strange temperature problem

Hello all,
i am new to this forum, so pls. have a little patience with me if i do/did something wrong ...
my disco 2, built 2000, ran absolutely fine (motorwise) for the last 8 yrs but since last december it shows a strange behaviour: it works fine under 2500 rpm, but if i rev up due to rise or on the motorway, temperature quickly goes up into red and the engine loses all power. if i react fast and drop to under 2000 rpm while the needle is a little below the red field, the temperature drops back to middle and engine runs fine.
if i am not fast enough red temperature light comes up and needle drops to zero - but driving 10 mins under 2000 rpm cures the problem as above (though i cant check the temp anymore, for that i have to stop and restart the engine)
mechanic checked and found the viscose fan defect, and replaced it, but no change. next he took out the innards of the thermostat (get a new one this week) and that keeps the temp at about 25 % if under 2200 rpm, at middle a little under 2500 rpm - and straight up to red again if i rev up.
there is no trace of water checking the oil-stick, no trace of water in the exhaust (so he doesn't believe in cracked head-gasket). i loose about a quart of water when things heat up and carry a full bottle to top it up all the time. no trace of any oil or diesel in the coolant - disabling the thermostat we drained all of it and looked. filling up water the plastic screw on the top hose releases the air until only water shows. this and the engine cooling when driving slow seems to prove a working water pump, doesn't it ?
heater is working only from time to time, and never when the temp gauge is/has been up.
i read a thread here with a similar problem on a 1999 td5, and there the temp sensor seemed to play a part, so i have that replaced next, but failling that we all run out of ideas, so it seems to be the time to cry for help here, if i may.
tks all of you for your ideas in advance !!!

If the temp sensor is changed and you still have the problem then I would the coolant system pressure tested, this will find any leaks or cracks if there are any.

As andyb66 has said, change the temp sensor first and then see what happens. Have you tried removing a hose from the heater matrix and filling the matrix and pipes up from there?

Are the coolant pipes going rock hard after being driven?

A simple pressure test 'should' show up any faults. My own landy had a crack in the hea on the inlet side which meant that everytime my turbo spooled up it overpressurised. This DID NOT show up on a pressure test though.

@andyb66
tks, i will try to get the garage to do this, though i am not sure he knows how to go about this. here in germany (where i work) the situation is that ford officially is not servicing LR anymore, and my usually LR garage is unwilling to touch it - they started off with ''new ECU'' and i don't believe it - three yrs ago for a failure in the air-spring back left side i had to buy a new ''computer'' for 800 euros and then after no improvement they found a little hole high up on the rubber sleeve and charged another 500 for changing that. the situation in the netherlands (where i live) is worse - no LR anymore for 250 klicks :-(
do we need any special equipment from LR to pressure check the coolant system ?

Alwin said:

do we need any special equipment from LR to pressure check the coolant system ?

Click to expand...

No it should be standard kit for testing cooling systems.

@andyb66
tks, spoke to mechanic today again and he'll pressure test the cooling system as soon as the new thermostat and the temp sensor are delivered, wich should be middle of next week

@ratty
today, car standing more than 24 hrs, removed coolant cap to top up 3/4 quart of water, hissing sound and rising level in coolant reservoir - that means coolant system has pressure, doesn't it ? drove to customer, temp gauge at 25 % for 5-6 min, then moves up in one smooth motion to 50 %. stayed under 2500 rpm, so no problem heating up. total ride took 20 min and i checked the top coolant pipe immedeatly, it was firm, but i wouldn't say rock-hard, no problem pushing a thumb into it.
your cracked head description doesn't sound good - maybe mine has a smiliar fault, i fear.
when i recall the whole story the engine DID overheat (or i believe so) once last december after a two-hour-long motorway trip, although of course i stopped at once and had the car towed to the garage, not having any idea what went on. they topped up coolant (i dont know how much) and checked the engine, wich was fine (in town, under 2500 rpm), and the search began, resulting finally in the discovery of the non-functioning viscose fan. but something seems to be damaged as the fan went initially ...

@all
mechanic will check price for waterpump (he has had some bmw's with similar problems, originating in defect pumps) and order it with sensor and thermostat, and we'll go from there.
I keep ya up to date, and really appreciate your help ))

Alwin said:

@andyb66
tks, spoke to mechanic today again and he'll pressure test the cooling system as soon as the new thermostat and the temp sensor are delivered, wich should be middle of next week

@ratty
today, car standing more than 24 hrs, removed coolant cap to top up 3/4 quart of water, hissing sound and rising level in coolant reservoir - that means coolant system has pressure, doesn't it ? drove to customer, temp gauge at 25 % for 5-6 min, then moves up in one smooth motion to 50 %. stayed under 2500 rpm, so no problem heating up. total ride took 20 min and i checked the top coolant pipe immedeatly, it was firm, but i wouldn't say rock-hard, no problem pushing a thumb into it.
your cracked head description doesn't sound good - maybe mine has a smiliar fault, i fear.
when i recall the whole story the engine DID overheat (or i believe so) once last december after a two-hour-long motorway trip, although of course i stopped at once and had the car towed to the garage, not having any idea what went on. they topped up coolant (i dont know how much) and checked the engine, wich was fine (in town, under 2500 rpm), and the search began, resulting finally in the discovery of the non-functioning viscose fan. but something seems to be damaged as the fan went initially ...

@all
mechanic will check price for waterpump (he has had some bmw's with similar problems, originating in defect pumps) and order it with sensor and thermostat, and we'll go from there.
I keep ya up to date, and really appreciate your help ))

Click to expand...

yes the hissing means it's pressurising. Have you had a headgasket changed on it in the time you've had it? They had nylon dowels which were prone to distorting causing head slippage. These were later changed to metal dowels. If you've had this behaviour since december I would prepare yourself mentally for headgasket or cracked head. If it turns out to be something less that's great. Judging by the strange behaviour it wouldn't surprise if he head was cracked or head gasket was blown.

A waterpump for a TD5 is about £25. I believe that they do some cast ones (although I've never actually seen one) which are supposedly better than the pressed steel ones but are pretty hard to find. I replaced mine with a pressed steel one back around xmas.

The cast impellor water pumps were fitted to pre 2001 TD5s. When they changed the engine to EU3 spec (ie. EGR has a cooler and an additional butterfly valve) they started fitting the cheaper water pumps which are prone to cavitation damage and rusting.

@ratty
Nope, the head hasn't been touched in the 8 yrs i own and love this disco, and its unlikely it has come off in the 2 yrs before, though i don't know for sure.
since its likely the engine overheated, i guess then the nylon dowels are f....d up :-(
Hmmmm, OK, let me be honest - i'm pretty fair with computers and programming them, but i'm at a loss with engine details - i had to google what a dowel in a engine head exactly is.
we'll try that filling up water from the heater matrix on saturday, tks for the tip. in another post here i read that happens sometimes, no water in the heater and therefor no heating in a disco 2. i was lucky getting the maintanance/service manuals as pdf's and now it should be easier for the mechanic to find his way around ...

@shifty1962
tks for joining this ;-) my disco was built before may 2000, so i keep the old waterpump in case it isn't defect and may be better than a new pressed one, right ?

@all
i drive her every day (gotta work) and the engine runs just fine keeping it under 2500 rpm, making me crazy to think the head may be cracked or the gasket gone and wondering what additional damage is done with every mile.
The parts should be here wednesday, thursday and then we'll see if i got lucky or not.

Alwin said:

@shifty1962
tks for joining this ;-) my disco was built before may 2000, so i keep the old waterpump in case it isn't defect and may be better than a new pressed one, right ?

Click to expand...

Not necessarily any better than a new one but it will last a lot longer because the impellor wont rust. They are not that hard or expensive to change so I would not lose any sleep over fitting a new one if thats all you can get. Keep the old one as a spare but if its OK dont bother changing it.

Hello all,
unfortunately no spare parts yet :-(