'Jasper Oxford' on MINI2 (link below) recommends more often than that, say once a year:
The cooling system on these engines is not particularly reliable. They have a reputation for boiling up.
I have an 01 Cooper which is about to have a new head. After a great deal of time looking at the MINI cooling system, I concluded it can indeed fail where others would be tolerant. This gen MINI uses a Chrysler PT cruiser 1.6 engine, basically. It's ok in that car, but in the MINI it's a lot of engine crammed into a small box. If the cooling system starts playing-up, it'll quickly overheat.
Air locks in top of head......this engine is inclined rearwards, heater matrix take off is at lower section of head. Hence air must be bled out via the thermostat's bleed valve, but here's the crunch....this valve can, and often does close while there is still air in the head.
Coolant properties 'worn out', causing the coolant to boil at a lower temperature, i.e. liberate gases...which are not necessarily expelled through the stat's bleed valve......hot spots = more gases trapped, etc. Coolant in these engines must not be allowed to 'bubble', since the system will not always bleed by itself. Degraded coolant WILL begin to gas-out in a really hot engine, ignore BMW's 4 year advice........change it once a year 50-50 mix.
I have followed an old remedy with my 01 Cooper: it has a thermostat with a hole in. At one time people used to drill a small hole in the stat, not large enough to over cool, but just large enough to make sure there is always some flow past the stat.........doing this on the MINI means it bleeds itself, works perfectly.
However, the MINI's stat has a (unreliable) bleed valve, or jiggle pin.....you simply get a pair of cutters & castrate the thing, leaving a hole which will be about 3mm, this is ok in hot climates, but in the UK it should be about 1.5 - 2mm. I have a 2mm reducer on mine.
Jiggle pins only really work properly when the stat is mounted vertically, which on a MINI the are'nt. Horizontal stats really require a slight but constant bleed, otherwise their bleed pins can slam shut while there is still air in the system.
Check also the stage 1 cooling fan dropper resistor hasn't gone open circuit, if it has, you won't get stage 1 fan cool down....and it'll boil up without it. Stage 2 i.e. full fan speed I've never seen come on, and I don't know anyone that has either. I can only assume stage 2 is to stop it bursting into flames. The stage 1 dropper resistors are notoriosly weak.
That's esentially why so many of these MINI's go down with head gasket failure. In any case, if you boiled it, chances are you've done the head gasket too. Hope action was taken quick enough so you don't have a warped head.
('Snip' the jiggle pin.....a constant bleed hole stops this happening again.)
More tips for early MINI overheating problems:
First, establish that the water pump is circulating coolant....if the interior heater is working this indicates the water pump is working. I don't think you need to pay further attention to the pump at this point.
Filling & bleeding.....check you're not running the engine on low coolant.
Now an important point:........the temperature gauge on some Coopers (mine included) is controlled by the ECU...and does not necessarily tell you when the engine is overheating. When the engine is started from cold the ECU will push the gauge up in unison with the coolant temp, HOWEVER!.....when the gauge reaches 'N' or midway the ECU holds the gauge at that reading REGARDLESS OF ENGINE TEMP. It is what's commonly referred to as a phoney gauge. Lot's of cars these days have such gauges, some new cars don't have a gauge at all.
BMW's philosophy is, basically: "we don't want the owner to know how hot the engine is, we'll decide what is normal".
Now, you have obviously done some testing of the radiator fan circuits. You say the engine reaches boil without there being any stage1 fan operation. You also say you are unable to jumper a stage 1 fan. I think you are on the right track....you must now suspect the voltage dropper resistor as having gone open circuit, i.e. 'burned out'
There are actually two components to this, a large dropper resistor, and a smaller thermistor in series with it. These components are located within the plastic fan mounting assembly.......they are accessible but you have to remove the radiator to get at them.
The dropper resistor has a resistance of about 0.5 ohm. It's wattage is generally reckoned to be underrated, they frequently burn out. There's also an issue with it's electrical connections.....it isn't soldered in place, it's 'tack welded'....the welds corrode & go open circuit.
You have to consider: you've got no stage 1. The engine boils but only if you turn the heater off. The water pump is ok and doing it's job.
Loss of stage 1 fan is the culprit. (even if the thermostat were stuck closed, stage 1 will still cut in).
You'll need to replace the radiator fan assembly, resistors are'nt sold separately. I suggest you remove the assembly, gain access to the resistor & check it. My money is on you finding an open circuit resistor.
I had the same problem on mine, but refused to pay BMW money for a weak replacement so I made my own resistor & wired that in.
and more tips including the importence of using the correct coolant/antifreeze:
Ok, if the A/C calls for, and initiates rad fan, that would indeed suggest 'some' cooling fan function.........but be careful to ensure this is actually stage 1 fan you are getting, and not stage 2.
Sounds like you have a can 'o worms problem.
The thing about these engines is they can throw so many 'red herrings' at you for the simplest of faults.
I believe you would be wise to methodically remove and inspect the systems associated with the fault.
1) Go over the wiring harnesses, paying particular attention to the connectors. start at the fan connector and work your way back as far as possible. Make sure all the connectors are good.
2) Check relays.......this is important!.....the aircon probably on your model uses it's own separate relay to switch in stage 1 fan, which might account for why aircon puts the fan on. The ECU signal for stage 1 fan will go via it's own dedicated relay.....this could be duff, which might be why engine temp doesn't switch the fan on, but aircon does. You need to make sure these relays are good, purely as a matter of procedure if nothing else.
3) Make absolutely sure for certain the fan voltage dropper resistor circuit is ok. Do not 'assume' that aircon is giving you stage 1, it might be giving you stage 2 full speed....which bypasses the resistor entirely.
4) Is your water pump working?......run the engine, verify hot air comes out of the internal blower....if it's cold......the water pump is knackered. If it's hot, the water pump is circulating coolant and is probably ok.
5) Are you sure the thermostat is opening?....for the sake of 20 bucks do this test: get a new thermostat....take a reasonably heavy pair of cutters and crop out the jiggle valve (the wiggly little pin) just leaving a hole. Fit that thermostat. Refill and run the engine. The cylinder head will now bleed itself, and continue to do so. IF everything else is ok and is verified......your engine will not have trapped gas pockets and shouldn't boil over.
6) Coolant system pressure cap, which on an 'S' I believe is on the expansion tank.....if this cap is opening too early because it's worn out, you'll get boil over, simply because the coolant is not being held at a high enough pressure and will therefore liberate gas at a lower temp....leading to gas pockets in the head and subsequent hot spots which liberate more gas, and so it goes. The pressure cap is often overlooked, but actually is a vitally important component.
Note: the three vitally important things you MUST FIRST verify are; water pump, thermostat, and radiator fan electrical circuits which includes coolant temp sensor.
I assume you are filling with a 50/50 antifreeze - water mix, and not just plain water?
Do not ever under any circumstances fill with plain water......trust me plain water in these engines WILL BOIL.
You are trying to fix one of the most cantankerous engines ever put into something that bears the MINI name.......it is going to fight you every step of the way...rise above it, at least keep yourself cool even if the engine doesn't.....being methodical and not rushing is what will beat this.
Bleeding cooling system:
DIY: How to change the coolant
Bleed Valve locations: