Strange cooling issues, help me wrap my head around this.

87 Scirocco 16v, so reverse flow obviously.

Book procedure for refilling coolant says fill, run to temp, cool off, top off.

Shade tree ways is the same, but it says fill, add while running untill almost at operating temp, shut down, allow to cool and top off.

Both of these cause overheating immidately.

I found in an act of desperation, squeezing the upper hose forces huge amounts air. Keep squeezing and adding coolant until it stops popping significant bubbles.

Car runs under temp while moving, a bit warm while running still. System seems to work, if maybe a thermostat staying open.

I think I found the reason to why I'm getting air in the system. A coolant flange off the head is contacting a serp belt tensioner I added from a different engine. It seems to have rubbed a hole in the side of a coolant bypass hose. Kinda weird in that its seeping coolant and allowing air in, but how its rubbing the tensioner, the pressure applied to the hose keeps it from just blowing completely.

It just seeps coolant and aerates the cooling systems, ok at least I figured out where the coolant air is coming from.

But why would this manual priming of the system allow it to be semi functional, while the normal ways of bleeding air don't seem to be doing anything.

I have a new coolant flange with a different offset so it won't contact any longer and a new bypass hose, but what would make the normal way of bleeding fail so badly? Keep in mind the hole found on the hose is pin sized.

In reply to neckromacr:

So you've got the euro front water neck now? That's what I had on my ABA 16V. Had to have it to clear the serpentine unit.

I think the hole in the hose is causing it to act like an overflow tank, that is it fills when the coolant is hot, then draws coolant back into the system when it cools. Except you have an opening to the air, so it sucks in air when it cools.

I've found the real key to filling any VW system and getting the air out is to leave the top rad hose off at the rad end, and fill the res SLOOOWWWWWLY. Painfully slowly. Like just a trickle. At least 5 minutes to pour in a 4 liter jug. Then when the top rad hose starts to drip coolant, shove it back on the rad and then run it and just keep adding until it stops burping. It's a slow process, but I've never had any serious issues bleeding them.

doc_speeder wrote: So you've got the euro front water neck now? That's what I had on my ABA 16V. Had to have it to clear the serpentine unit.

Yeah, ABF neck from Tectonics Tuning. It all started with the ABA tensioner swap that "looked so straight forward" of course the instructions were based around swapping it into a pre-ABA 8v. It has been a trail of errors.

I think the hole in the hose is causing it to act like an overflow tank, that is it fills when the coolant is hot, then draws coolant back into the system when it cools. Except you have an opening to the air, so it sucks in air when it cools.

Exactly what I was thinking, I forgot to mention the very bizarre nature of the coolant overflow tank erupting with coolant when dead cold. This was before the pinhole made itself bigger and well known. It was the first symptom, but the air getting trapped in the system was trying to bubble its way out, but had no means to escape.

It was just really strange to see before I knew why it was happening.