Mystery coolant leak - Solved?

[Old-Guy]

For some long time, the Green Lady has had a slight thirst for coolant; not enough to worry about, just irritating that every few hundred miles she needs a small top-up. No drips anywhere visible and it seemed to get better after I changed the thermostat a couple of months ago - leaky old seal?

Too simple, the thirst returned with the next long fast trip.

Then when looking for simple causes of the spluttery 'cold' starts, I spotted a drop (just one) of coolant way below the bleed cap behind the injector pump. Further investigation revealed that the Schrader-type cap had a slight split - swopped it with a new valve cap off one wheel.

The original blue cap escaped down the back of the Bosch pump several years ago and was replaced with a spare tyre valve cap of unknown age.

I suspect that when the engine was really hot, the split opened up under pressure releasing a tiny fine spray of steam. Changing the thermostat improved matters because the cooling system could cope better with hard driving and the engine now rarely goes above 83°C - full power up big hills (Haldon Hill A38 south of Exeter and Wells Hill on A37).

So, if your XUD engine starts to use a little coolant, check the bleed cap (or just fit a new one!) before looking for anything more serious.

[DickieG]

Coolant bleed caps are different from tyre valve caps as they have a small rubber o-ring gasket at the bottom so best to stick to the correct cap as after all they can't cost very much new from a dealer.

[myglaren]

They are about 30p IIRC and the plastic ones have been replaced with metal ones.

[CitroJim]

Good work Guy

Those are really hard to locate I know...

[Old-Guy]

They are about 30p IIRC and the plastic ones have been replaced with metal ones.

Plus the 20 mile round trip to get one! However, I have to go to Gloucester anyway on Friday...