Hey guys I have been trying to troubleshoot what's causing I assume a vacuum in the cooling system...

Some months ago I installed a new aluminum Koyo radiator along with brand new OEM upper and lower radiator hoses, a Mugen low temp thermostat and fan switch, and the matching size Mugen radiator cap.

Since then - I notice that when the car cools down the radiator hoses are shrinking/collapsing and if I pop the cap open, air gets sucked in and they expand back to normal size. The thing is, I've bled the system several times and while running it appears to be full - there is even a bunch of coolant in the overflow tank.

I checked the Mugen radiator cap and it doesn't seem to be stuck closed either. At the same time I did this I also swapped to a bored-out intake manifold and throttle-body and I have noticed that sometimes when letting off the throttle, idle will stick slightly high (like 1500 rpm) unless I blip it so I'm not sure if that's related to the situation.

What might be going on to cause this?

 

swapped a coolant and vacuum line somewhere.

 

wouldn't that suck antifreeze into the engine though? it's running fine and doesn't seem to be 'losing' any antifreeze... and it's not pulling anything out of the overflow tank either. I thought that maybe the cap was malfunctioning but I can squeeze it open with one hand so I don't think it's stuck closed...

 

Try purging the air again

 

I think this is caused by your cap not letting coolant back into the radiator.

Are you sure your cap works with that radiator? Try a different cap...like a Koyo cap. The Mugen cap may still be blocking off the overflow port when everything cools off and now allowing coolant to get sucked back in from the overflow.

The coolant is supposed to flow into the expansion tank when it gets hot and expands...and flow back into the radiator when it cools down enough.

If there's vacuum, it means the second part isn't happening. When you open the cap, air is what gets sucked in to fill the vacuum, rather than coolant.

You're introducing air every time you open your cap, in your case.

 

Thanks! yeah that's what I think too.... I got the Mugen cap that matches the size of the filler neck on the Koyo rad but I wonder if maybe the spring is too strong or somehow it is becoming stuck and like you said not opening to allow coolant to be sucked back in from the overflow - since that tank stays full even though the main hoses are collapsing from an internal vacuum.

I'll order a Koyo cap and see if that fixes at least that part and report back! on a side note there, for a weekend street car I guess there's not really a benefit to having that higher-than-stock pressure radiator cap right? since all that does it increase the pressure in the cooling system to increase the boiling point of the coolant?

 

These cars are super cold blooded from the factory (relatively speaking). So I don't think any of your cooling mods will have much benefit unless you're enduro racing it. It will just take it longer to come up to temp....so make sure you're not short tripping or long idling it.

 

oh yeah for sure - I only swapped the radiator in as a preventative measure - since the factory radiator has plastic end tanks. No plans yet to do anything crazy with it - just kind of minor tweaks here and there since all I do is cruise around on weekends or go on occasional spirited canyon runs.

 

Cool. The Koyo cap is 1.3 bar. Not sure what the Mugen cap is. Maybe the same?

Did you buy the Type F Mugen cap?

 

yeah i made the mistake of getting one of the fullblown rads way back when. not a good idea for a barely driven weekend car.