took the thought right out of my head...

Unfortunately it doesnt really clear anything up as i've just ran it for 5+ minutes with one of the sniff testers attached and the reaction agent remained blue...

I stopped as the coolant overflowed the tank into the tester.

On the residual pressure point... At the weekend, i went out to it, stone cold, 0.18bar of pressure on the guage. I did not open the tank. Ran it for maybe 10minutes, drove to the lockup, picked up some bits and drove home, couple of miles tops. This morning, stone cold, 0.35bar on the guage. Opened the cap and the guage immediately dropped to 0.01bar.

So something is leaking into the system, but presumably at a very very slow rate which is not enough to trigger the RELD fluid?

If coolant is overflowing that would suggest there is some air in there. Air expands far more than coolant and the only time the reservoir will overflow is when there is too much expansion. As you've got LPG on it, do the hoses to the reducer go over a hump so air could get trapped in there? Ideally the reducer needs to be mounted as low as possible so the flow and return hoses are going downhill so any air in the system remains in the cooling system and doesn't get trapped somewhere.

The LPG reducer is essentially replacing the throttle heater loop. Line leaves the manifold, goes to the reducer and then back to the expansion bottle. Its all 10mm hose and theres loads of flow, i very much doubt any air is stuck there. I've also left the LPG system off while testing as there was a suggestion the reducer could be leaking LPG into the cooling system.

I did wonder, if the "air" was actually just the leaking combustion gas, but the cars parked on a slight slope with the engine downhill and it was simply getting stuck somewhere and not making it back to the reservoir while testing. As a result it overflows. When driving normally it would find its way to the reservoir and burp out the PRV when the pressure got too high.

The fact that there was 0.35bar left in the system after two short drives means something new is getting in after the cap is closed, right? A "normal" air bubble wouldnt do that, it would return to 0bar as the air would return to its previous size.

I think i'm going to throw some of the "headgasket in a bottle" stuff in as a last ditch effort and see what happens.

I would be very reluctant to throw the “head gasket in a bottle” in, though I have used it successfully myself on several vehicles in the past. It will also partially close the small passages in the radiator and heater matrix. If you plan to keep the vehicle a long time you will likely need to replace the radiator and matrix after you properly repair the coolant leak.

Depends what he uses. Stuff like K-Seal, Radweld, etc will clog up the passages, Steel Seal or Sodium Silicate (the active ingredient in Steel Seal, but far cheaper) won't as it needs to come into contact with combustion temperatures when it turns into a solid. It will seal a slight leak around a liner for quite some time. Just buy water glass and follow the instructions for using it on the Steel Seal website. Sloth on here had it in his 4.6 for almost 3 years to cure a pressurisation problem before he swapped the engine for an M57 BMW lump.

yeah i ordered some steel seal, i was going to order the bare sodium silicate, but steel seal claim to offer a warranty, and will refund if it doesnt work... Granted they may well try to weasel out of it, but i figured that would be worth a shot.

I'll need to flush it a few times first though, as its not compatible with the Red/OAT coolant i currently have in it, so will flush thru a few times with fresh water and then run the steel seal with plain water and see what happens.

From memory, Sloth had very similar symptoms to me when we talked about this a while back.

Really its a sticking plaster either way. The engine is knackered, the bores are ruined and the camshaft is past it. But if the steel seal gets another year or two out of it then great.

I think longer term, i want to find myself a tidier shell. Ideally one without a sunroof. Then perhaps build that up with a nice BMW engine swap or something. If anyone has a non-runner with a tidy body and no sunroof let me know :D

I don't want to jump the gun, but steel seal certainly did something

Flushed the system 3 times with water to get rid of the old coolant, then filled with clean water and two bottles of steel seal.

On initial startup it did gain some pressure, but I've just driven 60miles of motorways, a roads and back roads and the pressure settled in around 1 bar, dropping to about 0.85 bar at idle. Still rises with engine rpms but don't think I saw it over 1.2bar, certainly not the 1.4+ that I saw the other day.

I'll let I cool right down, top off all the levels etc with it cold and have another run tomorrow, see where we are at.

That's good news and seems to confirm you have a slight leak around one of the liners allowing combustion pressure into the coolant passages. At least you can monitor the pressures and see if it stays that way.

I had all sorts of problems with overheating & in desperation I used steel seal which fixed the problem & it lasted for a couple of years & 20K miles before the head gasket started blowing so I then got the engine completely rebuilt by V8 Developments.