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Discussion Starter · #1 ·
ever since I bought my rabbit used in 2010, and after replacing my engine, and coolant expansion tank among other things, I have replaced the expansion tank cap seemingly incessantly over the years. perhaps twice a year most years, the caps fail, and I start losing coolant along the sides of the threaded hole in the expansion tank. I have tried just about every brand of expansion tank cap, from the $12 German auto parts ones to the $3 or $4 dollar Napa ones and everything in between. they seem to work fine for a couple months, and then that sweet smell comes trickling back. I can't remember how many caps iv been through, or how many jugs of coolant at that. it drives me mad. if I could just find that one last cap, precisely machined by hand out of gold and silicon, I would be willing to drop $xxx.00 if it meant I could just stop this awful cycle. please help rabbit people, please!
 

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Get a combustion leak tester. It will check the coolant for combustion gasses. I can speak from experience, a blown head gasket, cracked cyl head can cause over pressuring of the cooling system resulting in a failed coolant tank cap.
I would check that, and also do a cylinder leak down test. You might find something else is the culprit for having to replace the cap.
What coolant percentage are you running? To weak and I have found that it boils the water out of the coolant.

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As I was reading the post I thought OK, new engine so maybe not with a head gasket problem. I was thinking that there must be something you are doing with the antifreeze mix. I do a 50/50 and never had problems with hot or cold temps.

There is a way to take the caps apart and make sure the little hole in the top is allowing air in when it should and holding back the pressure when that is needed. I disassembled the one on my Rabbit and found it to be pretty dirty, even though it was working I was getting concerned about it.

Even if it is a new to you engine I would still be doing a compression check on it just to be on the safe side. That can be done real easy too. Just take one of those mechanics gloves or latex gloves and put it on the expansion tank in place of the cap. Do this prior to starting it up cold. Then start the engine gentleman! In about a minute do you see the glove starting to inflate? Likely that is caused by the air coming in from the cylinders and thus you have a leak on the head gasket.

Food for thought or an excuse to be under the hood.
 

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As I was reading the post I thought OK, new engine so maybe not with a head gasket problem. I was thinking that there must be something you are doing with the antifreeze mix. I do a 50/50 and never had problems with hot or cold temps.


Even if it is a new to you engine I would still be doing a compression check on it just to be on the safe side. That can be done real easy too. Just take one of those mechanics gloves or latex gloves and put it on the expansion tank in place of the cap. Do this prior to starting it up cold. Then start the engine gentleman! In about a minute do you see the glove starting to inflate? Likely that is caused by the air coming in from the cylinders and thus you have a leak on the head gasket.

Food for thought or an excuse to be under the hood.
This test is a winner for Head Gasket Failures, and they can occur at any time, as I used to run a Diesel that would blow one every 6months because of the bolt issue on the engine.

The CAVEAT is that you tie the Glove tightly over the open Expansion tank.
You have to Start the engine, then REV it above 3000K for 45-50 seconds any longer and the Glove will Auto Inflate from the heat and steam of the a/f.
If the glove inflates and gives you the finger, then you have a blown head gasket. On Gassers it can either be a blown head gasket, or a leaky water jacket oil cooler.

Another indication of a head gasket failure is that your Hoses will Balloon and be hard to squeeze but once the engine stops your Hoses return to normal.

Easy to see on a running engine that is warm.

If you do have a Head Gasket failure then I would strongly urge you to install the ARP stud kit.
These kits are re-usable and they when installed usually won't loosen and take care of the bolt loosening issue that the 1.5 engines can be prone too. If when I had found this out, (back in the Pre-Al Gores invention of the internet) I would if installed these. The Head Bolts are not re-usable they are a one time use only. They also have to be re-checked for torque at about 500-1000 miles. I suspect the best thing is that I got really really quick at changing the head gasket. I will also suggest that you place the engine in time prior to diss-assembly.

Causes of the Expansion tank issues (and Caps) is two fold, a blown head gasket or Crappy parts.
I just went through this on one of my Cabriolets, where the Cap wouldn't tighten down.... new tank and cap later (bought as a pair from O'Reilly's) worked.
 

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Discussion Starter · #5 ·
I did the head gasket dance a couple times with the original 1.6D back when I first bought the car in 2010. the block had already been helicoiled, so I got into custom machining threaded inserts... what a nightmare. in 2012 I decided to spring for a 0 mile 1.9 naturally aspirated diesel complete conversion engine called 1Y, designed for the 1990's European Jettas if I remember correctly. its been a charming upgrade. im not super concerned with head gasket issues as I have been babying this new engine ever since with regular oil changes and modest driving habits, but I do love the rubber glove test, I may do it anyway.

the expansion tank is a new(in 2012) Meyle, and I even got a second one thats on the shelf in my shop out of fear that the threads on my current one were the culprit to my leaks. I have found that after a few months, tightening the cap becomes a delicate operation that involves avoiding over tightening, as they start reaching a maximum torque (by hand) and beyond that get loose again as if they were stripping. new caps don't do this, they don't start skipping a thread when you go too far, they just get too tight to go any further, which is what I like to feel to know im done tightening.

the coolant I run is 50/50, always. the climate im dealing with these days in the Adirondacks is averaging between 0 and 25 degrees Fahrenheit. hottest summer temps average between 65 and 85.
 

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Just went through this on my 92, the new Cap lasted but a year before it started leaking under pressure, and it was TIGHT....I have seen on replacement jugs the same issue that you have described, the Threads don't seem to be as well defined as a OEM and they don't feel tight and or are a pain to get started correctly.

The leaking cap was diagnosed by using water, as I heard a hissing when the engine was off and cooling down. I sprayed water on the coolant level sensor, then the Cap, when the water hit the cap the hissing stopped......new cap no hissing and it was a German Cap less than a year old that I was replacing.
 

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This replace the replacement thing is not just confined to Radiator caps. Seems to me most all the parts we get our hands on nowadays carry zip to few warranty days. What happened to good quality parts? One more thing to add to the never ending list of variables now. Can't rule out that part you just replaced with a good one because it too might have gone bad.

More mechanics nightmares, just what I need.
 

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Design by accountants, lack of quality control, Chinese made parts that while in spec, aren't up to the tolerances that VW OEM'ed.
I went through this on a new Expansion jug a couple of years ago, mine has a leak, ordered new, it split the seam 2 days later, and well I opted to buy one from the Dealer, and it lasted for quite a few years.

It's like lets redesign all the metal coolant flanges out of Plastic. I would go through one to two of the various flanges until I found them in metal in a wrecking yard.
 

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Discussion Starter · #9 ·
Just as an experiment, I hooked up my new meyle expansion tank that has been on my shelf for 8 years, and I sealed it with a 6 month old cap that had been badly leaking prior. The old cap works fine with its new tank friend, no leaking as of yet!! This leads me to believe that the threads on the expansion tank, which look pretty fine on the tank I just replaced (except for the very first one that tapers to nothing at the top) are indeed the culprit of this chronic sealing issue.

Lesson: baby your expansion tank threads everytime you take the cap on and off, but especially when putting the cap on, by making sure the cap is going on properly, feeling the threads mesh with absolute ease. The first thread is more important to achieveing the proper torque for the cap’s sealing function than I ever imagined, and it’s the first to be deformed. Perhaps a creative alternative to replacing the tank could be to hack saw the top centimeter off the tank fill hole to get back down to good threads that won’t skip. The trick there would be tapering the first thread and making the lip nice and smooth and flat after the hack job, to mate properly with the squishy o ring on the cap.
 
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