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Ducati750cc · 14th March 2021, 16:40

As Steve says " Basically it comes out via the pressure relief valve in the cap."

If you fill the cooling system much higher than the max. when cold, even possibly to the bottom of the neck, there is no airspace for the coolant to expand into as it gets hot, as pressure increases to above the cap pressure the valve in the cap will lift to vent the pressure and water.

This can give the impression that there is a loss of coolant, which there is, but the coolant lost is the excess when overfilled, venting as it should and when re-checked when cold it will be around, but more likely just above the the max.

If there is antifreeze in the coolant there should be evidence and stains of this around the bottom of the cap, where it vents and possibly around the engine bay if it vents whilst being driven.

Can a faulty cap cause overheating, possibly for two reasons, 1) if there is a substantial loss of coolant which there doesn't appear to be, 2) if the cap can't maintain pressure as the system gets hotter approaching boiling point, the water needs to be under pressure to prevent the water from actually boiling and causing steam pockets around the hottest area, cylinder head and particularly the ex. valve area, but you would expect to see evidence of some steam or water loss from the faulty cap.

The remaining popular o/h causes are faulty thermostat, system not correctly bled on refill, blocked radiator core ( unlikely ) rad fan or rad fan sensor faulty, cyl. head problems.

The mayo you have in the oil cap can come about simply with lots of stop start short journeys but can also be a sign of head, or head gasket problems, there may not be the usual evidence of steam from the exhaust when hot with a small leak due to the heat of the cat turning it into superheated steam which won't be visible.

If it was me I would initially satisfy myself that there are no head gasket problems, first by getting a friendly garage to use a sniffer test to see if there are combustion products in the coolant, plus you can purchase, quite cheaply, some DIY kits, if you are then satisfied that the gasket/head is OK you can then progress to sorting the apparent overheating problem.

Finally, don't forget, that the engine may not actually be overheating, a faulty temp. sensor / gauge could be showing higher than actual readings.

14th March 2021, 16:40	#10
Ducati750cc Loves to post Rover75 tourer Join Date: Jun 2018 Location: Ashton Posts: 428 Thanks: 121 Thanked 206 Times in 145 Posts	As Steve says " Basically it comes out via the pressure relief valve in the cap." If you fill the cooling system much higher than the max. when cold, even possibly to the bottom of the neck, there is no airspace for the coolant to expand into as it gets hot, as pressure increases to above the cap pressure the valve in the cap will lift to vent the pressure and water. This can give the impression that there is a loss of coolant, which there is, but the coolant lost is the excess when overfilled, venting as it should and when re-checked when cold it will be around, but more likely just above the the max. If there is antifreeze in the coolant there should be evidence and stains of this around the bottom of the cap, where it vents and possibly around the engine bay if it vents whilst being driven. Can a faulty cap cause overheating, possibly for two reasons, 1) if there is a substantial loss of coolant which there doesn't appear to be, 2) if the cap can't maintain pressure as the system gets hotter approaching boiling point, the water needs to be under pressure to prevent the water from actually boiling and causing steam pockets around the hottest area, cylinder head and particularly the ex. valve area, but you would expect to see evidence of some steam or water loss from the faulty cap. The remaining popular o/h causes are faulty thermostat, system not correctly bled on refill, blocked radiator core ( unlikely ) rad fan or rad fan sensor faulty, cyl. head problems. The mayo you have in the oil cap can come about simply with lots of stop start short journeys but can also be a sign of head, or head gasket problems, there may not be the usual evidence of steam from the exhaust when hot with a small leak due to the heat of the cat turning it into superheated steam which won't be visible. If it was me I would initially satisfy myself that there are no head gasket problems, first by getting a friendly garage to use a sniffer test to see if there are combustion products in the coolant, plus you can purchase, quite cheaply, some DIY kits, if you are then satisfied that the gasket/head is OK you can then progress to sorting the apparent overheating problem. Finally, don't forget, that the engine may not actually be overheating, a faulty temp. sensor / gauge could be showing higher than actual readings. __________________ Common sense isn't a gift, it's a punishment because you have to deal with everyone who doesn't have it.