Help bleeding a cooling system

[eltax91]

Hi all

I recently stripped down my MK2 MX5 weekend toy to change the cambelt etc, as it was my first attempt and I've been doing it with free time its been stood in the garage for about 10 weeks.

I got the engine going for the first time last week and ran the car, rad cap off, heaters on full, for a good 20 minutes, having massaged the coolant pipes as I filled the cold radiator. I topped the rad off when cold and topped the expansion bottle up to "full". Hot air blasts out of the heater fine and the car comes up to temperature (well, as far as the mazda indicator is concerned, it stays bang in the middle)

So, having finished off the discs, pads and brake fluid change this morning i took it for a spirited 4 miles local blast to loosen the cobwebs and get all that condensation out of the exhaust.

Towards the end of the run I wound the window down when at idle and noticed an acrid smell wafting in, the exhaust was kicking out the slightest of smoke, I assume this is still the condensation effect.

Anyway, when I got back I popped the bonnet just to check for any major leaks or snapped belts or anything stupid and noticed that the expansion tank was about half an inch above the FULL mark, so somehow I have MORE coolant in the system. I suspected an air leak so I got the rad cap off and immediately the water rose slowly up the rad and bubbled out of the top. I assume this means there is air in the system still? i ran the car for ages, heater on full, keeping slightly more than tickover, lots of little bubbles came out but the coolant still keeps rising up and spilling out of the rad cap.

Whats more, the header tank has now dropped to the "full" marker, but I'm convinced more came out than what has dropped in the header, maybe not though!

So, is there another way to ensure I've purged all air from the system? I remember an old PUG that had bleed screws at the highest point, but can't find any on this old mm5.

Any help greatly appreciated, the sun is coming out and I want my toy back.

[Barryboyni]

Tried the coke bottle method?

Cut the bottom off a 2l Coke bottle, push the thin end into the expansion tank and wrap insulating tape round it to keep the water in. Fill bottle with water. Hey presto, this is now the highest point in the system and the bubbles will (should) come out the top. You'll obviously need to remove some water from the expansion bottle when you're finished and have removed the Coke bottle but all the air should be gone.

DON'T SCALD YOURSELF

[DKS]

Anything hot expands. Of course it will read over the FULL mark when hot. If it's OK now when cold it's sorted. It will also expand more when not under pressure, hence it spilled out,

[BritishRacinGrin]

Never remove the cap from a hot coolant system- doing so will cause any 'dissolved' air to expand and form pockets of air in the system, meaning that the system will need bleeding again.

The level of water in the expansion tank will rise when hot as they said ^ so when you re-fill and re-bleed the system do not set the level to 'MAX'.

An overfilled system will generally sort itself out as any overpressure will be vented through the relief valve in the expansion tank cap when the system is hot. However, if the system is overfilled it'd be preferable to wait until the system is completely cooled before removing the cap and siphoning off some coolant.

Edited by BritishRacinGrin on Wednesday 5th March 16:49

[motorhole]

As above, water expands when hot which is why the level will have risen. This is normal. The cap on the expansion cap is the pressure limiter for the cooling system, as the system is kept at above atmospheric pressure (normally anywhere between 1.3-2.0 bar) to prevent the coolant from boiling. As soon as the expansion cap is removed, you remove this excess pressure allowing the water to expand more, overflow and potentially boil too - forming air pockets, as mentioned above.

What you've explained is all perfectly normal Apart from perhaps the acrid smell.

[BritishRacinGrin]

motorhole said:

Apart from perhaps the acrid smell.

...which could be explained by the expansion tank venting, or by a leak, or residual coolant which was spilled in the engine bay boiling off.

If it turns out that you're having a problem bleeding the system, try again with the front of the car raised i.e. parking the car pointing up a gradient.

[eltax91]

o if I'm getting hot air out of the vents consistently and the temp guagae doesn't move, there's not likely to be any air in the system?

For reference, the pressure cap is on the rad, it's a simmple plastic "clip" affair on the bottle. certainly no seal/ pressure cap

[Rich1973]

Had you made sure the stat had opened while you were checking the levels?
Keep feeling the top hose to the radiator while you warm it up, and you will feel it get hot when the stat opens. This will cause a rise in the expansion bottle and then a fall.

[Mave]

BritishRacinGrin said:

Never remove the cap from a hot coolant system- doing so will cause any 'dissolved' air to expand and form pockets of air in the system, meaning that the system will need bleeding again.

Edited by BritishRacinGrin on Wednesday 5th March 16:49

That's not dissolved air though is it? I thought it was essentially water boiling into steam bubbles as the pressure is dropped. Should just condense back to water when it cools...

[jebus]

From what I remember the MR2 of that era was a right pain in the ass to bleed, the level will go over the full mark at times, head over to imoc really good owners forum they will help you out.