Coolant level - how low is too low?

[jammy_basturd]

Carrying on from my saga with changing the radiator and problems bleeding/expansion tank blowing out.

I finally got to the point yesterday where my expansion tank would no longer spurt coolant everywhere. However once I looked in the tank once everything was cooled down I noticed the coolant level was only just above the bottom of the tank.

This I thought was too low.

So I've filled it to just below the top of the 'doorway' between chambers that you can see if you peer inside the tank. This is still a good centimetre or two below the MAX mark. On driving through town this morning the coolant was again overflowing from the tank, cue many funny looks from people thinking my car was on fire.

I've not yet had a look to see where the coolant level has settled down to as I'm waiting for the car to cool down.

The thing that troubles me is that I think the engine itself is running hotter - the oil pressure is now a good 10 PSI lower than what it usually is for just normal town driving (at least at low revs).

Maybe draining the coolant has dislodged some crud and blocked some passage ways in the engine?!

I still don't think it's head gasket failure as the car hasn't seemed down on power or misfiring at all.

[Supateg]

Mine likes the level very low in the expansion tank.

The tank was designed for a car with around six litres of coolant and expansion room. A cerb has twice the coolant so by my layman thinking it should hade twice the expansion room?

I was worried initially by this. I used
Combustion testing dye in the coolant
And all was well (paranoid that's me)

The coolant temps should be taken off app or diagnostics so you can see truly what's going on.

Maybe oil pressure spring needs renewing?

[CerbWill]

Mine is the same, likes a minimal amount in the bottom of the tank. Like Supateg I felt the need to test the coolant for exhaust gasses which has allayed my fears of head gasket failure.

As for the oil pressure, the oil and water pathways around the engine are totally separate so a drop in coolant level will not affect oil pathways. At low revs (idle) the oil pressure on the AJP drops off rapidly, on mine adjusting the idle from 900 to 1000rpm will make a 10psi difference in oil pressure so if your engine is, for whatever reason, now idling slightly low that'd explain it. though as Supateg says, monitor the water temps around town with the diagnostic software and see whats what.

[anonymous-user]

Opposite for me. Previous owner ran it with the coolant just below the mark on the tank, which I stuck to. I've switched to waterless coolant following the rebuild but have it at the same level. Hasnt lost a drop yet and has barely any pressurisation.

[jammy_basturd]

Supateg said:

The coolant temps should be taken off app or diagnostics so you can see truly what's going on.

Maybe oil pressure spring needs renewing?

Coolant temperatures seemed to be OK when I was running the engine in the garage yesterday. They'd go up to about 92, fan would kick in, come back to 88/89, fan goes off, repeat.

I guess the spring could need renewing, just seems odd for it to start to fail over the course of a couple of days, right when I've changed the rad.

CerbWill said:

As for the oil pressure, the oil and water pathways around the engine are totally separate so a drop in coolant level will not affect oil pathways. At low revs (idle) the oil pressure on the AJP drops off rapidly, on mine adjusting the idle from 900 to 1000rpm will make a 10psi difference in oil pressure so if your engine is, for whatever reason, now idling slightly low that'd explain it. though as Supateg says, monitor the water temps around town with the diagnostic software and see whats what.

That's a very good point!! Whilst doing the rad I did some odd jobs which needed doing, one of which was adjust the idle down from ~1500rpm to a more suitable 1000/1100rpm. This could well be the reason for the drop in PSI.

djstevec said:

Opposite for me. Previous owner ran it with the coolant just below the mark on the tank, which I stuck to. I've switched to waterless coolant following the rebuild but have it at the same level. Hasnt lost a drop yet and has barely any pressurisation.

Shouldn't the coolant system run pressurised?

The coolant level pre changing the rad used to be JUST under the MAX mark, not sure why changing the rad would now change things? Maybe I'm being overly cautious.

[anonymous-user]

jammy_basturd said:

Shouldn't the coolant system run pressurised?

The coolant level pre changing the rad used to be JUST under the MAX mark, not sure why changing the rad would now change things? Maybe I'm being overly cautious.

Im no expert on engines....with much evidence to back that up....but I thought that it was the expansion/evaporation of the water that causes the pressurisation, so coolant systems have to cope with pressure rather than require it to operate? Pressure might increase the boiling point by a couple of degrees, but with water there's also the corrosion and cavitation issues, no?

It does seems thicker than water, so it might put extra load/pressure on the pump, but I had that rebuilt too so you'd think it would be ok.

[jammy_basturd]

Ah, I think we're talking about the same thing.

The cooling system obviously needs to be sealed to some extent to stop the coolant evaporating and steam escaping. Because coolant system are sealed off to atmospheric pressure, when the coolant heats up and expands slightly, the pressure increase, hence why, if you try taking the expansion tank cap off when the coolant is up to temp, the coolant erupts.

I think! This is how I understand it, but I may be talking out of my expansion tank!

[Jubag]

I think that if the engine can't retain the coolant at the max mark there is something amiss. It might be worth changing the expansion tanks cap if it keeps dumping to atmosphere.

[Mr Cerbera]

Sorry to be a philistine

BUT

Fill the tank to max
Fill Oil to correct level (I think the difference between min and max on the dipstick is 800cc)
Run the car till normal
park (with the nose up) on a hill
put a brick behind threar whell
bleed any air out of the system from the valves on the external, steel pipes (starting with driver's side if they are of equal height)
allow to cool
top up header tank - if required
Repeat process
Check ECU for true engine temp withy diagnostics

Job jobbed

[jammy_basturd]

Kind of my point. Before the rad change I used to be able to do that. Now, if I fill to the max, the thermostat barely has time to open before coolant is being chucked out of the tank. In fact I think the first time coolant came out of the tank at 60-70 degrees, well below 'stat opening temp.

[gruffalo]

It is irrelevant if the stat is open or closed as the water warms up it expands, if the cooling system is in good nick then the only place it can go once the expansion cap pressure has been reached is out of the cap.

As others have said the expansion bottle is off a 2ltr Audi 100, the max line on the expansion tank is set to allow the content of the Audi cooling system expand and not overflow. In a Cerbera with twice the coolant you get twice the expansion so just covering the bottom of the expansion tank is entirely normal, it is where mine sits when cold and it reaches the top when hot.

If it was different on the OLD rad maybe you had air in the system which is compressible or a leak therefore not allowing the pressure to build properly. The system being presurised is a good thing when hot as it increases the boiling point of the coolant to around 125C stopping hot spots around the liners by stopping the coolant boiling in that hot area.

Basically stop worrying and give it a good spanking;-)

[jammy_basturd]

Awesome, spanks!

Air/leaks in the old system is entirely possible given the coolant stains in a couple of places on the old rad.

[Jubag]

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

Same thing here.

[V8 GRF]

Jubag said:

I think that if the engine can't retain the coolant at the max mark there is something amiss. It might be worth changing the expansion tanks cap if it keeps dumping to atmosphere.

Not necessarily true. All TVRs find their own level, remember this is the expansion tank we're talking about it doesn't need to be full or even half full as long as there is some water in there to stop air getting in the system.
If you overfill the tank it will dump coolant to the air as there is no where for the expansion to go, it will find it's own level be it high or low.

If the engine is running hot I'd suggest the issue is elsewhere.

[ukkid35]

Simplest way to fill with coolant is to open the rad bleed hole with the expansion tank full. Once the air has gone close and fill the tank again with the water rail valves open. Close when they spill coolant. Job done.

[TheRainMaker]

Change the rad cap

I've just been through a similar thing.

[wiggycerb]

My coolant level sits between min & max on the expansion tank and always has done.

With regard to oil pressure, i always keep mine on maximum and have recently changed the pressure spring. The old girl sits at between 35-40psi on tickover (900rpm) when hot.

[Jubag]

V8 GRF said:

Not necessarily true. All TVRs find their own level, remember this is the expansion tank we're talking about it doesn't need to be full or even half full as long as there is some water in there to stop air getting in the system.
If you overfill the tank it will dump coolant to the air as there is no where for the expansion to go, it will find it's own level be it high or low.

If the engine is running hot I'd suggest the issue is elsewhere.

I'm just going by the fact the OP states his car pre rad change liked the level JUST below MAX mark. I'm sticking with a cap change and re-blead. Where would you look first? Just out of interest.

[jammy_basturd]

Nope! Something is definitely wrong. Had to drive home from a friend's house this afternoon. It's about 12 miles, 4-5 through town. Bear in mind that there was 2 stops within that first 5 miles, once to get food shopping then again a mile or so later to get petrol. Barely got out of the petrol station and the temperature needle was all the way around the gauge. Stopped a mile or so later, got the laptop out and the temp was about ~95 I think. Waited 10-15 minutes and then carried on, again the temp went above 95 and I pulled over and let the engine cool over about 30-40 minutes. In the last 5-6 miles I had to stop once, but the temperature stayed around 96-97, with both fans on.

So, air lock? Something wrong with the radiator? I can do a compression test on the cylinders tomorrow.

I don't *think* it is the rad cap. I'm currently using the old rad cap which was fine with the old radiator, and I do have a new rad cap (which I never used with the new rad), but I had problems with the coolant boiling over once I installed the new rad and rad cap.

[Supateg]

Dare I say thermostat?