Faulty thermostat
 

Hi sometimes when I start car up and let it idle the top hose and the bottom hose get hot as they should then when the fan kicks in it will be on for about 5 minute and all of a sudden the bottom hose will be cold if I rev it the hose gets hot again and the fan quickly turns off at 95 degrees this happens sometimes not everytime i've recently replaced the oil cooler and the lower coolant rail as the cooler was blocked with k seal when I bought the car 12 month ago i have since flushed all the cooling system countless times and replaced the thermostat about just under a year ago. The running temperature is around 90-94 degrees on an open road however driving slowly around built up areas and stopping at traffic lights it can rise to between 98-100 the heater gets red hot. Forgot to say I took it for a run earlier and when I got home I checked the bottom hose and that was hot the fan kicked in and went off in about a couple of minute.
Any advice thanks it's a 2.5 V6

Sounds like an airlock to me, have you tried the correct bleeding procedure?

I lifted the expansion tank up and had the bleed screw removed whilst I filled it I closed the bleed screw before lowering the tank then I started the car with the cap off until coolant started to rise then I put the cap back on got in the car and revved it to 2500rpm until the fan cut in and out twice

There's nothing wrong there Pete; those temperatures are spot-on. If you followed the MG Rover bleeding and refilling procedure exactly then you shouldn't have an air lock. Traces of K-Seal might still be causing you problems however, specifically to the correct operation of the thermostat. You're doing the right thing by monitoring the running temperature.

Simon

I think I'm going to try another bottle of speedflush then flush it again and see how it goes it doesn't overheat it's just that when the fan comes on it takes a while sometimes to drop to 95 degrees and if I rev it it drops quickly and the fan turns off. It seems to stop coolant circulating for some reason although earlier on when I went for a drive and got back the fan came on at 100 rose to 101 before it started cooling down slowly if I revved it it would cool quicker and the bottom hose was warm. Thanks all for advice

As said, there's nothing unusual in your findings. Radiator/fan cooling isn't an instantaneous process, the pump must ciculate hot coolant from the heads to the radiator top and downwards. It takes energy and revving the engine will simply speed that up a bit. There's also a high ambient temperature at present so the cooling efficiency will be lower than in cooler weather. Re-flushing the system will be a waste of time and money in my humble opinion.
Just to be clear, coolant temperatures of 100- 105C isn't overheating. It needs to approach 115C before you're getting into that situation. So while 100-105C seems pretty hot it's well within the normal operating band.

TC

Thanks for your advice I was probably just worried about it