Overflow pipe to ground - MOT Failure?

Mudgey · Post by **Mudgey** » Fri Apr 12, 2013 11:09 am

Hello,

I am running my radiator overflow pipe straight to the bottom of the car and I was wondering do I need an overflow bottle to pass the MOT, or is that sufficient?

Confused · Post by **Confused** » Fri Apr 12, 2013 11:42 am

I don't believe there is anything specified in the MOT manual that checks for the prescence (or lack of) this.

However - without an expansion bottle, when your coolant system gets up to temperature, and subsequently cools again, you will be drawing air into the system, rather than coolant. This can affect the running of the car if this air gets pushed around the coolant system, or at the least, will cause your radiator to not be as efficient as it should be.

Mudgey · Post by **Mudgey** » Fri Apr 12, 2013 11:54 am

Thanks for your swift reply!

Correct me if I am wrong, but surely the pressure cap is a one way valve and will not allow anything to flow back into the radiator?

BMCC · Post by **BMCC** » Fri Apr 12, 2013 11:54 am

I don't think they look specifically at the radiator in an MOT. However coolant is toxic to humans,animals & the environment so I would put and expansion bottle on.

Edit as has been said, coolant will go both ways, otherwise the coolant in your radiator would be permenantly low.

Confused · Post by **Confused** » Fri Apr 12, 2013 11:56 am

The radiator cap is actually 2 one-way valves.

When it gets to a set pressure, it will vent excess coolant out to the expansion tank, and when it cools, it will draw it back in again.

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Fri Apr 12, 2013 1:29 pm

Here for info Mudgy
http://www.howstuffworks.com/cooling-system7.htm

The expansion seal is the red highlighted bits (where it expels out to the expansion tank).
The return valve is the sticky down bits in the middle of the cap at the bottom.

Plenty of older cars don't have expansion tanks and vent as you've suggested above (e.g. my old Sunny has the overflow pipe simply venting down a pipe that finishes down by the bottom of the rad).

In practise, I don't know how often the Prelude system does actually vent/draw back when it's in normal operation and good nick? (ref other thread where I've said I've given up topping up my UKDM expansion tank because practise has shown that car just doesn't use it).
No doubt it will vary greatly individual car-car, depending on operating conditions/condition of the cooling system/mods to engine etc and that's why I'm still monitoring my JDM.

For that reason I didn't mean my other post to be a recommendation to everyone else to not bother topping up their expansion tanks, it was just a comment to re-assure the original poster there where they were saying theirs wasn't venting and asking whether that was normal (in my experience on one of mine, yes, that can be normal so don't worry...)

Supermarine Blues · Post by **Supermarine Blues** » Fri Apr 12, 2013 3:33 pm

It's a question of how do you know it's in good condition, if you don't keep an eye on the expansion bottle?

Risking a warped head.

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Fri Apr 12, 2013 3:52 pm

Ref my other post - the level in the radiator level has never dropped in 3 years (that I still check of course

)

Post by **Pushki** » Fri Apr 12, 2013 3:55 pm

Why not just fit a reservoir bottle?

Angus · Post by **Angus** » Sat Apr 13, 2013 9:46 am

IMO, it would be silly to not have the expansion bottle - because the theory of the system is that it's an integral part of it.

Wurly's experience is interesting (and not a little disconcerting!), though.