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mgb66 · 31st July 2021, 20:16

Having spent a good few hours reading all the posts on this subject I hesitate to write another one on this thorny subject. However I'm at a bit of a loss and don't want to go off in some wildy wrong direction with the overheating problem I'm having.
My ZT-T had been running fine until a week or so ago the temperature guage started fluctuating upwards, and then went all the way up with the red light showing and ENGINE OVERHEAT coming up on the trip computer.
I must admit I do not check the coolant level often enough, especially as when I do, the header tank always seems to require some topping up.
In fact I do always carry some coolant in the car just in case.
On this occasion it took at least a litre and a half.

However on my next trip out, the temperature guage went into the red after a mile or so.
I decided to check everything as best I could and particularly the thermostat and associated pipework. Having taken the engine cover off I had a good look in the V and although I could see some liquid it was more oily than evidence of coolant. I cleaned this up and it has not returned. The area immediately by the thermostat was completely dry.

Given that air locks seem to be an issue I decided I had better go through the recommended procedure for filling the system.
With the help of a neighbour I duly re-filled the system according to the Haynes manual. Two concerns during this process were that as soon as the engine started the expansion tank overflowed, plus the engine cooling fan did not come on when the temperature guage was indicating normal temperature.
I let the engine cool down and took the car on a short drive. Same story. At about a mile the temperature guage shot up and the red light came on.
I thought that the re-filling procedure might have been done incorrectly so I decided to do it again. I asked my son to help and following the procedure he kept the revs up until the fan cut in.
I then left the car for an hour and checked the expansion tank level which was a bit high. I syringed about 300mml out to just reveal the ribs.
I waited another four hours or so and took the car for a short drive. After a mile the temperature guage once more shot up and the ENGINE OVERHEAT warning appeared.
I was on a dual carriageway and so gingerly drove to somewhere I could stop with plenty of steam from under the bonnet and evidence that the cap was letting out coolant. After half an hour or so I filled the tank with about 2 litres of coolant ( I've got through about fifteen litres so far in this sorry tale) and limped home.
I have checked again and again for any sign of leaks but connot see any. The water pump was replaced along with the cambelt at 97,450 miles and the car has now done 107296. The thermostat is a plastic one and was replaced in 2017 at 90220 miles.

The oil on the dipstick is clean and there is no emulsion under the oil filler cap and no sign of steam from the exhaust.
I have not been able to check the engine temperature via the trip diagnostics as I daren't run the engine long enough.

Given loss of coolant seems to be via the expansion tank pressure cap and there is no evidence anywhere else of leaks I would be grateful for any thoughts. I'm wondering about a failed water pump or a sticking thermostat.

31st July 2021, 20:16	#1
mgb66 Regular poster MG-ZTT Join Date: Aug 2018 Location: London Posts: 41 Thanks: 5 Thanked 7 Times in 4 Posts	KV6 190 persistent overheating Having spent a good few hours reading all the posts on this subject I hesitate to write another one on this thorny subject. However I'm at a bit of a loss and don't want to go off in some wildy wrong direction with the overheating problem I'm having. My ZT-T had been running fine until a week or so ago the temperature guage started fluctuating upwards, and then went all the way up with the red light showing and ENGINE OVERHEAT coming up on the trip computer. I must admit I do not check the coolant level often enough, especially as when I do, the header tank always seems to require some topping up. In fact I do always carry some coolant in the car just in case. On this occasion it took at least a litre and a half. However on my next trip out, the temperature guage went into the red after a mile or so. I decided to check everything as best I could and particularly the thermostat and associated pipework. Having taken the engine cover off I had a good look in the V and although I could see some liquid it was more oily than evidence of coolant. I cleaned this up and it has not returned. The area immediately by the thermostat was completely dry. Given that air locks seem to be an issue I decided I had better go through the recommended procedure for filling the system. With the help of a neighbour I duly re-filled the system according to the Haynes manual. Two concerns during this process were that as soon as the engine started the expansion tank overflowed, plus the engine cooling fan did not come on when the temperature guage was indicating normal temperature. I let the engine cool down and took the car on a short drive. Same story. At about a mile the temperature guage shot up and the red light came on. I thought that the re-filling procedure might have been done incorrectly so I decided to do it again. I asked my son to help and following the procedure he kept the revs up until the fan cut in. I then left the car for an hour and checked the expansion tank level which was a bit high. I syringed about 300mml out to just reveal the ribs. I waited another four hours or so and took the car for a short drive. After a mile the temperature guage once more shot up and the ENGINE OVERHEAT warning appeared. I was on a dual carriageway and so gingerly drove to somewhere I could stop with plenty of steam from under the bonnet and evidence that the cap was letting out coolant. After half an hour or so I filled the tank with about 2 litres of coolant ( I've got through about fifteen litres so far in this sorry tale) and limped home. I have checked again and again for any sign of leaks but connot see any. The water pump was replaced along with the cambelt at 97,450 miles and the car has now done 107296. The thermostat is a plastic one and was replaced in 2017 at 90220 miles. The oil on the dipstick is clean and there is no emulsion under the oil filler cap and no sign of steam from the exhaust. I have not been able to check the engine temperature via the trip diagnostics as I daren't run the engine long enough. Given loss of coolant seems to be via the expansion tank pressure cap and there is no evidence anywhere else of leaks I would be grateful for any thoughts. I'm wondering about a failed water pump or a sticking thermostat.