Terracan pressure in expansion bottle

Guys I am confused with the following issue .
When I drive the Terracan it pushes air into the expansion bottle along with coolant .There is no overheating on the vehicle and it reaches normal temperature.The expansion bottle just has a press on cap which when driven appears to be pushed off by the pressure build up.

I have done a chemical test for the head gasket which comes back as normal.
Radiator Changed
Thermostat changed
Heater matrix changed
Rad cap changed
Can’t think of anything else .

I’m pretty sure it’s the head gasket but wanted to throw it open before I start to dismantle it .
Any thoughts

There shouldn’t be any pressure build up, there should be a vent to release the pressure, otherwise the cap would pop off.

I don
t think this was ever sold in the US but every car I am familiar with has either a radiator cap with an overflow bottle that is not pressurized with a push on sap or a system with no radiator cap but a pressurized expansion bottle with a sealed cap that won’t pop off.
If you have a pressure cap on the radiator, your overflow tank should no blow off the cap. You must have a clogged vent.

Hi Guys might be a bit of confusion hear I think .The system has a radiator cap and a overflow tube which runs into an overflow bottle which is not pressurised and just has a clip top to stop spillage. However pressure and fluid is going into the overflow bottle and sometimes this causes the click on cap to pop off.

All the symptoms of a bad head gasket.

At last someone who thinks the same as me I can’t find any other solution for the issues thanks

Are you losing any coolant? If not, I’d just drill a hole in the reservoir cap and watch the coolant level.

Or just leave the cap loose on the overflow bottle for a while and watch the level carefully. Coolant doesn’t evaporate very much on its own so if the level goes down it’s leaking out, boiling away or getting sucked into the engine.

Provided no coolant is being lost, I’m guessing the same as the above posts, just a vent somewhere is clogged in the overflow system. Good idea if it is possible on your car however to look into the top of the radiator (remove cap when engine is cold, wear safety goggles, etc) for bubbles coming up through the coolant when the engine is warm and idling. Check it also when bumping the throttle to boost the rpm briefly. If you clearly see bubbles coming up, most likely the coolant bottle pressurization cause is a faulty head gasket.

Thanks guys definitely worth a try. Will update when I have tested it .What I have noticed after giving it a hard drive that it pushes coolant into the bottle nearly to the top.After it cools down I have to tip the contents back into the radiator but doesn’t appear to have lost any fluid

That’s another indication that the vent is plugged,

Both problems could also be explained by using the wrong radiator cap

When it’s running and all warmed up, is there bubbling in the bottle? That would indicate head gasket problems. It can be confirmed at a shop that can test for exhaust gasses in the bottle.

I assume you mean you take the radiator cap off, then tip the contents back into the radiator. Ask yourself, “What took the place of the fluid that ended up in the reservoir?” (Hint: combustion gases.)

Possibly , but hot coolant will move to the plastic expansion bottle in normal operation too, as the coolant heats up. Presumably the driving force is the heat related expansion in the volume of the liquid coolant, coolant vapor, and the air in the cooling system. They all expand with temperature. Or is there something specific about OP’s problem that could only be caused by combustion gasses?

Popping caps off overflow bottles is not normal, and is a symptom of head gasket problems - the exhaust gases have quite a lot of energy.

Yes, coolant level in the reservoir should only go up an inch or so, not fill it up, right?

@Pdamo : A new radiator cap certainly wouldn’t hurt.

It depends on the volume of the reservoir and how full it is when cold. I’m not disagreeing with you that it shouldn’t overflow the reservoir if the level is at the proper cold fill, the cooling system is holding the correct pressure, and the venting system for the reservoir is working.