Coolant Tank Cap faulty

I have a 2001 TDI with ~340k miles. Recently, I've lost coolant, so I've had to occasionally add some. A few years back, I had to replace a cracked coolant reservoir, but didn't replace the cap at the same time.

I've recently noticed that when removing the cap after the car sits overnight, there is a vacuum on the tank/lid. Same process as a pressure cooker sealing, I assume. The center piece of the cap is always stuck in the opening of the tank, and I have to pull it out by hand, and pop it back onto the cap.

1-Is the vacuum created by the cooling of the coolant normal? Since this is a closed system, it seems normal.

2-What would be the symptoms of a faulty cap? Since it holds a vacuum, it obviously seals quite well.

The coolant leak is bothersome, and I wondered if the lid could cause any issues.

If you're relieving pressure in the cooling system after the car sits overnight you may have a leaking head gasket. Next time the car sits check the radiator hoses and see if they're hard. that is an indication that the system is getting pressurized. Also, run your finger around the inside of the coolant bottle. If it has soot on it after, that's another indicator.

1. Yes

2. Not sure! But, those caps are cheap.

Here is a "in the nut shell" experience I had 4 years ago. (2002 ALH in my 84 Vanagon)

On the first day of a very long (33 days) road trip, the coolant pressure line from the expansion tank to a gauge split on the back of the gauge. This is something I set-up to monitor cooling system pressure.

Well, I did not bother to make a repair............ drove the entire 33 days and almost 12k miles without the cooling system pressurized. The engine preformed just fine and no coolant leaks. Repeated the same trip two years later with the system working as it suppose to, engine ran fine.

But, you need pressure in the system to find the leak, assuming it is not at the cap. I'd start with a new cap.

Cap shouldn't come apart but is the system under pressure when it gets hot? If not you need a new cap and this might splain your coolant loss but it's probably going elsewhere.

The reason for a pressurized system is to raise the boiling point of the coolant. Doesn't seem necessary on these engines which are efficient with well designed cooling systems. But who knows whats going on deep in the engine on the cylinder walls or the top of the combustion chamber.

I know Andy's not recommending running unpressurized all the time but being the picky guy that I am I thought I'd mention it.

Sealed system is to keep air out.

Can't have one without the other when the fluid goes from ambient, which might be well below zero to maybe 200 or more.

spanky1 said:

The center piece of the cap is always stuck in the opening of the tank, and I have to pull it out by hand, and pop it back onto the cap.

Click to expand...

For sure it's not normal for the cap to come apart like that.

The pressure in the system is dynamic, depending on running conditions, ambient temps, etc, and a slight "whoosh" when you loosen the cap the next morning is normal.

If the cap is not working properly it may vent prematurely...then seal as the system cools, creating a slight vacuum. I'd replace the defective cap and then see if you have any further issues... collapsing hoses...etc.

Sure sounds like a bad cap. Replace it and then see how the coolant loss issue is.

Thanks all. Gonna replace the cap and see how it goes. I just noticed the relief "port" today. I always thought these vented around the cap, rather than diverting from the cap.

I have a feeling IBW is right on this one... but cross your fingers its just the cap.