Coolant expansion tank cap popped off - attempting to diagnose

I have a '99.5 Jetta ALH. While in stop and go rush hour traffic (0-10-0-10 mph) I heard a loud 'pop' and lots of steam started to eminate from under my hood. I got over and shut it off, popped the hood and the expansion tank cap was missing. Long story short my dad picked up a new cap from the dealer and some coolant and brought it to me. I filled it up, capped it, and drove nearly 2 hours back home without a hicup.
I did notice when it initially popped off, I glanced at the cluster and the water temp was a little over 3/4, unfortunately I don't know what it was just prior. Any ideas on what could cause this? I'm going to replace the thermostat today, coolant looked clean (what was left at the time) and expansion tank cap is now new. Defective relief valve on old cap? Maybe I even left it loose? (Stranger things have happened) I'm not for sure what caused it, and that makes me slightly nervous.

BTW my car has standard green coolant in it, has since the day I got it.

Maybe your cooling fan is not working

yeah, would be good to check that. I wonder if there is an easy way without VCDS...

Don't run around replacing parts until you identify what the problem is.

The dash gauge is about as useful as a dart board for accuracy.

First thing is to verify that coolant IS circulating. The Green coolant isn't really the correct stuff, though I suspect that the "proper" stuff was mostly a requirement based on the early plastic impeller water pumps (I can't speak with authority on this though); easier to just remove doubt and go with the correct/pink stuff. And if you're going to replace the coolant then, yes, that would be a good time to replace the t-stat (be sure to replace the housing WITH a GENUINE VW ONE [I've been lucky with a non-VW one, but in the future I won't take the gamble again]).

If coolant is circulating then check the temp in the fill tank using something that is fairly accurate.

I've got an UltraGauge II in both our cars. On the first display page I have programmed to display engine temps. My car's temperature sensor is flaking out, I get wild readings on my dash gauge, but the UltraGauge's display shows everything is fine.

Double-check that you don't have coolant in your oil (or oil in your coolant), as this could be a sign for more serious things going on.

Coolant is circulating, no water / oil mix anywhere. What does reading the coolant temp at the expansion tank achieve? And if it's at operating temp do you let it cool down before you open it up? Other than that one time the car does fine, which is why it's weird. I do need to check to ensure the fan is turning on though.

TDI-Guy said:

Coolant is circulating, no water / oil mix anywhere. What does reading the coolant temp at the expansion tank achieve? And if it's at operating temp do you let it cool down before you open it up? Other than that one time the car does fine, which is why it's weird. I do need to check to ensure the fan is turning on though.

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It's to check actual temps (rather than the random-function dash gauge). You might have a t-stat that is failing in the partially open state (= overheat). If your temps are fine then your t-stat is likely fine. And, as also identified, check that your fans are operating properly.

As far as when it's OK to open up the coolant tank, obviously when everything is cold/cool, in this case that doesn't serve the purpose of checking temps. Most will say to never open up the cap on a running engine, but that is stated to CYA in the event there's an overheating problem and you're going to get steaming hot coolant spewing out! As always, you can do things that are not "recommended" as long as you use caution.

What were your ambient air temps like on that day?

Lesson learned the hard way, thanks to previous owners of our '03: DO NOT mix the green and pink coolants. While either one will supposedly work ok, having a mixture will cause plastic components of the cooling system to fail.

Of your current interestingness... How does the plastic look on the expansion tank? Replacing the tank is cheap (~$30 part, IIRC) and not too hard to DIY, if it's starting to look old and potentially brittle.

UhOh said:

Don't run around replacing parts until you identify what the problem is.

The dash gauge is about as useful as a dart board for accuracy.

Click to expand...

3/4 of the way up the gauge would be about 235°F, which is indicative of a cooling system problem.

The cap includes pressure relief, and it could just be that the old cap had this part jammed up.

when it blew from excess pressure (good design, if you ask me)

coolant would boil until enough heat was absorbed in the boiling to have a stable temp at atmospheric pressure.

in the mean time, since coolant/water in vapor form is not as good at moving heat as liquid is, it would not be surprising to see an elevated temperature.

If you have a good quality cap, and everything is fine, press on and enjoy the ride.

Unless the coolant your dad brought matched what is in the car, I would advise a complete flush, and fill with the correct coolant.

not a terrible idea to do so anyway. how many miles you have on the car?

I have flushed the coolant in this car twice before, and it is clean. I haven't switched it over to pink yet. I probably will eventually, but have had no issues with standard green. You are correct though, if you add the green and there is still pink in the system (even a very small amount) you will have problems. I have flushed several of my customers cars due to this.

My guess is it was either a faulty valve in cap of expansion tank, or thermostat. I replaced the thermostat and cap both, flushed more coolant and refilled, tested the fans for proper function. Took it on a 2 hour drive today and she sung along sweetly with no hiccups. Problem solved for now

doctorkb - Expansion tank only has 25k on it

meerschm - Coolant is all the same - Car has nearly 300K