Replacing buff header tank with black.
 

Simple question for someone that knows; tried searching but failed. Bought a PPD black header tank to replace my buff one on 2001 CDT. The black one appears to have same pressure rated cap 140 but has an overflow pipe that the buff one does not and different min/max level markings (also a fitting on the base that appears to be redundant). Is that an issue with overflow pipe or still a straight swap? Many thanks.

The differences don't affect anything. The later buff tank vents any excess pressure from under the cap itself. The same cap fitted on the black tank will do the same. The 'overflow' pipe is simply to direct any venting down behind the readiator. Whether the earlier vent system used a slightly different cap design I'm not sure about. I suspect it may have. The fitting underneath the black tank was to take a coolant level sensor. For the 75/ZT system, that never actually made it off the drawing board unfortunately. It explains perhaps why the diesel warnings screen includes a low coolant icon (which also didn't get off the ground). So to answer you basic question, yes, it's a straight swap using the same (140) pressure cap.

TC

I've often wondered how does the overflow pipe work? I've got one on the PPD black expansion tank on my ZT. Now when I had a leaking cap on my bottle the coolant would leak out the cap with the pressure, so what stops the pressure from pushing the coolant out the overflow?

Not sure who did it but someone on the forum dissected a rad cap to see how it worked and also gave a full explanation about it, Sorry cant think who it was:shrug:

Twas T-Cut I believe:cool:

The overflow pipe connects to the annular space inside the filler neck. The current pressure cap is designed to direct a disharge up into the handwheel and release it from under the finger grips. I believe this will also happen when the earlier black tank is used with the current cap. Considering how the cap is constructed, I can't see that anything would be directed through the said overflow pipe. If that's so, then I have to assume that the earier pressure cap (designed for the black tank) was slightly different in the way it relieved pressure. If it was, I've not yet seen one. On the other hand could the overflow design be just another feature of the black tank that never actually functioned?

Much more detail here: https://the75andztclub.co.uk/forum/s...d.php?t=271894
TC

Steve, could you post a photo showing the inner neck of the tank? I'd like to confirm how the 'overflow' pipe is connected internally.
Cheers,
TC

I'll try :)

Original pressure cap
 

T-Cut,

I am still using my original pressure cap (marked '100') with my original black tank. On a few occasions in the past I have seen a few drops of antifreeze residue on the hose which lies directly beneath my tank overflow. I have never seen evidence of venting on the tank itself. It therefore seems that your assumption may well be correct.

If my cap ever fails I could offer it for dissection, but at present it's still going strong (with replacement 'O' ring seals) after 21 years service!

Simon

Simon, I've just checked my notes and the samples I have left from the earlier dissections. I actually have a 100 cap that I'd forgotten about. The HowTo does include some detais on it and basically, the design is consistant throughout the pressure range. On that basis, the overpressure venting takes the path into the handwheel and out from under the finger grips. So, the exact function of the 'overflow' pipe is debatable. It simply drains the annular space between the outer neck of the tank and the tapered inner retainer for the valve housing. This space is open to atmosphere when the cap's in place, so any excess pressure venting into that area won't take a path through the pipe. It'll go into the handwheel and out. So,my take at the moment is the 'overflow' pipe is exactly that. It drains any coolant that's spilled from the filler neck when topping up.



TC