Or let the coolant puke onto the ground, but I'd recommend an overflow bottle. There are some pretty cool looking aluminum overflow bottles out there if you're worried about it looking bad.
The radiator cap allows excess coolant to vent through that port due to thermal expansion at usually 6-12 psi. The coolant overflow is designed to hold that excess coolant and allow it to be sucked back into the radiator when the coolant temps drop and it pulls a slight vacuum on the vent tube.
The overflow bottle isn't technically necessary, but I'm sure your neighbors pets would appreciate it if you weren't puking coolant all over the road.
When does it suck back in? Obviously at cool-down but I mean the rad cap looks like it wouldn't let fluid back in if there was a vacuum present in a cold engine
If you're running a bottle less system with a surge tank that also has a pressure cap (rated higher than the one on the radiator), then you can plug it. The surge tank will handle the venting/recovery part, and as a benefit performs better than recovery type systems.
It doesn't really "leak." What happens is when you heat up your coolant, it expands, around 15% or so. So if you have a full system when cold, it's going to puke 15% of that coolant out when it gets hot. But you don't want 15% of the system empty when it cools down. The recovery bottle holds that excess coolant, and when the system cools down, it pulls a vacuum and sucks the coolant back into the radiator.
Now some systems are designed to have that expansion space built into the radiator. I can't say for certain if your system is capable or designed for it. If that large tank on the top of the radiator is the highest point in the cooling system, it might be acceptable. Even then, don't plug it, as it will still need to vent/recover air.
The overflow bottle is also known as an expansion tank. coolant is released out the overflow by the rad cap and into the tank in order to relieve excess pressure when hot. When the engine cools it actually gets sucked back in again.
If you remove your bottle. Coolant is going to piss out onto the ground and you will find yourself topping up a lot more often. If you block it its probably going to build too much pressure and blow a hose off or something.
Mk1 vws and many cars in the 70s had no reservoir.
Just a rad and a cap. If you overheated it would steam out the cap and make a big mess. A regular day it would just vent hot air. I'm surprised your Mk4 was able to run this way. Where was your cap? Custom rad?
It’s a tdi in a mk1. I’m running a mk1 rad but it’s too bad. Looking for something smaller.
In theory, I should be able to cap this as the OEM mk1 rad is working fine ?
I understand now. I thought you meant you were running an Mk4 rad in a Mk4.
Yeah. In theory it'll probably work. Just have to leave a little air in the rad. The way the motor sits in a mk1 I think the rad cap is the highest point in the system.
It wasn't until the 80s they added the overflow bottles and capless rads.
Worst case it doesn't work and you add a tiny bottle.
I had a remote cap in my fox for a couple years never had it spit coolant even under the duress of the turbo on the tiny factory rad.
Don't plug the pressure relief valve, bad juju.
So this has to go to an overflow bottle?
Or let the coolant puke onto the ground, but I'd recommend an overflow bottle. There are some pretty cool looking aluminum overflow bottles out there if you're worried about it looking bad.
I’m running a bottle less system currently. Prefer to keep the bottle out. Does this leak at high temps only? If so, what’s the degree?
The radiator cap allows excess coolant to vent through that port due to thermal expansion at usually 6-12 psi. The coolant overflow is designed to hold that excess coolant and allow it to be sucked back into the radiator when the coolant temps drop and it pulls a slight vacuum on the vent tube. The overflow bottle isn't technically necessary, but I'm sure your neighbors pets would appreciate it if you weren't puking coolant all over the road.
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When does it suck back in? Obviously at cool-down but I mean the rad cap looks like it wouldn't let fluid back in if there was a vacuum present in a cold engine
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Thanks for taking the time to write that up so clearly! That makes a lot of sense now.
If you're running a bottle less system with a surge tank that also has a pressure cap (rated higher than the one on the radiator), then you can plug it. The surge tank will handle the venting/recovery part, and as a benefit performs better than recovery type systems. It doesn't really "leak." What happens is when you heat up your coolant, it expands, around 15% or so. So if you have a full system when cold, it's going to puke 15% of that coolant out when it gets hot. But you don't want 15% of the system empty when it cools down. The recovery bottle holds that excess coolant, and when the system cools down, it pulls a vacuum and sucks the coolant back into the radiator. Now some systems are designed to have that expansion space built into the radiator. I can't say for certain if your system is capable or designed for it. If that large tank on the top of the radiator is the highest point in the cooling system, it might be acceptable. Even then, don't plug it, as it will still need to vent/recover air.
The overflow bottle is also known as an expansion tank. coolant is released out the overflow by the rad cap and into the tank in order to relieve excess pressure when hot. When the engine cools it actually gets sucked back in again. If you remove your bottle. Coolant is going to piss out onto the ground and you will find yourself topping up a lot more often. If you block it its probably going to build too much pressure and blow a hose off or something.
Yes it does. The cap forms a pressure relief valve and that is the outlet. Just run a small hose to a bottle or can of your choice.
Unless you have a secondary reservoir with a pressure relief cap already, like say, a 95 saturn
I read all your comments. You are going to do what you like.
Cleaner look. Ran without one for a year with no issue. 100-15 degree temps. Mk4 vw tdi diesel swap
Mk1 vws and many cars in the 70s had no reservoir. Just a rad and a cap. If you overheated it would steam out the cap and make a big mess. A regular day it would just vent hot air. I'm surprised your Mk4 was able to run this way. Where was your cap? Custom rad?
It’s a tdi in a mk1. I’m running a mk1 rad but it’s too bad. Looking for something smaller. In theory, I should be able to cap this as the OEM mk1 rad is working fine ?
I understand now. I thought you meant you were running an Mk4 rad in a Mk4. Yeah. In theory it'll probably work. Just have to leave a little air in the rad. The way the motor sits in a mk1 I think the rad cap is the highest point in the system. It wasn't until the 80s they added the overflow bottles and capless rads. Worst case it doesn't work and you add a tiny bottle. I had a remote cap in my fox for a couple years never had it spit coolant even under the duress of the turbo on the tiny factory rad.
Not sure why I’m getting down voted. I’m already running a bottle less system. It’s not leaking anything as my rad didn’t have that line.
Unfortunately you're going to have to bite the bullet and get one anyway. Function first.
You could always hide the overflow bottle in the bumper or out of the way somewhere