Vacuum Filling Coolant

I am curious to know - given the quirky temperament of the hijet's coolant system - whether it is useful to use a vacuum purge when refilling and bleeding all the air out of the system.
The reason I ask is that, although my pickup never overheats (and barely gets hot at all according to the temperature gauge) I suspect that there might be an air lock in the idle control valve as it idles at around 1600rpm, there is heat in cab blowers and the lower hose gets hot too.
Ever since refitting the lpg reducer, the gauge barely gets to 1/4. It's not really a major issue for me but a niggle and would probably help fuel economy if I got it sorted.
I'm not really that confident when it comes to mechanical issues and wanted to know what others thought.
Also, how would the purge work when the thermostat is not open?? Is it safe to say the coolant would remain in the engine as it cools and thus not be purged?

I like addage, if it works leave well alone! My temp gauge hardly rises these days, changed sensor, thermostat, gauge and still same so I leave well alone for the moment as it drives well and I get good mpg. I get heat like yourself and no loss of fluid. I intend to test the voltage to the gauge at some point but as it all works it’s not high on my list.

No idea about a vacuum purge but can the idle speed not be adjusted?

It should fail an MOT under:-
"is idling at a speed clearly above its normal idling speed"

I don't believe in vac filling as it crushes all the hoses flat and the coolant system and its pipework was never designed to be below atmospheric pressure.

but it should hold a high pressure relative to atmospheric pressure.
Surely it is designed to withstand the pressure gradient?
Or maybe i'm clutching at straws

Any system designed to operate below 1 atmosphere, e.g operate under vacuum conditions, has reinforced hoses that do not collapse. 40mm coolant hoses flatten by the vac pump, and its where these hoses join to rigid steel tubes is the stress problem. Folding hoses flat causes them to crease or split on the folds, maybe ok for a new car but 20 year old hoses need to be treated gently.

Hi HJ, in your extensive knowledge any idea why my temp gauge hardly rises and when it does after stopping (but does not go wildly up) goes down with fan on (obvious really) and also with just a few more revs (which I suppose is obvious). Had no other issues, as I said new temp sensor and different gauge does same. Just wondered what your opinion would be...leave well alone? Been like this for last 6,000 odd miles, including trip to Scotland, IOW and Cornwall.

I think I would start by pointing an infra red thermometer thingie at the rear of the cylinder head where the temp sensor is.

That way I would know if the coolant is really the typical 78 to 85 degs tickover running temp.

Or if its just a gauge/reading type fault.

If the water is the correct temperature, then I would probably explore the wiring and connectors for the temp sensor.

If the water is not the correct temperature, e.g. it is actually very cool, then the gauge is not incorrect and its actually a coolant problem.

The fact that your fan kicks in automatically says the coolant gets hot enough to trigger the fan switch temperature in the bottom hose. I can't remember now but I'm sure the fan switch kicks in at about 95 degrees or something.

So I would speculate that its a wiring fault to the temp sensor. If the wiring has a high resistance, e.g. a poor contact or corroded wire, then the gauge will always read lower than the truth.

Thanks, will have a look with the infra red sensor, got one of those, didn’t think of doing that...doh!
I have a feeling it a wire issue or a bad earth issue as it goes down with revs as it never did that before, fluctuates quite a bit.
Presume your restoration work on the house is finished? Good to have you back on here.

Just seen this topic. Although I have no comment on your problem I used a vac bleeding system to fill mine after head gasket job and it worked a treat.

Interestingly, the only time the van did overheat and I was able to notice it and pull up in time the coolant boiled into the header tank and all the hoses became crushed. It was because the fan relay switch had gotten corroded and this was two years ago.
Pickup hasn't overheated since.

Regarding vacuum pump purge and fill: contrary to Steve when I tried a vac on my Porter it simply didn't work. I was not happy with the way the hoses collapsed under negative pressure and when I switched to fill it only put back in about half of that removed. Sent the unit back and returned to the traditional method of filling from a height greater than the heater matrix and a good bleed. On paper these units appear to be spot on for our eccentric cooling system; perhaps others will have better luck but I'll stick to the 'old' way.