Now before you all laugh at my noobishness.

I have scanned the shed and it shows no fault codes.

But when travelling slow the temp needle drops to about 80 slowly then as I give it a little right foot it creeps back up too 90.

Very random really

Any ideas ....Dashpod maybe or thermostat on the blink ???

Clean and test the thermostat mate? a kettle of hot water should do the trick?

Any chance of a blockage in the cooling system anywhere? when was the coolant last changed?

Sticky thermostat, probably. The action of the needle is damped to screen out hysteresis in the system, and transient external factors like gusts of cold wind on the radiator (not the best example, but all I could come up with!)

When you're travelling slowly and at low revs, the engine obviously doesn't produce heat at the rate it does when you're travelling fast and at high revs. Normally, between the action of the 'stat and the damping in the gauge, the needle should stay permanently at 90 once the engine's warmed up. But if the 'stat is staying open longer than it should (especially if the fans are on when you're moving slowly), then the temperature of the coolant may drop sufficiently below 90 for enough time to register on the gauge.

In the Haynes, it goes on about testing the 'stat with some boiling water and a thermometer, but it's up to you whether you do that or just put a new one in and take things from there...

See ..... you took the words right out of my mouth with all that hysterectomy of the system stuff ........ good man yourself

Sticky thermostat, probably. The action of the needle is damped to screen out hysteresis in the system, and transient external factors like gusts of cold wind on the radiator (not the best example, but all I could come up with!)

When you're travelling slowly and at low revs, the engine obviously doesn't produce heat at the rate it does when you're travelling fast and at high revs. Normally, between the action of the 'stat and the damping in the gauge, the needle should stay permanently at 90 once the engine's warmed up. But if the 'stat is staying open longer than it should (especially if the fans are on when you're moving slowly), then the temperature of the coolant may drop sufficiently below 90 for enough time to register on the gauge.

In the Haynes, it goes on about testing the 'stat with some boiling water and a thermometer, but it's up to you whether you do that or just put a new one in and take things from there...

Cheers.

Only problem with testing the stat is ...........

On the Octy its in a right bitch of a location under everything engine wise. I see a trip to the garage coming on.

There some bits of my water pump left in there but that was three years ago. I wonder ???

See ..... you took the words right out of my mouth with all that hysterectomy of the system stuff ........ good man yourself

it could ba a water pump seen this happen before. if the other things dont solve it its a pump

so Dave, your pump went before? and there are bits stilll in there? that might be the prob then,

If the engine isn't getting up to temp, how is that a water pump issue? If the WP was failing then I imagine you'd be overheating, not running cool... Either way, I had exactly the symptoms you describe, the Octy would struggle to get up to 90deg especially if I hit the motorway before fully warming up the motor. Coasting down from speed (sliproads especially) would see the temp drop to 80deg. Hard use would see it hit 90deg for a while. Changed my thermostat a couple of weeks ago and it now sits at 90deg all the time. To be honest I couldnt actually see any problem with the stat that came out, but because you have to substitute them quickly to avoid losing much coolant you don't really get chance to compare the old vs new side by side... It's not that hard a job either, just fiddly. If you accept it will take an hour or so and take your time it's very straightforward. It's only 2 bolts fer chrissakes. Have a look here for a how-to. I didn't remove the dipstick like he shows, but I did remove the wires to the alternator. It only lost about 1/4 litre of G12 coolant and cost me less than 30 quid all in. http://forums.audiworld.com/showthread.php?p=16925102

I didn't think it would be the pump for the above reasons.

It doesn't look as bad as I thought tbh I had heard it was quite awkward but that looks easy enough just a little tight, but my symptons does sound just like yours.

New stat it is then, that said it ain't doing it now it just take a little longer too reach 90 atm.

Strange .................

If you're first few miles of driving is in stop/start or low-speed 30/40mph zones you won't notice too much difference with a failing stat. However, jump on the motorway within a mile or two and it will take an eternity before it gets up to temp. This is all costing you mpg as well btw. Honestly mate, if you're half decent with the spanners its a piece of cake. Yeah it's awkward to get at but if you have some 1/4in extensions and an angle drive or two, it's a doddle.