No pressure in cooling system

[31satara]

I've just replaced the cylinder head gasket on a 1998 TDi. Upon initial start up all seems fine. I ran it upto operating temp and it sat at that for several minutes. I switched off to check coolant levels. There was absolutly no pressure in the system which I thought odd. I ran it for another 10 minutes - again all seemed fine. Top hose was hot, bottom hose warmed up, but still no pressure at the expansion tank. (I had checked the thermostat opened in hot water when I had that out). I ran it with the coolant cap off. There didn't seem to be any coolant movement in the tank, and with my hand over the top no pressure at all - so I assume it's not to do with the coolant cap. It was getting dark so I stopped!
Not being familiar with the engine I would be grateful of any suggestions. How long should it take to build up some pressure in the system? Could it be the water pump not circulating the water? But if this was the case shouldn't the engine start to overheat and the temp. guage go up? What further tests should I do to try and find the reason? (I didn't replace the water pump because I was told it was replaced a few years ago).

[Lex Luthor]

The only source of pressure in the coolant system is the thermal expansion of the coolant.

If the coolant is already hot then sealing the system will not generate pressure. you must thermally cycle the system to see the pressure build up. Leave it overnight and then warm it up fully and you should have pressure and the best way of checking that is to test the firmness of the large coolant hoses.

Have you bled the system to remove any air from the high point at the heater matrix inlet ?

[31satara]

Thanks for the reply Lex. I'll try that. How long should it take to warm up? (Bearing in mind it can't be driven - no bumper etc and SORN)
After flushing I refilled the system as per Haynes manual which I assume bled the heater matrix - would the fact that it's got A/C make a difference to this?

[Lex Luthor]

Thanks for the reply Lex. I'll try that. How long should it take to warm up? (Bearing in mind it can't be driven - no bumper etc and SORN)

As it is a tdi..a long time if you cannot drive it.

After flushing I refilled the system as per Haynes manual which I assume bled the heater matrix - would the fact that it's got A/C make a difference to this?

So you raised the header tank as high as it would go after unfastening it and partially pulled off the heater hose to expose the bleed hole till coolant ran from it ?

[31satara]

I did fill it as you describe.

Last time I ran it for maybe two sessions of 5 minutes taking the cap off to check levels etc. Then replaced cap and ran for maybe 15 mins. Again no pressure. It ran for a few more tries but as you say, maybe it needs a much longer time.
When fully warmed up should both radiator hoses be hot, should the radiator be warm/hot, and should water in the header tank be hot? If I remember correctly the top pipe was very hot, the bottom pipe hottish but the radiator and header tank were not.
I'll try again when I get home.
Thanks a lot for your help.

[Lex Luthor]

I doubt you will get a tdi hot enough to test the system without driving it and loading the engine up. Diesels are very efficient when idling.

[Col]

Totally agree with Lex.

At idle the TDI simply won't get hot. Take it for run for at leat 15mins.

[31satara]

You're quite right. Took 15 minutes for the needle to start to rise. By 25 mins the radiator (not viscous fan side) was a bit warm. The pipes felt a little firmer. By 40 mins slightly warmer. I'd run it a bit faster now and again but not that much. Obviously very efficient cooling. On removing the cap I did get a tiny little hiss so I'm assuming all is ok on the cooling front.

Now I need to research about the smoking. It seems to produce rather a lot of whitish smoke, especially when revved. I know the smell of oil smoke - it's not that. It's also not steam. Something to do with the diesel I guess. (I've never had a diesel, and don't how much smoke this car used to produce - it belongs to my brother). I may well be asking for further advice.......
Thanks guys.