VN Cooling System problems

[VNHeartache]

All, I need some help with an old VN Exec with a S2 V6 in it that keeps loosing coolant and giving me overheating issues. It's a bit of a saga, but in short it was loosing coolant and running too cool so I put in a thermostat. Then it was still loosing coolant and ran rough when cold so I changed the inlet manifold gaskets. I bled it all up and it seemed fine but then the temp went high without the engine overheating before coming back down. Holding pressure so let it cool, low on coolant again. Ran it, bled it, did it again so took it to Natrad man. Put in a new radiator, bled it and it was good for about 3 days when the temp guage went over high before coming down when the cap let the coolant go. Took the middle out of the new thermostat and bled it up again. Now it runs cool of course but still seems to be loosing coolant. I thought possibly the new thermostat was no good? Maybe the inlet manifold is still leaking (didn't have it machined) or is there something I missed? I want to put a thermostat back in but can't be confident it won't overheat again? Holds pressure really well (still had pressure today after 9 hours not running)? Any ideas?

[Not_An_Abba_Fan]

Losing coolant from where? Are you checking the reservoir or the radiator? After the series 1 when they went to an expansion bottle they had a habit of getting an air lock and blowing the coolant into the reservoir then out the overflow and then as it cooled it would suck what was left in the reservoir back into the radiator. When you bleed it, cut the bottom off a 3L coke bottle (or what ever plastic bottle you can find) and put it upside down into the radiator. Fill that with coolant and bleed the system. The level of the coolant in the bottle is higher than the bleed screw so all the air should come out.

[savage1987]

Can't offer much here but I will say that running the car without a thermostat will ruin your fuel economy.

Find out where it's leaking from mate, get in there with degreaser or have it pressure cleaned by a detailer if you're really keen, then keep topping it up and it'll be easy to spot a leak.

[taskmaster]

loss of coolant could also be a head gasket although its not a common issue with the buick it seems to be the season for blowing vn head gaskets latly ive seen around 4 post regarding head gaskets in the last 3 weeks eeepp

although it seems to be keeping pressue ( 9hours wtf ) i would get a radiator pressue and cylindr compression test done ... but to start with just go have a good look at all your hoses and make sure one doesnt have a small hole / leak

[VNHeartache]

Thanks for your feedback guys. Logically, I'd have thought that if it had a leak it wouldn't hold pressure for so long (held it for over 12 hours yesterday) but I've not seen this in a car before and I've been around a while? Checked it thoroughly for leaks anyway and none to be found. When we checked it yesterday it was under a bit of pressure but there was coolant at the cap so I reckon it's still full? If it had done the head gasket, it would get hot every time and as taskmaster says, it's not common on these motors so I don't think it's that. The recovery tank doesn't gain or loose coolant so it could be a dodgy cap too I guess? I have had it under pressure with a tester and it comes up OK. I'm thinking that now I've bled it up using the bottle trick, it may have been the new thermostat as has been known to happen (fortunately it's running on gas so fuel economy is not such an issue? a thought - could it be the gas that's keeping it under pressure through the diaphragm in the converter, even though it's a completely new system?)