Help with cooling system issue diagnosis

[marksx]

Hi all,

I have a 2014 Mondeo, 1.6 tdci and I'm having issues with cooling/overheating.

I've noticed at times, that the car cools below normal operating temperature when stood in traffic.

At other times the fan will be on after shutting off, even after a normal journey in normal weather.

The most concerning is that sometimes the temp will swing around to around 3/4 on the gauge then drop back. Weirdly, it only seems to do this if I hop straight on the motorway from a cold start. If I drive around town I don't see it it all.

I'm also losing coolant with no leaks. No mayo in the oil. Water smells a bit sooty but everything does with a diesel!

My initial thoughts are thermostat, but the coolant loss is making me wonder if it could be something else.

[colin_p]

I've seen this on VAG TDI's.

Believe it or not, the culprit is the head gasket.

I'm not saying that is the case here without detailed knowledge of that (Peugeot / Ford) engine but the symptoms fit.

My own personal theory on this is...

HG fails between a cylinder(s) and water passage, this gives rise to;
Combustion pressures (at high revs / motorway speeds) effectively stall the coolant circulation, causing rapid overheating.
Combustion pressures (at low revs / around town) overcome the spring resistance of the thermostat, causing cool running.

I'm not saying this is the case on your car with that engine, but is worth considering if all others investigations come up with no explanation.

Also, don't these 1.6 Pug / Ford engine have a "mapped" cooling system i.e. electronic thermostat etc, which would be worth looking into?

Good luck with finding a fix.

[GreenV8S]

I suggest you carry out a block test aka sniff test. This will tell you whether you have a leak between the block / crank case / oil cooler / EGR mixer cooler / manifold heater / trans cooler etc and the cooling system.

[marksx]

I do worry it's the head gasket.

By sniff test do you mean the chemical solution that goes in an airlock on the expansion tank?

[GreenV8S]

marksx said:

By sniff test do you mean the chemical solution that goes in an airlock on the expansion tank?

That's the stuff.

[stevemcs]

Have a look at the top of the gearbox for coolant staining, chances are its the thermostat (sits on the RH side of the engine)

[marksx]

Finally got around to testing, I never got a good flow through the airlock (no bung), but this seems pretty conclusive.

[SystemOfAFrown]

colin_p said:

HG fails between a cylinder(s) and water passage, this gives rise to;
Combustion pressures (at high revs / motorway speeds) effectively stall the coolant circulation, causing rapid overheating.
Combustion pressures (at low revs / around town) overcome the spring resistance of the thermostat, causing cool running.

The pressure across the thermostat won't change even if the cooling system is being pressurised by a leaking head gasket, and it's not obvious why higher pressure would cause the circulation to stall since the pressure difference across the pump wouldn't change either.

Most likely cause of running cool is sticking thermostat, it's always possible to have more than one fault.

[PaulKemp]

Start cheap, swap out the thermostat if you still have a problem test for combustion gasses in the coolant

[GreenV8S]

PaulKemp said:

if you still have a problem test for combustion gasses in the coolant

Seems to me that test has already been done and gave pretty clear results.