I seem to have a problem with my Header Tank on my 940 Turbo 2 litre. I have checked all the hoses in that area and they are all sound, and the tank itself seems to be OK.
I replaced the cap on it recently because the original (grey)
cap had spilt around the top.
The one I got to replace it is a Black cap which I didn't worry about at the time, but ever since then I have been losing coolant. It seems to be blowing from the header tank, as the inner wing of the car down by the side of the turbo is always wet after a run, but I can't get it to blow it out in the driveway, so it must only be doing it when I'm actually driving. At the end of most 20 mile journeys I have to put in about a pint or more of water/antifreeze. This is getting pricey!!
The grey cap has 150 kPa stamped on it. The black cap has 75 kPa stamped on it.
Not knowing what these stamps mean, could this be my problem ?
I got my old cap (the grey one) and put some heavy duty sealant on it overnight and it seems to keep the coolant in. If this is the problem, then I'll get a new one. Can anyone give me some advice please.
Thank you in advance. This is becoming a real pain.

Right: I've sorted this particular problem so here's the solution for anyone unfortunate enough to have the same problem.
It is ESSENTIAL that the cap is the one with 150 kPa stamped on it.
The black cap (with a rating of only 75 kPa WILL NOT replace this original spec cap.
The bottle was fine, but I was rather surprised that my local Volvo dealer told me that now there is only one cap which fits all models!! This is totally wrong.
Having rung another (much bigger) Main Dealer in a major city, I was told that ONLY the correct cap would solve the problem. I tried it with a black cap and it still lost coolant, but with the grey (150kPa cap) all is well again.

Thanks to all of you who helped me to look at a few things that I might have missed. I didn't think it was the head gasket as it is still running beautifully. The higher pressure rated cap has solved the problem. The original one had developed small cracks all around the top.

Your car is designed to work with 150kPa caps, so that could be the problem. As Brian notes, take a close look at the expansion tank connection around the hose, which can crack and leak.

I think there's a good chance your expansion tank may have a small crack, or there is a leak where one of the hoses attaches to the tank.

I had a small crack develop on the top of the original tank on my '93 945T. I ordered a new tank (around $30 US, Volvo part from a mail-order place, FCP Groton)... when I pulled the old tank to replace it, the plastic nipple at the bottom of the tank fell apart, thus:



Check the FAQ for more info on this failure, and other troubleshooting tips. Once you have your leak fixed, you might want to consider fitting a low-coolant-level sensor & warning light, as described in that section of the FAQ.

As far as the cap goes, you should be OK with the new black one.

- Brian '93 945T