Cam belt and Water pump

[SuperV8]

I'd think that pulling a vac on the cooling system might annoy a compressor unless you change the oil a few times after using it for this, as the pressure drops below ambient the coolant will gas off and that will end up in the compressor oil.

Surely small garages and DIYer will continue to use the old method which will include easing off every pipe in turn to try to let any trapped air escape, I'd think by doing that you can be sure that none of the pumps will run dry, okay it will take some time to clear all the trapped air which is not an issue for a DIYer, these vac coolant systems will allow a dealership to get it right first time, well almost as many new VW Group models seem to continue getting rid of air for a few weeks after delivery new.

Or is there more to this?

Interesting point about the gassing off of coolant, my old compressor has never had an oil change so maybe a good time! Do you think running it in free air after use will clear-out gasses from the compressor crank case? I will be flushing out the coolant system several times before so hopefully minimal coolant will be left - but might be a good idea to have a water trap or coalescing filter on the inlet to the vacuum pump!

The official VAG method for working on these cooling systems is very thorough and the system is so blooming convoluted I think gone are the days of just easing and burping a couple of hoses to the radiator. Even after vacuum filling they still want you to run a bleeding process via diagnostics! so surely there 'must' be a reason for being so thorough?

I have heard BMW had lots of failed electric water pump issues from incorrect coolant filling! that's from a BMW master tech friend, and the same guy then went to VAG as a master tech and when I told him I was planning on working on my coolant system he pulled a worried face! VAG even say to use distilled water for coolant system flushing!

Is there more to it?
Who knows? maybe:
VAG are just trying to cover there backs from warranty issues?
or being overly cautious?
or wanting to keep all the servicing money for themselves?
or maybe these cooling systems are just so bloody complicated - (my TDI= 2x electric pumps, 1x electro/mechanical pump, lots of one way valves, lots of heat exchangers, 3x coolant flow paths and of those there are multiple flow paths within each of those 3 main circuits!) under the engine cover is a spaghetti mess of pipes - the thought of trying to manually bleed all that lot must be virtually impossible, and an airlock in any of that mess could drastically reduce coolant flow.

Yes i'm sure lots of people will just fill up the cooling system and try and burp some hoses and run the engine whilst topping up the coolant - and this may work fine? but this will i'm sure increase your chance of having coolant problems.

 

[SuperV8]

The water pump on my 2018 Leon FR 1.4TSI (150) was replaced under warranty at 15 months & 16K miles as it was leaking. I never realised it had a plastic body - stupid attempt at cost cutting/weight cutting.

Completely agree from the end user point of view but they (VAG) will just see a spreadsheet with a large multi-million saving for using a plastic pump casing vs some warranty claims! And those warranty claim costs will just be passed on to whoever supplies these plastic pumps.
we can't always assume new versions of cars are better versions, ALL manufactures and ALWAYS trying to cut costs, sometimes successfully so the end user doesn't even notice - sometimes not so much!

 

[RUM4MO]

@SuperV8, coolant getting into the compressor oil, I would not think that you could drive it out by running it with free oil, yes the body and oil would heat up, but even driving only water out of oil is not easy, so I'd be replacing the oil a couple of times.

The value of the achieved vacuum will not be too important in this application, especially when as suggested, some of the system will not have been designed to operate under vacuum, this need has, I'd think, originally appeared to, as I said ,get the backfilling right first time and so the result being very predictable - only now that most new IE engines has 3 coolant paths does this what was just a useful method to aid backfilling, become a lot more of being the "best practice" way to backfill a coolant system. I would add that as well as venting as much as possible, using a diagnostic tool to manually run the electric pumps for short periods, and in some cases open valves must also be carried out. Remember the mixed coolant being backfilled into these cars will not have been treated to remove air in suspension, so some air will settle out - and get purged through into the header tank and drop the coolant level.

The need to use only distilled/deionised water will be more important for folk in certain areas, where I live our water is very soft, but buying quite lot of distilled water once every 5 years is no big issue.

I do accept that replacing coolant on modern cars is not just a case of "dropping it out" and "dropping new coolant back in" - so if anyone is unsure, I'd say avoid doing that job.