16 replies to this topic

[Rusty Nut]

Good morning all

 

2001 MPI - and it keeps overheating in traffic. Expansion caps are rubbish and even after two changes, its still doing it. The trouble seems to be that the PO must have overtightened the cap to compensate so the thread doesn't hold any more so of course its going to blow off (sorry!)

 

I've flushed the system, changed the thermostat and sensor and the fan collector.

All are working properly - opening or coming on at the right time -  but the system is just not holding the pressure so boils over.

 

So if I buy a new regular expansion tank will it just do the same?

 

Or get a steel one made?

 

Or a roll of PTFE tape?

 

Any permanent fixes for this anyone?

[cal844]

Get an alloy one made, you can then use a better quality cap. Try flushing the radiator etc through

[Steve220]

Are you using an eBay cap or a genuine BMW mini one??

[Quinlan minor]

Check your expansion tank very carefully.

This pinhole, on the side close to the wing, kept me puzzling for a while.

Attached Files

•  Header Tank Pin Hole (small).jpg   30.56K   2 downloads

[Midas Mk1]

Ive got an alloy header  tank on ebay, this is the reason I went for it when I ran an MPi block.

[gav]

PCF101240ALLOY - Mini expansion tank for mpi alloy (minispares.com)

[Midas Mk1]

Pm if interested, pretty sure mines from Minispares, used for about 3k. but would be under £100.

[gazza82]

Check your expansion tank very carefully.
This pinhole, on the side close to the wing, kept me puzzling for a while.

Unlikely to be popping the pressure cap if it had a hole .. there wouldn't be enough pressure.

[Quinlan minor]

 

Unlikely to be popping the pressure cap if it had a hole .. there wouldn't be enough pressure.

 

"Ye cannae change the Laws of Physics, Captain!"

 

The pinhole causes the drop in pressure towards atmospheric.

The drop in pressure allows the coolant to boil at a lower temperature, close to 100° Celsius.

The pressure cap is designed to hold the pressure against air/vapour, not to stop the egress of boiling liquid coolant.

 

QED

[gazza82]

It will also take the route of least resistance .. ie the rusty hole rather than try to force the pressure cap off ..

"O-Level Physics pass" when O-Levels were a thing! ;) (I hated Physics .. more specifically the Psychopath that was mascarading as a Physics teacher, but the more mechanical stuff I was interested in plus I blew up a radiator once when the seam in the header tank started to go .. that let go under pressure before the rad cap.)

[Quinlan minor]

 I blew up a radiator once when the seam in the header tank started to go .. that let go under pressure before the rad cap.)

Hydraulic pressure will always win, because liquids are incompressible. Witness what happens when there's a bubble of air in the braking system.

[roblightbody]

Why would a mini need an alloy expansion tank, when every other car on the road has a plastic one? Genuine question!

[Quinlan minor]

Why would a mini need an alloy expansion tank, when every other car on the road has a plastic one? Genuine question!

Because the quality of the plastic ones is problematic, to say the least.

 

I'm now on my second new replacement in seven or eight years.

 

The plastic,IMO, is too thin and the welded joint is insufficient.

 

If you look at plastic expansion tanks in other vehicles the plastic is more than 5mm thick and the shape, more often than not, sperical.

The MPi's one is about the same thickness and shape as a brake fluid reservoir/ice cream tub, less than ideal for repeated temperature/pressure cycles.

[sonscar]

Quite a lot of other elderly vehicles with plastic tanks have them fail.Age heat cycles vibration etc all take a toll.As mentioned new stuff is of variable quality.Steve..

[Steve220]

Why would a mini need an alloy expansion tank, when every other car on the road has a plastic one? Genuine question!

Packaging and cost.