27 replies to this topic

[Ethel]

At least it sounds like it's more likely to be mechanical if it is actually boiling. Especially if the fan is cutting in as you'd expect.

[lsto]

Is the car running ok or is it lumpy when you first start it then clears for example.
You've done everything right so far.
If you have flushed it, changed the thermostat and the fan is working next thing id check is the belt tension, the water pump isn't slipping is it, silly as it seems. Next option is either take it to a garage and get it sniff tested to see if there is any exhaust gases in the coolant which would point towards a head gasket failure, if that is ok then water pump will need looking at, or just do the water pump first and see what happens, they have been known to fail.
Good luck.

Edited by lsto, 22 July 2026 - 08:02 PM.

[john2502]

Runs completely fine, which is what then led me to thinking the sensor might be reading wrong but then a bit weird for it to be going to the centre of the gauge as expected then rising slowly? Which exactly mimics genuinely overheating.

Thanks yeah, I did wonder about belt but seems OK, alternator runs fine but then I'm not sure if that's an indication, will take a look to see if its seems to be slipping.

None of the usual signs of head gasket failure, it did cross my mind in dread, I know its not fatal but not something I don't have time to do myself right now (3kids and into summer holidays) so would be a bit more pricey as I'd have to pay for that.

I am going to run it up again today with the old overflow cap on, I should have mentioned last night the top hose was sucked in like it had drawn a vacuum so could it be that time (when it did actually genuinely overheat) the cap wasn't sealing or not on tight enough so it lost pressure.


Feel like I'm stumbling around just throwing parts at it at the moment. Haha.

[RWD3M]

Runs completely fine, which is what then led me to thinking the sensor might be reading wrong but then a bit weird for it to be going to the centre of the gauge as expected then rising slowly? Which exactly mimics genuinely overheating.

Thanks yeah, I did wonder about belt but seems OK, alternator runs fine but then I'm not sure if that's an indication, will take a look to see if its seems to be slipping.

None of the usual signs of head gasket failure, it did cross my mind in dread, I know its not fatal but not something I don't have time to do myself right now (3kids and into summer holidays) so would be a bit more pricey as I'd have to pay for that.

I am going to run it up again today with the old overflow cap on, I should have mentioned last night the top hose was sucked in like it had drawn a vacuum so could it be that time (when it did actually genuinely overheat) the cap wasn't sealing or not on tight enough so it lost pressure.


Feel like I'm stumbling around just throwing parts at it at the moment. Haha.

Hi John,

 

I feel your pain. I've recently been through pretty much the same scenario with our '97 MPi and to be honest, I'm still not convinced I've resolved it.

 

In the last couple of years I've replaced, for new, the following:

 

Radiator

Header tank

Cap (with a BMW R50 one)

Water pump (and belt)

Thermostat

Temperature sensor

Fan relay

 

Unlike you, my main issue appears to be that the fan doesn't come on so I have also now added an override switch for MoT time which is when mine seems to repeatedly boil over!

 

Ours is a 50k mile example and is only in use in the summer months. That's not to say it doesn't get around mind you, it's just been up to the Shetland Classic Motor Show!

[john2502]

So wanted to start fresh today:

Ran the car up, came up to temp as normal, then a while later electric fan kicked in as expected stayed fine in the middle of the gauge for another 5-10mins then temp gauge began to slowly rise.

I shut the car off and the electric fan shut off 2 seconds after as normal. Opened the bonnet and could hear a slight hissing from the overflow cap i think. Then suddenly while I had my head by it the electric fan kicked in again (scared the crap out of me while I was trying to listen where the hissing was coming from! Haha).

Fan remained on despite car being turned off, I turned the ignition on (without starting the car) and then the fan turned off.

Top and bottom hose are hot so is radiator, heat comes into the cabin but not as strong as before maybe. Hoses leading to cabin feel hot.

So..... my thoughts are

1. New overflow cap is also faulty and not holding pressure? Would that cuse these symptoms?
2. Overflow tank itself leaking pressure? (Although can't see anything)
3. Water pump weak and not able to "keep up" (no evidence for this really)
4. Air lock still (despite me running it up with cap off and squeezing bottom hose)
5. Head gasket (slight film on coolant in overflow), oil looks fine and exhaust is clear though.

I am now thinking the temp sensor is OK as seems to be reading what is actually happening.

[john2502]

Runs completely fine, which is what then led me to thinking the sensor might be reading wrong but then a bit weird for it to be going to the centre of the gauge as expected then rising slowly? Which exactly mimics genuinely overheating.

Thanks yeah, I did wonder about belt but seems OK, alternator runs fine but then I'm not sure if that's an indication, will take a look to see if its seems to be slipping.

None of the usual signs of head gasket failure, it did cross my mind in dread, I know its not fatal but not something I don't have time to do myself right now (3kids and into summer holidays) so would be a bit more pricey as I'd have to pay for that.

I am going to run it up again today with the old overflow cap on, I should have mentioned last night the top hose was sucked in like it had drawn a vacuum so could it be that time (when it did actually genuinely overheat) the cap wasn't sealing or not on tight enough so it lost pressure.


Feel like I'm stumbling around just throwing parts at it at the moment. Haha.

Hi John,

I feel your pain. I've recently been through pretty much the same scenario with our '97 MPi and to be honest, I'm still not convinced I've resolved it.

In the last couple of years I've replaced, for new, the following:

Radiator
Header tank
Cap (with a BMW R50 one)
Water pump (and belt)
Thermostat
Temperature sensor
Fan relay

Unlike you, my main issue appears to be that the fan doesn't come on so I have also now added an override switch for MoT time which is when mine seems to repeatedly boil over!

Ours is a 50k mile example and is only in use in the summer months. That's not to say it doesn't get around mind you, it's just been up to the Shetland Classic Motor Show!

Ah wow, what does (or did) your gauge do? Did it also warm up, stay central afor a while then start to rise slowly? Or was it going straight up?

Thanks. Frustrating when you can't find the route cause isn't it! Haha

Edited by john2502, 24 July 2026 - 02:59 PM.

[RWD3M]

 

 

Runs completely fine, which is what then led me to thinking the sensor might be reading wrong but then a bit weird for it to be going to the centre of the gauge as expected then rising slowly? Which exactly mimics genuinely overheating.

Thanks yeah, I did wonder about belt but seems OK, alternator runs fine but then I'm not sure if that's an indication, will take a look to see if its seems to be slipping.

None of the usual signs of head gasket failure, it did cross my mind in dread, I know its not fatal but not something I don't have time to do myself right now (3kids and into summer holidays) so would be a bit more pricey as I'd have to pay for that.

I am going to run it up again today with the old overflow cap on, I should have mentioned last night the top hose was sucked in like it had drawn a vacuum so could it be that time (when it did actually genuinely overheat) the cap wasn't sealing or not on tight enough so it lost pressure.


Feel like I'm stumbling around just throwing parts at it at the moment. Haha.

Hi John,

I feel your pain. I've recently been through pretty much the same scenario with our '97 MPi and to be honest, I'm still not convinced I've resolved it.

In the last couple of years I've replaced, for new, the following:

Radiator
Header tank
Cap (with a BMW R50 one)
Water pump (and belt)
Thermostat
Temperature sensor
Fan relay

Unlike you, my main issue appears to be that the fan doesn't come on so I have also now added an override switch for MoT time which is when mine seems to repeatedly boil over!

Ours is a 50k mile example and is only in use in the summer months. That's not to say it doesn't get around mind you, it's just been up to the Shetland Classic Motor Show!

Ah wow, what does (or did) your gauge do? Did it also warm up, stay central afor a while then start to rise slowly? Or was it going straight up?

Thanks. Frustrating when you can't find the route cause isn't it! Haha

 

Gauge behaved normally, i.e. as it warmed up it showed it and continued to tell me it was hot while it was piddling its contents over the MoT testers ramp!

 

I did have a suspicion that it was a lack of pressure causing the overheat (as you know, under pressure, coolant has a higher boiling point) hence a new cap from BMW (far better quality than aftermarket items from the myriad of Mini specialists - no offence). But this year even that failed to get the fan to cut in.

[Quinlan minor]

If you can hear hissing, there is a leak somewhere. As the pressure drops, the boiling point reduces.
Not much use at this time of year, but I had a Jag doing this, years ago. We could not find the leak until, finally on a November night, with it up on a ramp near the open door, we could see the plume of condensing water vapour. It was a pinhole in the bottom hose, too small for water to escape but enough to allow vapour out.

Edited by Quinlan minor, 24 July 2026 - 03:42 PM.

[john2502]

Hmmm, this is really stumping me currently. Tried again today with new (3rd) overflow cap and still does the same, no hissing I can hear this time so maybe 2nd cap was rubbish or just not quite on tight enough. 

 

So, again heated up as normal (5mins on a hot day idling), stayed fine for 5-10mins, electric fan then comes on, few mins later the gauge slowly starts to climb, i turn it off around 3/4 and the fan goes off and stays off. All hoses feel very hot as does rad so must be at least some circulation.

 

My thinking is:

 

1. Faulty thermostat- been replaced and top hose feels hot when gauge reaches the middle so dont think so, most unlikely for me

 

2. Blocked Rad- possible i guess, it does get hot all over though so seems to be doing it's job....

 

3. Overflow Cap/Loss of pressure- seems unlikely as tried 3 caps now, could be from somewhere else but all seems tight and leak free to me (still possible I guess)

 

4. Temperature Sensor- gauge behaves kind of as i would expect for what's happening so i imagine this is fine, (goes to middle then slowly climbs seems unlikely behaviour for faulty sensor)

 

5. Air Lock- all hoses feel hot when I’d expect them to including those going to the cabin and have run the system while squeezing bottom hose and cap off a few times now, warm air in the cabin is fine (maybe not as warm as it used to be), 

 

6. Water Pump- could be I guess, would have to be the impeller failing as belt tension seems fine and spins ok, all looks normal from outside

 

7. Head Gasket- bit of a last resort, hope not, oil looks ok, bit of a "film" on the coolant in overflow, no smoke/steam from exhaust. 

 

I am at a bit of a loss really, what would be best to try next? Still feel like I'm randomly just throwing parts at the cooling system to be honest, haha

[lsto]

Either take it to a garage or buy a cheap combustion gas tester and see if there is any exhaust gas in the coolant, the kits are less then a tenna on eBay. Yes you have a head issue, no then try the water pump as you have already flushed, done the thermostat and cap.

[Designer]

Hi,

 

This could be a stupid thought but could you have reversed the wiring to the fan and it is spinning the wrong way therefore not removing heat properly from the engine but adding to it.

 

Paddy

[john2502]

Either take it to a garage or buy a cheap combustion gas tester and see if there is any exhaust gas in the coolant, the kits are less then a tenna on eBay. Yes you have a head issue, no then try the water pump as you have already flushed, done the thermostat and cap.

Thanks so much, yeah think this is my plan going forward.

[john2502]

Hi,

 

This could be a stupid thought but could you have reversed the wiring to the fan and it is spinning the wrong way therefore not removing heat properly from the engine but adding to it.

 

Paddy

it's a thought but no, fan has stayed as is, it started doing this after a particularly hot day at the MOT centre when left running and has then done this ever since (I use it daily as well and have done for 3.5 years)

 

I did wonder about the fan but yeah it hasn't changed, cheers though!

Edited by john2502, 30 July 2026 - 11:10 AM.