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Nearly a year since i've owned this one and I haven't gone near the LPG system in all that time. Anyway, the coolant hoses for the Bigas unit that are T'd into the regular hoses looked a bit manky, so i replaced them today with some fresh coolant hose. In the course of bleeding the coolant system and running the engine at revs then i noticed that the LPG Unit started running at high revs ( i had never heard it do anything before) even with the LPG switched off at the dash... I assume that it has some kind of circulation pump in there that was probably full of air, hence the noise it was making. Is there something special I need to do to bleed the circulation system when LPG is fitted? Or it will eventually displace the air when running at full temp and pressure?

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There's no pump or any moving parts in there, just a coolant passage that heats it up. On a GEMS it is far easier, and tidier, to run the reducer in series with the heater rather than in parallel with Tees. Hose from the inlet manifold, through a bend to the reducer then out of the reducer to the pipe going to the heater. It gives a better flow through both the heater and reducer rather than it taking the path of least resistance and neither of them working at optimum. It also makes bleeding a lot easier.

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The ONLY issue I found when I had my (albeit Thor) LPG in series with the heater, is that sometimes in winter, it would take forever to get heat in the cabin, as most of it seemed to be sucked out by the vap when I was running on LPG.

I don't know if part of that was the size of the vap on mine, or the Audi heater core I had at the time had slightly smaller hose inlets, or the flow through it just wasn't enough to warm the heater core enough for me to get hot air inside - but since I swapped it back to being in parallel, I get warmth from the heater again, and I've had no issues with the LPG vap freezing up. Mine does also seem to have (what I would have thought) a smaller vap on it, so it could also have been restricting flow through the circuit to the heater.

After my heater core exploded when the PS pump failed and the serpentine belt got shredded, I replaced the old Audi core with one of the Nissens ones with the 19mm hose outlets, and it definitely has a lot more heat than the old one with 16mm hose.

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I had the opposite. When I first got my Classic, the reducer was in parallel and, as it was a single point system that would run on LPG from cold, the reducer would ice up within 300-400 yards of starting it. Changed that to series and never had another problem. When I got the P38, that was the same but I got the opposite. Reducer didn't freeze but at idle in traffic the heater output dropped to nothing. Again, changed it to series and never any more problems. I suppose the difference was which flowed easier, if it was the heater, the reducer would freeze, if it was the reducer, the heater went cold. Your theory that a more restrictive heater matrix would make sense, but I would have thought that would make it worse with them in parallel?

Thor heater pipework makes getting it neat harder than on a GEMS though.

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I've stripped mine just now for head gaskets, was pondering re-routing the coolant for the vaporisor to use the 1/2" feed that used to supply the throttle body... Anyone looked at that route? The mess of T'ed pipes really bothers me, i bought new hoses and am loathe to hack them up, but series mode also gives concern as the vaporisor itself only has 10mm inlets, which feels like it would be very restrictive for the matrix.
Using the throttle heater loop seems like it could work?

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I reckon whether series or parallel plumbing is best depends on all the factors mentioned above (flow rate of reducer, flow rate of heater core, etc). But the height of the reducer is also important... The heater core is always going to be low but some reducers are mounted higher than the expansion tank (are the highest part of the cooling system). If the reducer is the highest part of the cooling system then if it's plumbed in parallel it won't see any flow at idle because all the heater water flow will go through the matrix, if it's plumbed in series it's forced to flow through both. But if we think of the heater circuit as a whole (the matrix plumbing and the reducer plumbing) we can be pretty sure that parallel will flow more through the whole than series could... It's just that as a whole we don't want 95% to go through one unit (matrix or reducer) leaving only 5% to go through the other.

I reckon a 10mm feed is a bit small for an LPG reducer on a V8, fine at low engine loads but may not flow enough for sufficient reducer heating at higher engine loads when the reducer is losing a lot of heat due to evaporating a lot of liquid gas to vapour inside. You could try it but it would be worth monitoring reducer and gas vapour temperatures at high engine loads. Most LPG reducers (with the exception of a few well knowns such as Bigas which have 10mm water feeds).