Level in coolant bottle - overheating

I have a strange set of symptoms. I recently changed the head gasket on my 1.8 as part of a head rebuild (snapped cambelt). Everything has been fine for 3 months.

Now the engine overheats within the first couple of miles & the bottom hose stayed cold. I checked the, new type inline stat & it's working ok. The strange part is that the coolant bottle fills & when I release the cap the level drops. After this the engine temp stays OK & the bottom hose heats up, until the next cold start.

I did have a problem bleeding the coolant as the bleed screw on the metal pipe is impossible to remove but I have followed all the other bleed procedures including the jiggle valve. I can see that a faulty bottle/cap would allow the bottle to fill but then why does the level drop when I undo the cap ?

Any ideas please ?

Have you fully flushed the system inc radiator then filled and got rid of air locks?

Did you check liners?

Bleed with heater on hot, if bleed screw not undo-loosen nearest hose clip and allow some coolant to flow out

Liners were good 3 months ago - could be knackered now

I have just had the engine running for over an hour & bubbles are still coming out of the heater bleed screw & out of the top of the rad via the bleed into the bottle. But I don't know if they are air, steam or combustion gasses. When I rev the engine the flow from the heater bleed stops & then gushes once the revs drop. I can even run the engine at 2000 rpm stationary & it doesn't overheat.

The engine will run, idling with the cap off, & the levels don't change & the temp stays normal. But take it for a run & the temp rises as does the level in the bottle. If I stop & release the cap the level drops back to normal & then the temp stays ok.

The odd thing is why it is worse on the first journey of the day.

Is the coolant being pumped round? Can you see it being pumped back into the header tank fing?

Yes coolant is being pumped.

Save us all the pain and get the local garage to put exhaust gas analyzer in engine bay- start engine then place probe over coolant tank- if hydrocarbons jump you have exhaust gas in coolant.

But the symptoms without seeing it point to exhaust gas in coolant

If the bubbles keep coming but the coolant level isn't dropping it sounds like exhaust gasses. As fanatic says an exhaust gas analyzer or sniff test should provide the definative answer.

That might be easier said than done here in the West of Ireland !

Apart from the obvious dropped liner is there any other likely area where exhaust gas can enter the coolant ?

I might get one of these as they appear to be essential equipment for any petrol Freelander owners

wow 2 threads for same prob havent seen that before............The test fluid is the best idea if you cant get a garage to help

There wasn't meant to be two threads !. I thought that it would be easier for others doing a search in the future if I asked separately about testing the pump. It seems like an obvious thing to check but no obvious way of doing it.

Really the only way to check the impeller is to take off the pump and ave a look. Then you might as well put a new one on as you got that far

was the jiggle valve free when you had the head off, and bleeding will be a pain if you cannot remove that bleed screw.

Yes jiggle pin was free. I don't fancy drilling out the jammed bleed screw & re-tapping.

This is getting stranger by the minute !. I have removed & checked the water pump & it is fine. There is no sign of exhaust gasses in the coolant & the thermostat & fan are working.

So after about 10 minutes of running the engine overheats. The bottle fills with coolant & the temp rises to 2/3rds. Then the fan cuts in & the temp drops back to just above normal. If I release the bottle cap the coolant flows into the engine.

The fan will then keep running & the temp stays just above normal.

sounds like it is air locking on exhaust gases- run it without expansion cap and see if it blows everywhere

It will idle all day with the cap off & the temp/coolant levels stay normal. Eventually the bottom hose will heat up but it takes for ever.

Take it for a run & it overheats quickly, the bottle fills & the bottom hose stays cold.

Once you release the, I am assuming vacuum, in the bottle the level drops back to normal. You can then drive for quite a while without the engine getting too hot - although the fan stays on & the engine will be a bit above normal temp.

I have just been through the Landrover manual & it says that this indicates a thermostat failure. I checked the stat but because the problem continued I manage to remove the internals so that now there is no stat. So it can't be a stat issue.

If it is exhaust venting into the coolant then that wouldn't explain why it is better & stays better when the cap is released. If it is only taking 10 mins of driving to create an airlock then surely it would carry on creating another one pretty soon.

Could a faulty bottle/cap cause this ?

I've had similar on a rover 400- it was head gasket- if your sure the waterpump is good as cap on may stop thermo siphon effect if pump impeller failed

Yes I spent yesterday removing the pump & praying that it was faulty .
But it is fine. There are no other signs of HGF, no mayo, no coolant loss etc but after changing two since I've had it, I know that this isn't proof.

what thermostat system have you got, and fanny certainly has a point about the head gasket.