2005 Cooper facelift - coolant woes - any ideas?!
From the UK and recently purchased a facelift R50 Cooper as a cheap run-around and project.
Car on the whole is generally v sound, has fairly good (though not complete) history and has only covered 78.5k miles.
Love the car and have sorted a number of its minor problems but I have one big one I am really struggling to sort.
The car generally runs and drives OK, hasn't outwardly demonstrated that it's tried to overheat at any point, the stage 2 fan definitely kicks in...but.....it keeps losing coolant from the thermostat housing cap. It tends to weep slightly from the cap and when hot, light pressure on the cap tends to cause hot coolant to flow out from the seal and run out the bottom of the car. Seems to be affected more by the incline it is parked on. If I park it on my steep driveway nose up it doesn't really drop other than maybe a slight weep. Parking nose down it tends to pee out.
Now as to what's been done.......
First noticed a problem when going to fill up the washers. I noticed the expansion tank was more or less bone dry (having been very full previously) and the lid had popped open. Can only assume the coolant was forced under through the thermostat and out through the expansion tank. I knew this wasn't normal. I therefore changed the thermostat cap. Note the cap that was on it was not original, v v tight and I suspect the rad would have blown before it would have vented any coolant if ever!
Following the cap change, no more issues with coolant coming out of the expansion tank but now losing it from the thermostat cap as described above.
Car goes to friends garage, diagnoses lazy thermostat. The stat, housing and cap are all renewed. Expected it to be fixed but soon presents the same exact issue.
I have two theories...but am leaning more towards the 2nd.
1. There is an airlock in the system still (despite being bled) causing pressurisation and for the coolant to leak from the thermostat cap. Note I can hear some glugging from behind the dash indicating there may be some air in the heater matrix at least...but the heater does work just fine.
2. The head gasket is probably goosed. Would explain the coolant pressurisation. Strangely though the coolant doesn't seem to be becoming in any way contaminated and not noticing any mayo in the oil filler or on the dipstick. The car does however run rough on starting from cold or if not run for a period of time (which if it's not fouled plugs could well be coolant in a cylinder I guess).
My mate it a top flight mechanic, but he does seem fairly adamant it's not the head gasket but my gut feel is it most likely is. He hasn't really been able to replicate the symptoms I've described but I don't suppose it's been left on an incline of any sort whilst with him and that could well be the difference?
I've suggested perhaps parking it on an incline and also doing a compression test.
Does anyone on here have any ideas...given the rad cap and thermostat and housing have all been changed, also the rad is quite new and it's integrity has been checked that most will probably say head gasket?
Appreciate any thoughts or comments anyone may have......frankly it's driving me spare!!