928 coolant overflow

[TeeSee]

Hoping for some advice on my 1987 928 S4. I just returned from a short drive and as i pulled in the drive stream started coming out under the bonnet.  I opened it up to find a lot of coolant overflowing out of the filler/reservoir.  After it cooled down i checked and noticed the neck on the filler turns independently of the bottle and it looks as though it was coming out through there.  I've heard it's common for these resevoirs to crack but I'm still concerned at the volume of coolant that was coming back out through it.  The car was just at normal operating temp and the fan only kicked in as the car was turned off. 

I guess my question is,  does the crack allow a high volume pressure release or is it more likely a cooling system issue?  

Any thoughts or advice would be greatly appreciated  ☺

[TeeSee]

Update.  I've realised the neck is independent of the container so i don't think it's cracked so it looks like it was coming out around the cap itself. After a bit of a play I'm thinking because of how the neck turns as you tighten it i may have had the cap tight but not fully sealed to the point where it hits the metal stopper.

Would this allow the system to pressurize but then still potentially provide an outlet for it to escape from?  

Thanks again. 

[Buchanan Automotive]

If you have an old plastic 928 expansion tank ( made worse if its 29 years old ) then the metal neck is not meant to move at all , it is meant to be bonded to the plastic tank within the tank , if the neck moves then it will not hold pressure , or put another way it will never be able to hold the min pressure required = 1 Bar pressure , and with no pressure you will continue to loose pressure / coolant

PS } we stopped using the 927 plastic tank ( meaning R/H/D) version well over ten years ago , we only use custom made expansion tank/s & we get them anodised black to minimise corrosion at the alloy neck area & on the outside of the custom tank that you will get if they are not anodized 

Regards

Bruce Buchanan

[TeeSee]

Bruce,  thanks so much for the explanation that seems to be exactly what has happened and by the look of the tank it is likely to be the original 29 year old item.   

Do you stock the replacement anodized tanks you mention and are they a bolt in replacement? 

Thanks again. 

[Buchanan Automotive]

I / we do not normally sell "over the counter " as I / we are not geared up for that at all , in fact positively dislike it , we only keep parts( custom made or Genuine ) in stock for our own customers as we can control ( quality control ) the fitment & what else the 928S/S4 needs in the regards( in this case ) to the cooling system .

I can give you more details on options if you email us at work on  } buchananautomotive@netspace.net.au 

Regards

Bruce Buchanan
Buchanan Automotive