G40 losing coolant after long run

[GSBellew]

My G40 has me a bit stumped at the minute 

I had to take it for a 50 mile motorway run a while back, it was perfect the whole way until I got to the end of the motorway, as soon as I slowed down from motorway speeds the coolant light came on, I still had heating and the temperature was not unusually high, luckily I was as far as I needed to get so I was able to park up and leave it.

Two days later I had to do the return trip, so I topped the coolant up and set off, the exact same again, perfect the whole way until I slowed down at the end of the motorway, light on again.

I was a bit annoyed so just pulled it into the garage and have left it alone till now.

I gave it a good checking over tonight, the oil and water are not mixing and there are no bubbles coming up into the expansion tank with the engine running, I ran it for a while and everything seemed fine, nothing unusual from the exhaust, and no signs of leaks anywhere, but when it hit temperature and the fan kicked in a little bit of coolant came out of the expansion tank overflow 

I'm not sure if this was just because the level was slightly over the max level as I know I had accidentally put a bit too much in or if this is where all the coolant has been going 

Does anyone have any suggestions as to how to figure out where it has been going or what to check next?

Any suggestions really appreciated!

[hayesey]

check the condition of the seal on the header tank cap, or just replace the cap anyway as they're not expensive.

[g40willa]

Also check the water junction at the right hand end of the head, the o-ring is shaped into an oval, i've had a few of these go on various polo's inc the g40. One occurance was that the plastic water junction had cracked.

[PeteG40]

yeah - the thermostat housing ages badly!

[GSBellew]

Thanks for the tips, I have a new header tank cap on order which should be in tomorrow morning, I'll check the water junction when I get home, would I be as well to just get a new one and new thermostat too?

[GSBellew]

Only got a chance to work on this today, unusual way to spend Saint Patricks day 

I drained and flushed the coolant system, then refilled it bled it and fitted the new cap, ran it up to temperature a few times and it seems fine, I could not do a road test as the tax is up at the minute and being a bank holiday I couldn't do anything about it today.

I think I might still fit a new thermostat and housing anyway and then just keep an eye on it 

[GSBellew]

This has me completely stumped, I had come round to thinking it was the headgasket, but before doing it I decided to change the thermostat and those O rings, found that the one at the head was in very bad shape and was showing signs of having leaked at some point, thought I was on to a good thing so I changed them. Ran it for half an hour at fast idle and it seemed perfect, fan kicking in and the temp dropping as it should

Changed them and took it for a run, perfect for about 20+ miles untill I decided to push it a little on the motorway for a few minutes after I let off the temp gauge started climbing and kept on going, red light on and I stopped, the cooling fan was not running and it had pumped a bit coolant out of the reservoir, I shorted the fan and limped on slowly as I was nearly home, the temp would drop to 100 and go back up to 110 then back down, into the garage, took the shorting wire for the radiator fan out, plugged it in and nothing even though it was a new sensor and working earlier!

I'm lost, do I just take the head off now?

Its had:

New fan switch

New Header tank cap

New Thermostat

New O rings on the housing

Help please!!

[GR40]

I had a kind of a weird problem like that a little time ago and it was the most unlikely to happen scenario! In my case it was the exhaust manifold seriously leaking exhaust gases! It had the nearest cylinder blowing hot gases directly to the thermostat housing and that really pushed the temperature sky high. The bolts from the exhaust looked fine but as soon as I pushed them they snapped. And when I got the gasket out it was burned at that point! So I am just saying, do check for the impossible before you do the head gasket! The oil pump not working could cause the temp in the water or a small leak from the water pump! My mechanic has a small manual pump with a gauge that connects to the expansion tank and you can pressurise the system to see if its leaking, you could try one of these!

[liam]

just run a simple compression test on your cylinders aswell, that should tell you if your head gaskets gone.

all of this happened to me not so long back and i ended up changing everything but it turned out to be the rad fan was not operating properly and wasnt staying on for nowhere near long enough.

[GSBellew]

I took the thermostat housing off again today and found it had cracked near the seal on the cylinderhead, so not sure if that was a cause or result of last nights high temperatures.

I've ordered a new housing and the screws / bolts for it.

I'll try a pressure test on it tomorrow too, would one of the halfords pressure testers be good enough?

[GSBellew]

Just did a compression test.

I got:

Cylinder 1          8.5 Bar

Cylinder 2          9 Bar

Cylinder 3          8.5 Bar

Cylinder 4          9 Bar

According to the Hanyes manual new is 8 to 12 Bar with the wear limit being 6 bar and a maximum pressure difference of 3 bar between adjacent cylinders.

So does that say that the headgasket should be ok as I am not getting low compression as such or oil in the water or water in the oil?

[liam]

when my head gasket went, cylinders 1 2 and 3 read 10.5 bar (150 psi) and cylinder 4 was just over 7 bar (100 psi).

my engine is a 1341 with wossnor pistons i dont know if that changes the compression though.

[breadman]

when my head gasket went, cylinders 1 2 and 3 read 10.5 bar (150 psi) and cylinder 4 was just over 7 bar (100 psi).

my engine is a 1341 with wossnor pistons i dont know if that changes the compression though.

Compression ratio has nothing to do with a compression test mate.
I'd say the figures that GSBellew got were ok, perhaps a tad on the low side but that could just be the gauge used. Obviously a very low reading or on one or more cylinder like Liam had would show there was a problem.

[breadman]

Just a thought, I see you say that you've flushed the cooling system but did you take the rad out to give it a real good go? It may pay to reverse flush the radiator particularly if it's the original as they do silt up over time. I reckon it's also worth doing the heater matrix (pipes disconnected) then reflush the engine (thermostat out) to give it all a really good clean.
It'll only cost you a bit of time (apart from another bottle of antifreeze!) rather than keep replacing bits uneccessarily, might be worth a try?
Good luck.

[Andy]

when my head gasket went, cylinders 1 2 and 3 read 10.5 bar (150 psi) and cylinder 4 was just over 7 bar (100 psi).

my engine is a 1341 with wossnor pistons i dont know if that changes the compression though.

Compression ratio has nothing to do with a compression test mate.

Compression ratio DOES affect compression test results, as does the cam profile and cam timing -as well as the obvious sealing ability of the rings/valves/headgasket etc.