Bleeding the cooling system of a single seater

[jedrinck]

In the cooling system of my car, an 8V FRenault, the cylinder head is by far the highest point for the coolant to pass through. Albeit only the radiators have bleed valves. So even when they're nicely flooded, assuming there is trapped air in the cylinder head, I guess it will stay there. I am asking because after engine shutdown the car tends to spill some water out of the reservoir, suggesting that hot air is expanding somewhere, pushing the water out. Am I missing something?

[tristancliffe]

Most cylinder heads are either designed not to trap much air or have a bleed point built in. Perhaps it is possible to add your own additional bleed hose back to the header tank (or even just a manual bleed point) to your head?

Is the header tank not the highest part of the system?

[Peter Dunne]

Originally Posted by jedrinck

I am asking because after engine shutdown the car tends to spill some water out of the reservoir, suggesting that hot air is expanding somewhere, pushing the water out. Am I missing something?

This is quite common in single seaters and usually occurs when there is too much coolant in the system (provided you don't have an airlock) and by chucking some coolant out of the overflow, the system is simply finding its own level.

If you have an airlock and feel that the radiator bleeder is not sufficient to dispel all the air, try raising the front of the car as high as possible so that the radiator is the highest point in the system. A simple check is to feel the return hose from the radiator. If this is only tepid when the other side is hot, then you have an airlock.

[jedrinck]

Originally Posted by tristancliffe

Most cylinder heads are either designed not to trap much air or have a bleed point built in. Perhaps it is possible to add your own additional bleed hose back to the header tank (or even just a manual bleed point) to your head?

Is the header tank not the highest part of the system?

You`re right, the header tank is the highest point. It's just that due to the way the hoses are arranged for the air to be moved out of the head it would have to travel downwards before it could escape through the pressure cap of the tank or the radiator bleed points. The head doesn't have a bleed point but I guess I could fit one. Would a permanent bleed line to the header tank be preferable to a manual bleed point? If there was a bleed line to the header tank I reckon some water would bypass the radiators flowing straight to the tank from the head that way.

I'll try raising the front, I would assume one shouldn't run the engine during that as the lubrication system wouldn't really work?

Is it advisable to use a higher rated pressure cap, like 1,3 bar?

Thanks for your suggestions so far!

[tristancliffe]

Yes, I think a permanent bleed is better, just in case you get localised boiling creating more air. A small amount of water will bypass the radiator, but if you use something like -3 hose then it's pretty minute compared to the coolant flow through the radiator. Maybe just use a manual bleed if you're REALLY worried about temperatures.

As long as the oil pickup point is covered (both in the sump and the oil tank in a dry sump car) then running at an angle shouldn't be a problem - cars work happily on 30% slopes normally!!!

A higher pressure cap will reduce the tendency for localised boiling, so might help with your problem. You can then also run a bit hotter before problems occur, although the optimum temperature will still be around 70-80°C

[the-enginetuner]

Have you thought about a swirl pot ?

[jedrinck]

No, I didn't even know what a swirl pot was. I looked it up and as I understand it is for cars which have hot spots where localized boiling occurs. My car is not exactly a prototype, and hundreds of them have been run for more than a decade with no such problems. I guess there must be something elementary I'm missing. Just got me a new pressure cap this afternoon and if that cures the overheating I will feel a little stupid but also happy.

[tristancliffe]

Can you track down and speak to owners of similar cars? They might have had and solved the same problem as you in the past.

[jedrinck]

Yes, they say it sounds like I have air in the system. In the F3 car in your avatar, can you run the engine with the car being stationary for an extended period of time without it overheating? Mine doesn't have fans either, naturally. And is it necessary to drive around some for cooling down after going fast?

[tristancliffe]

The car will idle for about 5 minutes before it overheats, but it is overcooled at the moment due to the engine being less powerful than an F3 engine, yet having the same thermal capacity in the radiators.

My in lap usually gets the temps down to a sensible level. It wouldn't do wonders for the engine to shut it down and come to a stop too soon after driving hard as hot spots form inside the water jackets - always best to keep the water circulating as long as possible.