Keep getting air in cooling system

[DinoSorcerer]

This is driving me crazy. B6 1.8t.

I had no heat at idle, very little under load, and eventually, NONE at all. The reservoir was bubbling whilst taking the cap off, and eventually spilled over while I was driving down the highway from too much pressure. A trusted mechanic told me to replace the heater core, and so I did, along with the coolant tank and cap. Then I was having trouble getting the air out until it finally burped and sucked a ton of coolant in. Success! Then I had heat! However, one week later now and I've already lost most of my heat at idle, once again.

I know I'm getting air into the system from somewhere and I have to drive 500 miles in 2 weeks. I don't think it's a head gasket because I'm not losing much (if any) coolant, the car idles smoothly, and I don't have any excessive exhaust smoke. I'm trying to look for leaks, but I'm not having much luck visibly locating any. Any ideas?

[StriktlyAudi333]

Make sure your coolant return line isn't clogged/not returning for whatever reason. Disconnect that top line to the tank (remove it from the tank) and have someone idle the car at 3000rpm while you aim the hose at another buddy. It should shoot out. If not, then that's your issue. I've seen it happen more than twice.

[DinoSorcerer]

Worth a try. I'll check that tonight and update.

[incep]

Well, the obvious place to look would be to the left rear of your engine, at the coolant flange. Check for drip marks on your downpipe. It's a known bad part but an easy repair. If not, look closely for wet spots and crusty white scaling all around the loop.


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[Dillon Clark]

Make sure your coolant return line isn't clogged/not returning for whatever reason. Disconnect that top line to the tank (remove it from the tank) and have someone idle the car at 3000rpm while you aim the hose at another buddy. It should shoot out. If not, then that's your issue. I've seen it happen more than twice.

Lmao, I love how you said aim at another buddy!

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[Kyüshutsu]

Any update on testing the coolant return line Dinosorcerer?

[monty2982]

Any luck on getting heat? I'm having the same issue. Did a pressure test. Fine. Did a leak down test. Fine. Flushed out heater core, replaced coolant flange, replaced j plug, replaced thermostat, spent multiple hours bleeding air out of the coolant system. Get the heat to blow hot at idle after bleeding air so I think I'm good. Wrong. Go drive and slowly the hot air dissipates. Pop the hood and I'm able to bleed more air out of the system. Every time I drive I have to bleed out more air.