I've had a persistant coolant leak from the inlet manifold for a while. There was always a puddle of coolant underneath the big fitting on the right side of the manifold that feeds the heater matrix. I had actually assumed the threads were leaking (theres signs of PTFE tape).
Inspecting it closely earlier i noticed some slightly worrying signs, firstly, it think the hose looks noticable swollen. Its ~19mm at both ends, but it bulges out beside the inlet manifold in a rather unnatural manner, and replacement hoses on ebay look nothing like that, they appear to be 19mm all the way along. Secondly i think its leaking from the cords... I tightened the clip up but it continues to leak. I think the coolants actually seeping along the reinforcement cords and out the end, suggesting the inside of the hose is compromised. A disaster waiting to happen for sure!

The small complex that feeds the heater/expansion tank/thermostat, also has signs of pink coolant leaking from the joints, so i'll replace all of those, and i think actually i should just replace everything. The lower thermostat hoses are drenched in oily ming which also isnt good for rubber hoses, and the main top rad hose looks a wee bit "uneven" though not swollen to the extreme of that heater hose.

So that leads me to the question. The heater o-rings. They arent (obviously) leaking, but i believe removing the heater hoses runs the risk of disturbing them? I've noticed when inspecting the pipes earlier that the top heater pipe that pokes thru the bulkhead was covered in the usual tell-tale pink deposits, suggesting coolant is leaking either from the pipe or the o ring. Theres no obvious dampness inside though.

I dont really have time to sort the o-rings at the moment, but i'd like to get this clearly knackered coolant hose sorted out before it leaves me stranded... What are my chances!?

Some pics:

https://photos.app.goo.gl/Rx6XVy1MpAfhVNqq9

https://photos.app.goo.gl/23KA7ySVYqL9qRej7

Heaving on the hoses to the heater core is a pretty sure fire way of causing the O rings to start leaking so the way I always do it is the remove the hose clip and then use a Stanley knife to slit the old hose lengthwise. That way it just pulls off without having to give it any grunt. The hose is scrap but as you are replacing it anyway it doesn't matter. While you are at it, I'd suggest altering the plumbing for the LPG reducer so it is in series with the heater. Instead of the Tees, I run the hose from the manifold to a 19-15mm reducer and then run that to the reducer. The return from the reducer then goes through another reducer and connects to the heater feed. Where you have the Tee in the return, just use a 19-19mm connector (or a new hose). That way you get full flow through both the reducer and the heater rather than giving the coolant a choice of taking the path of least resistance resulting in insufficient flow through one or the other. It'll warm the reducer quicker too so it will switch over sooner.

I've never had a problem with running my P38 vapouriser for the LPG in parallel They heat up quick enough anyway - if I start up at home and drive from cold, it's still switching to gas within half a mile.

I found when I DID run it in series, based on suggestions on a couple of forums that my heater ran colder because of the restriction in flow in the smaller pipes and the bends it went through. I went back to parallel when I put the new engine in and had it all out to replace hosework etc at the same time. I've not had one case where the vap has frozen over, or performance on LPG has suffered. The only thing I notice is it might take a couple more minutes driving in winter for it to change over to gas, but other than that...

Personal preference at the end of the day - they both work - but I found that my heater took a lot longer to warm up and never got as hot.

Running it in series is a lot less neater on a Thor because of the way the hoses are laid out, on a GEMS it's pretty simple (and leaves you with less joints and potential leak points)

Running mine in series tidied them up nicely, might get the lpg working this year,lol

My vaporiser has 10mm inlets which seems quite a restriction to the coolant flow so I think I will keep it parallel for now.

I guess I'll be very careful with the fittings and hope it doesn't start leaking!

And down the rabbit hole we go....

Swapped the dodgy looking coolant hose on sunday. Tried as best i could to not disturb the o-rings and filled it all back up.

Did a few miles around town and it seemed to use the whole expansion tank, but i figured that might just have been some air working its way out, so topped it up. No obvious damp carpet that one might expect for almost a litre of coolant having gone missing.

This morning it was about an inch down from the level so i filled it up again. looking underneath there was a few drips, which seemed to be directly underneath the heater hoses on the bulkhead. Had a feel about and the LPG T Piece was damp, so i tightened that up, refilled the coolant and set off to work (35miles).

Arrived at work and parked on an area of clean concrete and by the time i got out and looked underneath, there was already a puddle forming. Expansion tank was empty. Underneath the car there was active drops dipping off gearbox bellhousing. At the front, the steering bars and damper were also covered in drips of coolant.

Not convinced this is heater o-rings, theres no sign at all of leakage inside the car. Any ideas?

Is it blowing back from the water pump maybe?

Water pump or leaking rad if it's all over the front.

potentially, but the manifold looks dry and doesnt have much sign of any coolant. Theres a small puddle on top of the water pump, but i suspect thats made its way there from me changing the radiator top hose at the weekend. No signs of coolant anywhere near the actual water pump.

Rad is new and theres no signs of coolant on the rad itself.

Things get weirder though!

I parked it at work at half 9, and could see some drips off the gearbox bellhousing and rear of the engine block. They formed a small puddle (maybe a 6" diameter circle of wet) after being parked for about 2-3minutes.
Around 10 i went back to the car to fetch my coffee, the drips that were there previously had stopped and were clearly drying up on the concrete.

I decided i would go down to the car at lunch, start it up and have a good poke around looking for a leak. So around 1 i went out to the car, and found a massive puddle of coolant under it and stretching across the road. Opening the bonnet, there was a very obvious leak coming from the LPG T piece that i mentioned earlier. It was dripping off that, running along the edge of the rocker cover and then down the back of the engine and dripping off the starter motor.

So i topped it up with water (it took a kettle full) and then fired it up.... And the leaking T piece stopped leaking... WTF?

I've ordered a pair of new (metal) T's as well as some meters of 16mm hose for the LPG, so i guess i will change those out and see what happens :/

Seems to be one of those situations that the more i look, the more confused i get. I took some photos at lunch, i will see if i can post them.

I'd suspect from what you've said, that your vapouriser is leaking gas into the coolant and overpressuring it. First test to try and isolate it is to run it on petrol and see if it stops, if so then at least you know what your looking at.

I've known some types of hoses to distort when exposed to LPG that wouldn't normally be expected to cope with it (coolant ones and vacuum ones) - they tend to balloon up if left like it which looks very much like your second photo to me.

It will probably do it at its worst when the engine is stopped, as there is a more constant pressure there then - its got nowhere else to go at that point, but will stop leaking once the pressure has dropped far enough, so doesn't usually lose much.

If it is that, you might be able to get away with a rebuild kit on whatever vapouriser you have, or it might need another one - depends whats wrong with it.

hmm i'm not sure it is leaking LPG, its just a shit plastic T piece that has got all distorted and isnt sealing properly against the new hose and fresh jubilee clip.

As it happens, the vaporiser itself IS leaking coolant though a very minor leak, and its on my list to replace. I didnt actually consider it could be mixing LPG and coolant though, maybe i need to bump that up the priority list a bit.

If the vaporiser is leaking gas into the coolant passage, you'll get pressurisation in the cooling system. To the point where you will probably conclude that you've got a blown head gasket or slipped liner. If it doesn't have pressure in it after it has been run and then allowed to cool down again, then it isn't that. I'd say water pump. Any coolant split on it will evaporate off almost instantly so if there is a puddle laying on it, it's pissing out. My SE did just the same as you describe, river running away from underneath and that was because the Britpart water pump the previous owner had fitted just before I got it had done nearly 2,000 miles and was completely shot. Take the serpentine belt off and try wobbling the fan, I'll put money on slack in the pump bearings

righty, so i swapped that dodgy T piece out last night. It was definitely the cause of the "river", i managed to visualise it leaking a few times and it was pissing out of there. Filled it up and had a 20 minute drive around, mixture of town and dual carriageway then back home. All seemed fine, no obvious leaks, no puddles of coolant on the ground.

Coolant level had dropped about an inch after the drive, but i figured that was probably just the air bleeding out of the hoses i'd changed etc. After being parked for a few minutes, i noticed there was a few drips appearing off the gearbox bellhousing. It didnt form a puddle, but engouh water dripped out that there was maybe a 6" damp patch on the ground. I wasnt sure at this point if it was a leak or not, as i'd blasted the underside of the car with the garden hose before my test drive to get rid of all the coolant drips.

This morning i filled it back up to the line (no pressure in the bottle when i opened it), and then drove to work. On arriving at work, again the level was down about an inch, maybe a wee bit more. And again, a small number of drips off the rear of the engine/bellhousing flange.

There doesnt appear to be anything leaking at the front of the engine that i can see (well, i lie, its pissing engine oil and ATF/PAS fluid everywhere, but not coolant!).

Rear Core plugs? ;(

Had another look at lunchtime. Really cant see anything around the water pump bearing itself, its just grubby with oil, no dampness.

Removed the round hatch from the gearbox bellhousing and had feel around inside, couldnt feel anything wet.

The panhard rod and steering damper had drops of coolant on it again, suggesting the leak is at the front. but the only drips on the ground were at the rear of the engine in the bellhousing area. I cant see any coolant on the actual engine at the front.

ran it up for 10minutes and had a good look around. No obvious leaks anywhere, and no additional drips on the ground either...

Easiest way to find a non-obvious leak is to put some pressure into the coolant system - either via a proper kit designed to fit the pressure cap, or with a modified cap and a low pressure pump. Someone else on here managed to modify a cap and use a bike tyre pump (I think) to find their leak. You don't want a lot of pressure on it, around 15psi should be enough.

yeh i guess that sounds like a plan.

Things are becoming clearer.

There was a small puddle of coolant on top of the water pump, in a small nook between the pump/front cover casting and the block. I assumed it had gotten stuck there when i changed the rad hose and coolant doesnt seem to dry out like normal water and can sometimes hang around for ages, so i had kinda discounted it. So earlier i dried it up to make sure, and also dried up the coolant mess on the underside, making sure it was all nice and dry.

Puddle: https://photos.app.goo.gl/gwMSxFKvf2CgNjv78

Got home, and a) the puddle is back, and b) the coolant drops are all back on the panhard rod and damper etc.

So i guess that means either the leak is at the front of the engine, and some of its just tracking rearwards with wind etc, or there are two leaks. One leak spreading around seems more plausible, but i cant figure out where its coming from.

The pump itself doesnt appear to be leaking, and for water to get up into that puddle, i think it suggests the gasket on the pump, or the gasket on the front cover is leaking? Theres also a lot of oil around the offside front of the engine which again might point to the front cover gasket (or at the very least a bad crank seal.

I will order a new cap, and then modify the old one to take a tyre valve as mentioned and see if i can actually pinpoint the leak on a cold engine.

And to make it even more confusing, did 150miles today, including 20-30miles of greenlaning, and a total driving time of about 6 hours, and it didnt use a drop of coolant?!