not overheated yet. I checked the expansion tank cap and it is holding pressure and I don't lose coolant providing when cold, that the level is not much above minimum. When hot, the expansion tank is mostly full of bubbling coolant with a slow leakage of gas (steam I suppose) from the cap. It doesn't look right but other than that is not causing a problem. As above, water pump and thermostat have been replaced, system holding pressure, sniff test negative. Suppose I'll just carry on and see if something breaks or I could go back to running with no thermostat which has been totally fine for 4 years apart from poor heater in winter?

[EdGasket]

Update if anyone is interested or has a similar problem:

I was not happy with the way it was running as the coolant in the cylinder head was boiling whereas the thermostat did not appear to be opening soon enough to cool the engine. The thermostat was working as I tested it before fitting however it only starts to open at the stated temp. of 82 degrees and bearing in mind it is halfway down the engine, does not appear to get hot enough to open in time to stop the coolant boiling. So I found a similar size (48mm) thermostat for a Ford Focus which opens at 74 degrees (Febi 105992), drilled a 3mm bypass hole in it at approx 10 o clock, and fitted that.

Also changed the coolant to Comma XStream G30 though could not get all the old coolant out despite emptying by bottom hose and refilling/running 4 times. Only about half the coolant comes out each time . But anyway I mixed some old and new coolant in a jar and nothing bad happened, just changed colour to purple.

So far so good with this arrangement but will see how it goes.