Focus Mk2: Engine temperature isn't constant

[RichHorrocks]

I recently had to top up the engine coolant on my Focus. It had dropped quite a bit, but I was unable to find any leaks anywhere. Since topping it up, the level has stayed constant.

I've noticed over the past couple of weeks that the engine temperature doesn't stay constant. It never gets too hot - i.e. the needle never passes vertical - but it does drop and go back up. I've noticed it gets cooler if I'm moving fast (probably when air is passing over the radiator?), and I've also noticed that the temperature drops when going uphill, regardless of how fast I'm going.

I'm thinking this might indicate an air lock, but wouldn't the engine overheat sometimes if that was the case and coolant wasn't circulating? Any ideas?

(Perhaps also worth noting is that a couple of months ago I blocked off the EGR valve. It's a 1.8 TDCi.)

[Povo]

The temperature gauge on the dash isn't showing real temperature. It basically has about 3 levels that it shows: cold, up to temperature and overheating.

I drove my car (same engine) on a track day with my phone showing temperature using a bluetooth OBD dongle and the temperatures varied from 75 degrees to 100 degrees Celsius. The gauge on the dash stayed in the middle the whole time.

[james1089]

There will be a leak on the thermostat housing at the back where it connects to the engine.

[RichHorrocks]

It doesn't appear to be the thermostat housing.

I couldn't see any residue from a leak around the housing, so I taped some white paper towel around it and drove for a few days. I checked the coolant level, and it had dropped by a couple of centimetres, but the paper towel was completely clean and dry. (And not red, from the coolant, and wet.)

Could it be the seal on the filler cap? Is there a good way to check the seal is still okay? (I might tape another bit of paper towel over the cap to try to catch any evaporation...)

[artscot79]

a new pressure cap costs a tenner and should be renewed every 3 years I always start there first as its always over looked if that doesn't cure it then there's a leak somewhere so its time to check every hose and clamp

[RichHorrocks]

Thanks for the reply.

A couple of days with a white paper towel taped over the filler cap, and it doesn't appear there's any evaporation, but the level has dropped a couple of centimetres again. (I made sure there was a good seal around the whole cap. The paper towel has no sign of being wet or drying out. Bone dry.)

Is there a good place to start when checking all the hoses? Any usual suspects? (Other than the thermostat housing, which I've already discounted.) Any tricks or tips I can use? (A big bit of cardboard under the engine, then let it run for 20 minutes? See where the drips are? Or will the coolant evaporate rather than drip, if it's hot?)

[RichHorrocks]

Just to close this thread, the problem turned out to be a knackered coolant pump. The seal was leaking, and eventually the whole thing went, to the point where it dumped the contents of the cooling system all over the M1...

All fixed now