Expansion Tank Cap Bad, or Something Else? (No Prior Issues)

[zv211]

Morning, everyone. Looking for a little bit of feedback. 2004 S4 6MT, 110K, no oil or coolant loss/consumption.

Went for a cruise with the family on Saturday and all was good. Stopped for coffee and left the car running when I went inside. Maybe 5 or 6 minutes. Came out to steam/smoke under the hood; coolant was dripping out of the bottom of the expansion tank breather vent--temp needle was squarely in the middle, where it always is.

Drove home (20 min) and didn't notice any smoke continuing. Temp needle remained in the middle for the entire drive. Parked and let the car idle while I looked around. The drip had stopped, but I could still feel a little wetness at the bottom of the breather vent. Shut the car off and let it cool for a bit. Slowly unscrewed the cap and that caused more coolant to come out of the breather vent before the cap was completely off.

Emptied and removed the expansion tank yesterday. It was replaced 5 years ago, but still looks brand new and has no markings. This is not my daily driver. I don't believe I replaced the cap when I did the tank since the cap does not come with the tank and I was replacing the tank due to a crack.

I've read that the pressure seal on the cap goes bad and can open up earlier than normal and under normal operating temps, causing coolant to escape from the breather vent. The car doesn't leak anything (aux radiators done 25K ago) and I've never had a hint of the temp needle moving past halfway. I'm hoping it's just the cap, but as a long-time 4.2 owner, I also know it's rare to have an easy/cheap fix. I'm also open to doing the tank again since even a non-detectable crack can impact the pressure in the system.

All that said, it's frightening to poke around and read what could be based on others' experiences. A head gasket problem can increase pressure in the system due to combustion gases, but wouldn't I have *some* type of other issue indicating a head gasket issue?

I'd love to hear from people who have had a similar issue, or where your cap went bad and created the problem I'm describing. Thanks in advance!

[Alexztt]

If I were you and had no other issues I would just put a cap on it and drive it again. I would just monitor the temp gauge and coolant level your next couple trips, if it stays where it is you should be fine.

[zv211]

An update and additional question. Grabbed a new OEM expansion tank cap, filled the reservoir and brought the car up to temp as it idled. Temp needle stayed perfectly in the middle and I had no coolant coming from the tank's breather vent. Let the car continue to run at temp and still no coolant was escaping. However, the fans never turned on and the level in the expansion tank began to rise. Not to a crazy high level, but enough to notice. Since nothing was coming out of the vent, and the fans weren't on, I turned the car off.

I don't have any codes stored or warning lights on. I'd have guessed, though, that the fans would have come on during this experiment since the car was at temp for what seemed like long enough. I need to get to my shop to see if I can get the fans to turn on manually, but in the meantime, if anyone can suggest the best place to look for why the fans may not be turning on (sensor, relay, fuse, etc.), I'd appreciate it.

[Alexztt]

Chances are the fans did not come on because you did not get the car hot enough. Most cars will not turn on the fans at idle unless it is a really hot day or have been driven around some. If you just started and let it idle and never took it down the road the fans probably will not turn on. The coolant level would rise I would expect because the coolant is going to expand as it gets hot. No offense by this but I think you are making yourself worried. Take it for a ride and see if the fans come on.

[924Carrera]

Chances are the fans did not come on because you did not get the car hot enough. Most cars will not turn on the fans at idle unless it is a really hot day or have been driven around some. If you just started and let it idle and never took it down the road the fans probably will not turn on. The coolant level would rise I would expect because the coolant is going to expand as it gets hot. No offense by this but I think you are making yourself worried. Take it for a ride and see if the fans come on.

What he said. There is a relay that supplies power to the coolant fan modules, but it also supplies power to the coilpacks so I think you'd notice additional problems if it were failing. The modules themselves can go bad as well, but since there are two separate modules, it would be highly unlikely for both to crap out at the same time. Cooling system performance will be impacted if one is out, but the other fan set will run overtime to try to keep things under control. Of course, the fan motors can go bad as well, but since there are 2 fans it is again highly unlikely that they both died. Go for a drive and keep an eye on the gauge, and then see what they're doing, before you get too concerned.

[zv211]

I appreciate all of the feedback and don't mean to get concerned prematurely. I had a bad experience with my Grand Cherokee a couple of years back before I converted it to a mechanical radiator fan and don't feel like going through anything like that again. Relays, fan motors, coolant sensors and that stupid sending unit that's behind the bumper...

I didn't have a chance to take it for a ride yesterday, which is why I brought it up to temp in the driveway, hoping that would do the trick and fans would kick on (or not kick on and have coolant escape again). Hoping to find the time today and see where that leaves me. My laptop **it the bed, so I can't use my cable. Just left a message for a friend to see if we can get-together in the next day so I can use his Vag-com and gather more info.