Bongo Fury

Heres a new one,my bongo is totally fine at normal speeds in traffic ect.
heres the prob when i hit 85mph the temp gauge creeps up from its 11 o clock position to half way and beyond, if i dont back off ,if i do or sit at 85 it stays rock steady at 11 ,it drops as fast as the speed back to normal,if i do 85 or below, even though i over service it and do the two year flush,
it does lose a slight amount of coolant from the res overflow if i drive over 85 , but not alot,it has not boiled over ever,as you would expect,
so ive done a rad flush checked for leaks hoses ,belts ect, thought it may be the dreaded sludge or partially blocked rad,thermostat works fine,rad coolant header, caps look ok but have ordered new ones to eliminate,
towing or steep hills have no effect on the temp at all ,only 85 plus,
i do follow and know the correct bleed process,ive also been to europe and back with this problem,and it was fine no coolant loss as i was in touring mode plus i never went over 85
any thoughts ideas would be greatly appreciated,

my thoughts are maybe a partial head leak pressurising the cooling system/
radiator/heater partial blockage/coolant cap fault/


cheers

The obvious answer is slow down a little, better fuel economy I think you may be correct about your last thoughts -expansion cap fault etc

Although i haven't actually checked, apparently there is a summer /winter setting on the intake somewhere, but i havent noticed it...I,m sure dandy or somebody will know when they read this post...

hi the first thing to do is take out rad and get it flow tested.sounds like its part blocked ?

THE SOLUTION ;REPLACED COOLANT RES CAP,
It was the diaphram in the coolant master cylinder cap ,
all rubbers were in sound condition, and the cap looked in good condition,
i hope this is helpfull for anyone who experiences similar,

Interesting solution TEZ - especially as the "coolant master cylinder cap" hasn't got a diaphragm. - at least mine hasn't!

I think you will find you have, all pressure release type valves/caps have
its how they seal yet vent under pressure ,

TEZ wrote:I think you will find you have, all pressure release type valves/caps have
its how they seal yet vent under pressure ,

Beg to differ TEZ. The cap has a cap to tank rubber which seals it at the mouth of the expansion tank/radiator and another rubber seal at the pressure relief valve which seals the valve in the neck until it is lifted by excessive pressure. There's a 2nd valve which is the vacuum/syphon back valve which stops the hoses collapsing when the cooling system cools again.

None of them are diaphragms. Anyway, glad the problems sorted - sounds like the cap wasn't sealing properly which meant the system wouldn't pressurise properly and the boiling point of the coolant lowered as a result - especially when the engine was labouring.

yes i agree, the action of the cap works like a diaphram/IT IS KNOWN AND CALLED A DIAPHRAM VALVE, the syphon back seal works, to stop the hoses from collapsing,you have saved me a job as i had just taken pics to explain,ive also disected the cap so you can see where the fault occured,which was as i stated in the diaphram


also as you will know in motorcycles ,coolant header tanks are none pressureised and its the diaphram like action of the rad cap/diaphram valve seal which keeps the rad full,

thanks for the diagram

Not going to argue with you TEZ cos the job's done ........maybe I should rewrite my 17 years worth of teaching notes - personally I've never called it a radiator diaphragm, a radiator diaphragm is found in speakers IMO

dandy nobodys calling your 17 years into question rewrite away if you wish, ive served the same time teaching and working on motorcycles at woods motorcycles in north wales we have been voted the best dealer by mcn readers for the past six years in all departments,


that aside we call it a diaphram valve inside the radiator cap not a radiator diaphram ive never heard of one of those either,

anyway job done,