Ford Fiesta 2006 1.2 Zetec cooling system pressure issue

[DirtyBarry]

Ford Fiesta 2006 1.2 Zetec cooling system pressure issue since head gasket failure caused by overheating and “Steel Seal” was used in an attempt to repair it.

Steel Seal did not fix the head gasket leak, the head was removed and measured using a straight edge and found to be warped by eight thousands of an inch. The head was skimmed and refitted. The car ran perfectly for 200 miles (a couple of short journeys and a long one). Between each journey the coolant level didn't move. It then overheated again, this time the engine was stopped before any damage was done.

Back on the driveway the engine was run up from cold and ran perfectly until it got to running temperature. The top hose from the head to the top of the radiator started to pressurise, the hose from the bottom of the radiator to the thermostat housing was cold. The thermostat bypass pipe back to the expansion tank was hot relieving the pressure from the system when the cap was removed. All other hoses warmed up including the hoses to and from the heater matrix and the return hose to the header tank. The only cold coolant hose was the bottom hose from the radiator to the thermostat housing. When the cooling system pressurises the header tank fills up.

As the cooling system doesn't pressurise until it is warm I didn't think it was combustion leak into the cooling system, but to test I warmed the engine then stopped it, removed spark plugs one at a time running the engine with individual spark plugs missing and the cooling system pressurised the same as before when all four cylinders were tested.

I thought the thermostat wasn't opening so replaced it, I found some white sludge in the block between where the thermostat sits and the water pump is located, I washed it out (but how much of this sludge is in there?).

So is the radiator blocked by some of this sludge as it is hot one side and not the other? To test the flow of coolant with the engine cold I attached the garden hose to the expansion tank coolant hose (perfect fit) and had water flowing through all hoses clamping some to divert the flow to test. Water was coming out of the bottom of the radiator when the bottom hose was removed eliminating a blocked rad.

I took the thermostat out and cut the back off so it will let water pass through but still hold the seal in place. The engine warms up to running temperature correctly on the drive way and has stopped pressurising the cooling system and stopped overheating (but as expected runs cold when driving due constant flow of coolant cooled through the radiators airflow).

So this has proven the engines combustion is not leaking into the cooling system via a crack or gasket problem causing the cooling system pressure issue, both thermostats new and old opened and closed simultaneously when tested with boiling water so the thermostat is not faulty. The radiator and water pump are functioning correctly. Pressure was not stopping the thermostat from opening, the side of the thermostat that pressurised would assist it to open.

The only other thing I can think of is some kind of blockage of a coolant journal in the either the head or block that is not allowing enough hot coolant near the thermostat to open it, but other journals allow a sufficient flow of coolant without the thermostat in to cool it?

Does anyone know of anything else that could affect the way this Fiesta 1.2 Zetec cooling system is behaving?

Thanks
Barry

[neiljohnson]

Likely the small return to the coolant header tank is blocked
If you follow the pipe you will see it goes to a small pipe on the head which can be unscrewed it often will get blocked in normal use so steel seal will probably have clogged it up
Normally just run a drill though to unblock it & away you go

[DirtyBarry]

Thanks for your reply Neil, the return pipe flows back to the header tank as normal - no blockage there. Coolant flows perfectly without the thermostat in, but pressurises the system when it is fitted, but thermostat opens when tested with the kettle, is very odd.

[DirtyBarry]

Well, after a lot of playing around, head scratching, tyre kicking and swearing I found the issue with the Fiesta was actually an airlock not a physical block in the system. Airlocks do however prevent coolant from circulating as if there were a physical blockage in the cylinder head or block. (More commonly found in your home central heating system than your car cooling system).

The Fiesta 1.25 Zetec (56 plate) was running sweet as a nut up to running temperature, as soon as the engine was up to temperature the top hose pressurised and was very hot to touch as expected but the bottom hose was stone cold (the thermostat is located between the bottom hose and cylinder block on the Zetec engine). As the thermostat wasn’t opening the coolant wasn’t flowing through the radiator and the engine was overheating. (Thermostat changed, water pump removed and inspected, sniffer test passed, radiator and all parts of the cooling system tested for flow with my garden hose – no faults found).

The root cause of the issue was air trapped in the highest point of the engine (where the top hose comes out), as the engine got hotter the air expanded pressurising the system preventing it from functioning. To resolve the issue I filled the cooling system from the heater matrix pipe allowing the engine to be filled directly from a high point with the expansion tank lid off to allow air to escape from the return pipe located just above the heater matrix pipe on the head.

I must admit I did this when the engine was hot (not running and had cooled slightly – pressure carefully relieved from expansion tank), it is extremely dangerous to play with a pressurised cooling system at running temperature and caution must be taken. If you do not know what you are doing seek professional advice / help! After removing the air from the system I was very careful removing water from the system by removing a pipe from the expansion tank to be able to add antifreeze without letting air back into the system.