Duratec (2.0 Caterham): coolant tank, level?

[Austin David]

Hi!  2021 Caterham 360, Duratec 2.0.  I think it's the same coolant tank as the 420, a pressurized tank up and behind the radiator.  

 

My car seems to want to push a little coolant out the top of the tank, past the cap.  If I top it up to the min/max level at rest, and bring it to temp, it runs just fine and the level may rise a little, but mostly seems to stay around that max mark.  Add a thousand miles and all of a sudden the bottle is mostly empty when cold, and there's residue around the cap.  No other leaks on the floor or anywhere I can spot, but clear wetspots on the bottle and below it.  The cap remains tight, but there's also residue under it -- clearly a little leak / blow-by.  The leak is on the "low" side, RHS.

 

It doesn't take much to top it back up.  The motor is running fine, temps are cycling normally, and I have no reason to think it's throwing much more than "a bottle's worth" of coolant.  I've repeated this cycle 3 times now, and my only interesting coolant loss is right under that cap, ever time.

 

Is that sort of normal?  Do I have a bad-sealing cap?

[MV8]

Any pressurized system needs to expand when heated. There must be enough air in the system to allow the coolant to stay in the system when hot, compressing the air. I suggest retesting with the coolant "topped" to the mid point between max and min. If there is still coolant loss, consider a pressure test for the system to check the caps (done cold); basically a hand pump and a fitting adapter.

 

If you need to fill to max or completely fill for more coolant volume, consider adding an unpressurized expansion catch tank connected to the small 1/4 inch port near a cap in the system so when it vents, it will flow into the tank. The hose extends into the bottom of the tank and stays covered at all times with coolant, so the system will siphon the coolant back into the system as it cools so no coolant is lost.

 

 

[Austin David]

in other similar cars (like a 2015 NC with ~ same motor) the tank stays about half full when cold.  I guess I'm asking if it's normal for the tank to be mostly-empty when cold, and those marks on the side are because this is an off-the-shelf part that makes more sense in a Focus or something.  

[KnifeySpoony]

Coolant level in my 420 fluctuates a bit cold to hot as expected but I am not losing any coolant over time.

[sf4018]

I'm sure you've done all this already but make sure lid clicks during tightening, check hose clamps are tight, remove the top bolt of the radiator to make sure there's no air trapped, and make sure the cooling fan switches on before you hit the red line on the temp gauge.

[Austin David]

Yup.  Almost 6k miles by now, it's great except for the coolant seemingly leaking out around the cap.  It does click tight, there's no air in the system, etc.  My modine connections were the last to really seal up well, about a year back.  

[jbcollier]

+1 on checking that the cap is tight.  many tighten in two stages so press down while you turn.  I know you probably know this but do check to be sure.  If it is tightly closed, then it sounds like either a failed cap or a damaged sealing surface on the expansion tank.

[Austin David]

An aftermarket cap costs less than $10, I'll try that first.

 

Tha ks everyone for helping confirm that the cap is being weird and I'm probably not just overfilled.

[Austin David]

For the record it's a $8.49 cap (new) from a 2014 Mustang.  I'll know in a month if it helps 

[Austin David]

FYI I'm still struggling with this.  It seems to not leak if I keep the RPMs down below 4-5k.  If I'm enjoying it more, it pushes out the top more.  I have not tried to do that without the bonnet on, so I'm not sure if somehow all the coolant is overflowing the tank or what.  But the volume does appear to be coming out around the cap (the NEW cap), and if I don't refill, it stops happening.  Like if the cold level is basicaly "no coolant in tank but definitely in engine", it overflows less or none. 

 

So it feels like the expansion tank might be too small, but that seems crazy to me.  This persists over very many heat/cool cycles over many miles; I'm over 13k now.  it's not a HUGE problem but I do top it up with a few pints, less often than I refill the fuel tank.

[MV8]

Pressure test the system cold and see what vents first and at what pressure. Is this the only cap on the coolant system or do you also have a traditional type radiator cap with a vent port? Systems specs vary so post a pic of your system components/plumbing. The opening pressure is usually indicated on the cap.

 

EDIT: If you don't know what adapter may be required, the full set is not that expensive and you can use it on any car.

https://www.ebay.com/itm/335869269586?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D284064%2C283795%2C284136%26meid%3Ddef62f6199f34ed6a04e1e503ba48a24%26pid%3D101875%26rk%3D4%26rkt%3D4%26sd%3D186850068386%26itm%3D335869269586%26pmt%3D1%26noa%3D0%26pg%3D2332490%26algv%3DSimVIDwebV4WithCPCExpansionFastIMARanker032025eCPM&_trksid=p2332490.c101875.m1851&itmprp=cksum%3A335869269586def62f6199f34ed6a04e1e503ba48a24|enc%3AAQAKAAABMKaEcTHAUF%2B%2FFR4OHhWiUku5hIsQWp7fwGDMMkUVoKldyxhxN25%2BWKNssJUKVG9e02gVRepi30iCbg3e%2BnXeMjnUHvdoXWXLAvVMinoZNsPbKkbwJkpH2m5ocpYMWMP9naH87cAEiE%2F7DrwrbQexSmvRYaAKB%2FPREBgpqn%2Bs6SAOtQn09cDdeK7v3Jqp5Mu0BDJH09Tu7PjhU5FX1%2FDexGS3Fx68C82FgygWIjklC4dgrSc0RwCm2%2FLtGnGa%2B2Iuq1q6J1pE3sZc8FFiW%2BQs97VmhwYx0aRhUxQt1jtO%2F9PzeSt7m3oBgy1LpEpUoveua1D%2F0wMU4HG9eD1Y%2FJVB7mrWoc%2BDpwQtUV07mV0bXl7aIIML1Xcf0fvOCGJjr9BgWU7Z%2FqBTl1twqEwxUESN4Xs%3D|ampid%3APL_CLK|clp%3A2332490&itmmeta=01JRGY6S64RAX3V0XV5799M2PQ

Edited April 10 by MV8

[sf4018]

Have you checked for any nicks or scratches around the top edges of the tank where the cap screws down to?