Coolant Reservoir Cap - a warning!

[RonR]

As part of my engine rebuild I replaced my coolant reservoir cap with a nice new yellow after-market one.
Out for a drive the other day I heard a small "Pop" while on a dual carriageway followed by a coolant smell and the temp gauge started to climb. I pulled off at the next junction and found somewhere to park, opened the bonnet and.... Where's the coolant reservoir cap?
It had blown completely off the reservoir and was nestling behind it, coolant had erupted all over my newly-restored engine bay.

Problem No.1:
Here's what I found when checking the cap:

Reservoir cap.jpg

The pressure relief valve had separated, seems it was only held together by push-fit into a small lip, with no glue or any other means of location.

Problem No.2:
The new cap was not a good fit on the reservoir thread, possibly due to both items as the reservoir was replaced with a new pattern part a few years ago. When tightening the cap it would do up reasonably tight then slip a thread.

So what I think happened is: The pressure relief valve failed, lowering the pressure in the cooling system enough to make it boil, which then forced off the cap.
My fingers are crossed at the moment that the head gasket survived and all I need is a new cap that has better valves than the last and fits properly.

So if you've replaced your coolant reservoir and have a nice new yellow cap, please check the relief valve and that the cap is a tight fit!
I'll be plastic-welding my valve when the new cap arrives.

[FunElan]

I had something similar happen with a yellow cap, so I put my original black one back on.

[RonR]

So I'm not the only one!
Had a look today for the original cap, but it didn't turn up in my "Replaced Parts" boxes. I've now ordered a different type which looks like it might be a better fit - should be here in a few days...

[Rambo]

I thought all you LEC guys were aware of the "yellow cap" header tank problem. This issue reared its ugly head years ago.

Also the cheapo header tank problem whereby it blew itself apart at the welded seam

Caveat Emptor.

[chrism]

Thanks for the 'heads up' Ron R. I haven't heard of the 'yellow cap problem'; probably few 'newbies' have.
I'll certainly be checking mine tomorrow - and looking to find my old black cap to keep in the glovebox...

[FunElan]

I tried a new yellow cap after I noticed small streams of dried coolant on my header tank after most drives. I think this is by design - the car always ran within correct temp, but after I shut the car off when hot, the coolant in the system would keep expanding, since there's nothing really cooling it down (and it's a tight hot engine bay).

After swapping to the yellow cap, I didn't get any streams, but like Ron experienced, the cap blew off after I shut the car off and parked it.

I deal with the small dried coolant streams now.

(yes, I've done the exhaust gas test and it was fine).

Stick with black, folks.

[Tuga2112]

I thought all you LEC guys were aware of the "yellow cap" header tank problem. This issue reared its ugly head years ago.

Also the cheapo header tank problem whereby it blew itself apart at the welded seam

Caveat Emptor.

"A few years ago" from you could be anything between 2004 and 2022
I dont recall ever coming across such information. but there has been a time when i was rather dormant in the forum.

although there has been attempts to organise, summarise and categorise information across this forum and wiki, there is a lot of hidden gems that overtime less and less active member are aware of.

ive shared things i learned over the years in multiple places in this forum. but oftentimes they are burried in the middle of other topics. and the project threads, although may appear on the surface as "i done x y and z and now the next thing is A B and C " often have some extremely valuable insights that should really be put into their own threads if not wiki pages.
i have been following RonR and Pinky's projects over the past year and both have information that i think would be worthy of its own wiki page and if possible calling to the attention of the widespread comunity to further investigate possible alternative solutions to the things they come across.

[Tuga2112]

but after I shut the car off when hot, the coolant in the system would keep expanding, since there's nothing really cooling it down (and it's a tight hot engine bay).

i would suggest you check if your recirculating pump works.

we in the UK have weather that is too cold where it never really makes a difference and they seize up from lack of use.

if yours has seized up. you may find it would be working when you switch off the car to avoid the issue you are experiencing.

[muley]

...but

The recirc pump is operated by the same switch that operates the fans, so if the fans are not operating when you stop - the recirc pump won't go

[dapinky]

"A few years ago" from you could be anything between 2004 and 2022

Oh come on now - it was literally just the other week....... a quick search shows Scott talking about it in 2017...... but it was an 'old' subject by then.

I have had experiences varying between "okay" and "bloody poor" with yellow caps on the tanks - biggest thing seems to be that they are not deep enough to have enough thread engaged for a good. strong seal.

I have had "new" caps which just didn't work, and "others" that appear to work, but leak a bit..... however, as I have long-since got rid of the black cap for my car, I have little option but to keep trying different ones.

I gave up buying cheap aftermarket ones, and only but Febi ones - it appears to be okay, but not perfect - and I gave one to Nick when he was complaining about a leaking yellow cap on his car.

On the project car, it still has the black cap - and it is staying!

(Joao, the reason we keep all the 'good' knowledge dispersed is to make us old gits look good when a problem re-surfaces after many years - if anyone could find the answer, we wouldn't look like we actually knew things!!!!) - oh, and it gives us a chance to yell "Why not just search for it?" again

[RonR]

The new Borg & Beck cap that's winging it's way to me looks like it may have a few more threads to engage with the reservoir, it also looks a bit like the original black one.
It's listed as fitting the Vauxhall Nova amongst a plethora of other cars, and the pressure relief is set at 17PSI (1.2Bar) (the yellow ones i've seen are 20PSI (1.4 Bar)).

new cap.jpg

Could anyone confirm the pressure setting of the original black one?

Edit: My yellow cap was a Quinton Hazel FC500 - or at least that's what it said on the box!

old yella.jpg

[dapinky]

The Febi Bilstein 01211 is a 1.2 bar unit...... according to the bumph! (but it is shallow, like the QH one you picture).

Interestingly, Amazon UK say it doesn't fit the Elan.......

[chrism]

I'm fairly sure my black one's an original. It has '120' on it (I suppose the unit is centipascals cPa?)



I'm keeping it in my glovebox as a spare since the newer yellow one seems to be substantially-made and is doing okay atm...

RonR, I reckon the push-fit into a small lip, with no glue or any other means of location should still work IF there are enough threads inside the cap to tighten it down sufficiently.

Perhaps not-enough-threads-to-tighten-it-down-fully might be the issue here?

[Saltire]

I’m running what is definitely the original black cap on the S2. There’s no record anywhere in the service history of it being changed. This one says it’s rated at 120, and there’s no part number on it.

IMG_1927.jpeg

My yellow cap, which came as a spare with the new header tank but has never been used, is also rated at 120, and there’s a part number too. They both appear to be the same depth, although I’ve not had the micrometer on them to check.

IMG_1928.jpeg

IMG_1929.jpeg

I’m sticking with the black one for now - if it ain’t broke . . .

[RonR]

Thanks guys, things are becoming clearer!

Chris and Jonathan, your yellow ones look like they're copies of the black ones whereas my yellow one is a different design altogether, so I think your yellow ones should be OK.

The outer cap of my yellow one is slightly domed, so the push-fit valve retainer has a void above it of around 2-3mm, enough for the spring to slacken and reduce the valve's working pressure if it pops out. To work out how much the pressure reduces by I'd have to revise my O-level Physics!

The one I have on order looks like a similar design to your four, so fingers crossed it'll fit and work properly.

[FunElan]

but after I shut the car off when hot, the coolant in the system would keep expanding, since there's nothing really cooling it down (and it's a tight hot engine bay).

i would suggest you check if your recirculating pump works.

we in the UK have weather that is too cold where it never really makes a difference and they seize up from lack of use.

if yours has seized up. you may find it would be working when you switch off the car to avoid the issue you are experiencing.

The pump works fine. I rarely drive the car in a situation where the fans come on and I have to park immediately. It's usually at the quarter hash or just above. But the temp still rises when it sits.

Thanks guys, things are becoming clearer!

Chris and Jonathan, your yellow ones look like they're copies of the black ones whereas my yellow one is a different design altogether, so I think your yellow ones should be OK.

I have one of these yellow ones that's a copy of the black also, but just a spare, like everyone else

[jgbff7]

...but

The recirc pump is operated by the same switch that operates the fans, so if the fans are not operating when you stop - the recirc pump won't go

Is that right? Yes, the sensor is the same (single wire to sensor on thermostat housing) but the fans and the pump have different fuses and relays. I have had the fans working and the pump not working because of a blown fuse. Would not the same apply the other way round?

But what do I know......

John

[Saltire]

John

The short answer is “yes” (fans not working, cooling pump working).

Let me try and explain, with the help of the wiring diagram:

IMG_1931.png

Assuming all fuses and relays are working normally, and the car doesn’t have aircon.

When the ignition is ON, the cooling fan relay operates, disconnecting power from the cooling pump relay (terminal 30). If the water temperature rises, the temp switch closes and activates the fans.

When the ignition is turned OFF, cooling fan relay terminal 87 becomes live, supplying power to the cooling pump relay. If the temp switch activates in those circumstance, it provides an earth path to the cooling pump relay terminal 85, and switches on the cooling pump.

In other words, either fans or pump can be running (assuming fuses and relays are all OK), but never both at the same time.

If the temperature rises when the car is parked, the pump may still switch on - I’ve had this happen to me.

[RonR]

Had another search for my original cap, but it's hiding somewhere, so here's my temporary solution until the new cap arrives:

1. Plastic welded and glued relief valve retainer in position.
2. This:

Temp Solution.jpg

Heath-Robinson would be proud!

[chrism]

Oh...poor Pinky...how embarrassing for her...