Defective New Expansion Tank? Leaks at cap

I just had some work done by a very qualified shop. They dropped the motor and did the AOS, associated vacuum lines, intake gasket and a new coolant expansion tank while in there (the water pump was done 2 months ago). Now with the new coolant tank, the cap will not seem to stay tight. It leaks out of the top and coolant runs down the drain line. I HAVE CHECKED THE LEVEL COLD and filled it to the MIN line several times, thinking this only happened due to over filling, but it is not above the MIN when cold. I bought a new cap and that didn't help. The strange part is that if I over tighten the cap, it will loosen and then tighten again, like it's stripped. You know, like it goes "around" again and then tightens. This seems to be worse when it's hot. In fact, when it's hot, I can hear the air escaping. The threads on the tank look ok, and remember, I tried two different caps. Is the tank defective? Is this normal for it to be able to let fluid out of the cap (I'm thinking it is designed to allow some to escape in the event that it is over filled) but why is it continually doing so? I will say it seems to stop if the coolant level is below the MIN mark. Is it possible that the calibration of the lines is somehow off for my car? Should I just let it self correct and as long as it runs normal temps and doesn't throw a low coolant light, leave it alone? Hmm, I just realized that my garage floor is SLIGHTLY inclined, but I do mean slightly. Like, I can't leave the car without the parking brake on or it will roll out of the garage, but again, it's slight. And I pull in forward, meaning the nose of the car is SLIGHTLY elevated. But honestly, I mean SLIGHTLY.

Top it off again when cold. Then manually open the expansion valve and drive the car about 5 miles until 180+ F temp. Then let it cool off again and check the level. I wouldn't worry if after removing the expansion screw top you can still feel the coolant on your finger. Close the expansion valve and drive again.

Yep. Sounds like a bad tank. Fluid should almost never leak unless it�s overfilled (even steep inclines or declines should not matter or else everyone in San Franscico would be leaking coolant). The cap will only open up under severe pressure due to overtemp or overpressure situations. The cap should be tightly sealed and not so easy to come off.

This is crazy. Who is going to pay the Indy's labor to RR a defective tank? As for driving with the valve open, I assume you suspect air on the system. I can try it, but I don't think that's the case because would air pockets cause the cap to come loose? Also, I've put about 400 miles, including 100 DE miles on the car since the new tank and probably added easily 2 quarts of coolant over that time. I would think any air pockets would be gone by now, no?

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Did they use a Porsche part numbered tank or an aftermarket one. What did the invoice show? Often you'll see a Porsche part number where the parts supplier actually provided an "equivalent". Are you using the latest version of the cap - I believe it ends in 04. If tank is defective it should have at least a 90 day warranty - I'd think the shop would honor that including the R&R costs.

Good comments above. Confirm that cap is on tight...repeat tight. It should not loosen up. Early in my ownership I put on new cap to the 04 version due to info in this forum. First drive to temperature water was puking on garage floor on return. Let car cool then TIGHTENED the cap. No problems since(4 years). Indi should stand behind his work if it is the tank, especially if he supplied it. I have read much bad about after market tanks.

I think it's a Porsche part. It looks like it, and the cap is blue. The invoice shows 996 106 147 56 and the cap is blue. The aftermarket cap I tried is black, made in Germany and the part number ends in a 4. I cannot tighten the cap cold or hot without being able to over tighten them have it loosen then spin around again. Sounds like a defective tank. Bad threads.right? PS. I want to expand upon my earlier comments about who will make good on this repair. My mechanic is very honest and also a friend. I'm kind of wondering who's gonna make HIM whole? Manufacturer? parts supplier? I have a feeling I know the answer and I don't like the idea of him getting screwed.

It's hard to say for sure from the photo, but it looks like the part number ends in 00 which is the old part number. My upgraded Porsche cap is blue too. That being said, it still shouldn't slip after tightening. If the threads on the tank are stripped you should be able to see it. I've heard that some of the cheaper tanks have problems but the ones made in Germany, with or without the Porsche part number, are good. If you can get a good seal on the cap, open the vent valve and drive around for a few miles. You might have some air in the system and that will purge it. I'm a Long Islander too. Where are you located?

That blue cap is not the 04 version. Not sure how you managed to buy the "old" 00 one brand new, but I would get the 04 cap and try it before doing anything else. You are going to want/need one anyway.

I bought this cap yesterday and it exhibited the same symptoms: stripped and leaked. It's the upgraded 04 number. Yes, that blue one ends in 0. Odd. It looks brand new too. Here's a pic of the one I bought from Napa yesterday. Dealer didn't have any on stock, which seems weird.

"West Germany"? I'm guessing that it's pretty old. The stripping symptom must be with the tank. How much did they charge you for the tank? Do they say if it was OEM? They should replace the defective tank for free. If they just hand you a new tank, let me know. I just installed a tank in my car last week and would be happy to help you with the install.

That cap is not stripped the tank is.

Are you sure it's the cap and not the pressure release black cap. I went through 2 new after market tanks and both black pressure release caps were bad and the system would not hold pressure. I put a genuine Porsche pressure release cap on one of the after market tanks and everything was fine. I used an Airlift system to refill.

I'm 99% sure it's the cap leaking. I see fluid around the rim of it and the pressure valve is dry. Plus, there's that issue that I can't tighten the cap and have it bottom/stop, instead it goes around (both caps). I know my shop will replace it. I just feel bad that he's going to be out the labor for a defective part.

The tank is defective. Replace it. The black cap is wrong, what PSI does it release at ?, replace that also.

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Day in the life of a mechanic, I'm afraid. If he's been doing this for a while I can guarantee you it's not his first time doing 2x work due to a defective part. Bring him a case of beer or something when you pick it up if he's a friend and you feel bad about it. That will cheer him up!

Do you have a picture of the threads on the tank? Is it a genuine tank? Perhaps the cap was put on incorrectly and it destroyed the threads?

You could try to put something that�s similar to pipe thread on the threads of the tank which could help seal the cap. I mean if that tank is shot anyway.

I'll take a close up pic of the threads tonight. I looked at them once and they looked ok. Definitely didn't strip them installing the cap. It goes on fine, but at the end it won't bottom out, rather it spins around again on the last thread. I wonder if there's too much space between the last thread and the body of the tank, like maybe some sort of giant washer would help. (and I checked both caps, they have the rubber gasket inside) Not that I'm doing that sort of thing. If it's defective, it needs to be replaced.

Bizzar! ​​​​Pics would help though. The cap does have ridges the rubber gasket should sit all the way in. Odd that it spins when it gets to the end??

The tank is defective.

Agreed. Spoke w/my mechanic this morning. He said to bring it right back so he can replace it (which I knew he would of course). We'll get to the bottom of things when he yanks it out next week. I still say someone should be reimbursing him for his time. I also know he pressure tested the system before giving me back the car, but maybe that's not the same PSI has a fully heated system generates on a 95* day. I'll bring up the issue with the updated cap as well. I'll reply back in about a week to update the thread.

Was the tank a Porsche or aftermarket part?

I thought it was Porsche, but I'll confirm.

Just as an update, the tank is a German Original Equipment Manufacturer tank. It is made by the manufacturer that supplies Porsche. (That is why it was cheaper than Genuine. Retail on the factory tank is $646.29.) It comes with the same 2 year warranty that the Genuine part has. Oh, and of course my tech immediately said to bring it back so he can replace it at no cost to me. (As I knew he would.) I'm just bummed that he's out that labor time. I still think he's entitled to something from the manufacturer assuming it's clear that the part is defective. The blue cap is my original cap. Hence the old P/N. We ordered an updated one. What's the difference with the updated cap?

Good to hear. I had a tank crack after one year and I went with a Genuine Porsche part in the Porsche box, I think it was about $375 from one of the online dealers. I also recently replaced my aftermarket window regulator after 2 years with a genuine Porsche part.....I'm 52 and been doing repairs my whole life, still have a few things left to learn. Not to say you always need a Porsche box, but sometimes it does make a difference.

The cap was redesigned because the original ones leaked. I'm glad you had a good outcome.

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I wouldn't say "had" or "outcome" just yet since this will happen next week! lol But I have 100% confidence.

So - funny thing.... exactly the same. when I had my motor out - we looked at the "while you're in there" list and the expansion tank was one of the items... long story short - the aftermarket one that we got leaked - just like yours. So we got a second one. It leaked. I put my old one back in. It doesn't leak. Sometimes OEM is the only way.

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As did mine, only 3 months after installation by a shop. I lowered the engine down and replaced myself. Leak was not caused by the cap. YouTube Video of the Leak (from 2013)

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That is MESSED UP! Why can't they make a product that doesn't leak?!

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Yeah, i always wonder the same but for these - window regulator - oil pressure sensor - ignition switch - headlight switch - door lock actuator - headlight wires - taillight wires - starter y-cable

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I seen reports of AOS's bad right out the box. That's a really gotcha.

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I'd just note that reading through this thread, you have an atypical problem. Some tanks (like mine, replaced by the PO), develop small cracks that leak like the video above. Others have the cap fail (like mine), where the cap doesn't hold pressure and coolant comes out once the system is pressurized. Luckily for me, this was an easy fix. You seem to have a cap that doesn't fit. Since its supposed to be OEM (but not Porsche) I would guess that the manufacturer intended for a different cap than Porsche supplies. Probably a cap with a different design altogether, given that the threads appear to be too short. Point is, I would discuss with your mechanic replacing this with the genuine Porsche part and not just replacing it with the same part he gave you in the first place.

You think the threads are too short? I'll measure them. Anyone have one handy to measure and share those numbers?

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Well, I'm speculating. As far as I know, the failure mode for most people is not "the cap doesn't stay tight." The failure mode for the cap is that although the cap is still tight, it doesn't hold pressure. I am assuming there is something wrong with the threads or the neck of the tank such that the cap can't be fully tightened. The cap isn't your issue, since you've tried more than one. Measuring the the number of threads on the neck may help if it's different than the Porsche part for that convo with your mechanic.

Yea, the threads/neck are/is clearly bad. I just don't think it's too short since it gets tight, but I can go beyond if I keep turning and it slips. I'll see what the mechanic says for sure. And I'm sure everyone (both the mechanic and supplier) will want to check this issue before installing the new tank of any brand.

I realized I never circled back here with info that may be helpful for others in the future regarding replacement expansion tanks... I had mine done when the AOS was replaced as preventative maintenance (along with a new water pump, just for grins). Old one was not leaking. New German part was installed by a very highly respected indy. He pressure tested it and the system was fine... that is, until I got it really hot. The fluid was perfectly filled to the cold line and checked A LOT (each morning during my issues). Yet once it got really hot, it seemed the coolant would leak from the cap a little bit, and then the cap would also be able to turn past fully tightened (like a medicine bottle safety top). When cold, it was fine. I replaced the cap, twice. OEM. Same symptoms. Mechanic replaced the tank under warranty. Same symptoms! We suspected that the system just might be operating normally for a system that was a little over filled (even though it's not). If over filled, once hot, the cap is designed to allow pressure/fluid to escape to avoid bursting something like a hose. That dribbles out onto the "balcony/shelf" of the tank where it runs downhill to the drain hole and out the hose behind the rear wheel like rain gutters on a house. So what did we do? We let it naturally level off without constantly topping off the tank. Guess what? PROBLEM SOLVED. So my tank has a "new normal." It's just a few ounces lower than before (see pic) but for the rest of the summer it's been fine. And fyi, it NEVER ran hot during all of this testing. So I don't know if something is slightly different on the new tank design (remember, two German made OEM replacement tanks did the same thing) or what, but all I know is it's fine now. Just thought I'd share. Happy cold weather motoring.

One thing I've discovered is that an "OEM" part may not be the same as a genuine Porsche part even though the OEM may be the Porsche vendor. I had both a new RMS and a new AOS fail recently that were OEM parts. Subsequently replaced with Porsche parts and no issues...yet.

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I was thinking that myself. But seems odd that the same vendor/manufacturer would make two different expansion tanks, with different specs, for the same car. On the other hand, maybe there's a reason the Porsche part cost over $600 when the other one was around $350 (don't quote me on that figure, I don't recall exactly, but it was a less for sure). I assumed that was just Porsche mark-up, but who knows. My indy said he's installed MANY DOZENS of these tanks and never experienced it, despite my experiencing it twice on my car. So maybe there's another factor at play we're not considering, some other variable. All I know is that it works now. Dry, runs normally, etc with the "self correcting fluid level" design. lol

I had the same conclusions as you in another thread from a few years ago. My tank was leaking and I did replace it with an OEM tank.....and it was all good after that, but I think leaving some expansion room in the tank is good practice and prevents the system from over pressurization during extended runs.

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I ordered my expansion tank directly from a Porsche dealership. They crossed referenced the tank with my VIN. I got the tank and coolant level sensor for $190.00.

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$190?!!! For a Porsche brand 996 tank? Wow is that cheap!

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Just for clarity going forward, the MK-I Porsche brand tanks are in the $600 range while the MK-II tanks are about $140..

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AHHHH! Thanks for that info.

OP, the thread problem is definitely due to the non-genuine coolant tank. An example here .

For 996 in general, OEM=bad and OE=genuine=good

Some things can be genuine and OEM - for example clutch master cylinder I bought from Sachs had the Porsche part stamp but the part number ground off! However in this case I agree - go genuine on the tank especially for such a critical part - "OEM" I feel is a term that is very loosely thrown around by parts suppliers...

If your cap is still turning past tight to loose again, I�d say you still have an issue. This is not the way this is designed to work. How do you know it�s pressurized to Porsche spec? Pressurization raises the boiling temp, which helps the coolant work obviously. Overheating on in these cars are no joke. They have weak spots already, they don�t need help from us. It�s super expensive, but I�d go with a true OEM.

I tend to agree that the cap shouldn�t turn past a certain point. In fact, the day I bought my car and drove it 200+ miles home, I lost the cap somewhere along the way (I suspect it wasn�t tight enough and rattled off). I bought a new cap (version 04 is the newest, correct cap) from a Porsche dealership that tried to charge me $75 (LIST Price). Once I negotiated a resonable price, the cap went on and is tight. To be honest, if I were ever to have to replace my coolant tank, I�d fit a new cap to it before installation to make sure it all fits...

The cap doesn't turn past right any more. It only did that when it was over filled and not to allow fluid to escape. Both my original OEM cap and two new replacements (one OEM brand, one genuine Porsche cap which is what's on it now) all acted the same way. And based on the fact that there's a drainage route built into the tank, it's clear to me that the cap allows excess fluid out when overfilled and hot. The mechanic pressure tested the system properly and it held pressure for a long time. He checked it to spec initially (10 minutes or something) and on the second tank he checked it for a long time. Finally, note that the car NEVER ran hot on me. I swatc it to make sure, especially with this going on.

Would a bad tank leak over to the air box and alternator? I just replaced the cap and noticed a slight coolant smell. I opened the lid and saw this.

No, that's from one of the hoses or clamps leaking while under pressure (ie, higher RPM) and then spraying. I had a factory clamp fatigue on me and it caused that. If the tank over flows via the cap, it's designed to allow it to drip around like gutters on a roof, to a hose that drains behind the left rear tire. In this pic, you'll see how it's wet around the cap area, but the "gutter" directs coolant around to the lower left area where there's a drain hole. You'll also notice coolant stains from my prior leak on the hoses when my clamps weren't sealing properly. (it's all fixed now)

Thanks I'll check out the hoses and clamps. Dave

I found a bit of coolant below the three way coolant tube. I thought it was replaced during the PPI. I know they told me that the coolant smell I noticed was coming from there. It doesn't look like a new hose to me. I can't get a hold of the mechanic today to double check. Oh well, I ordered a new hose and figured I might as well go for new expansion tank too. I don't think mine is leaking but it served its time.

My thought on the expansion take as preventive, based on my experience, if it ain't broke, don't fix it. That's probably contrary to most stuff/people here, I know, but my original tank was working fine until I put a new one in. I forget, what happens if a tank fails? Does it just leak and then you do the job then or is there catastrophic failure that leaves you stranded? That being said, I did mine at the same time I did the AOS and water pump, so it's not like I was going to drop the motor and put it back with the 18 year old tank in there.

OEM dealer tank and cap about $260.00. /X3

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I think that's for 996.2, not 996.1

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Depends on where the failure is. If it�s just a crack at the top, it might just leak. That is, until it gets really hot out, and that�s when pressure can build up and cause a really big leak. If it�s at the bottom, leaking coolant can increase. Im with you, though. If it looks good, ain�t broke, don�t fix it.

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^^This is correct^^ 996.1 tank (Genuine Porsche) is $663.95 or some silly price like that... SunCoast has one for around half of that - they say it's genuine Porsche, but I'm not so sure.

Just to follow up, more trouble. I’m obviously not driving the car and I noticed the that fluid was continuing to drip. It’s the water pump. Ordered a new one and hope to get it installed this week. Maybe I should have realized it sooner. Anyway, doesn’t look like it’s too difficult.

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looking at your photo, I'd get a new belt as well. my new wp, low temp thermostat, belt and extra wp gasket just in case will be here Monday.

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Belt is two weeks old. There's just a lot of dried coolant I guess in the engine compartment. Shame too, it was super clean when I bought the car. I've decided not to do the thermostat. I'm wondering if I should or not. Does it fail often?

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Yes, they (I meant water pump) fail. It's a maintenance item. The plastic blades can crack and be circulated throughout the engine. There's a service interval, something like x years and 60k miles, I forget. It's on this forum somewhere. Had mine done since the car is a `99 even though it only has 37k miles. It wasn't too expensive. Did thermostat also (went with stock since this is a street car in the Northeast) and of course it's fresh fluid. The car has now had every single maintenance item done except for timing chain work, which my mechanic said they sound fine. But every fluid has been changed, AOS, IMS, RMS, clutch, spark plug tubes, coils, water pump, belt, coolant tank, entirely new brakes (rotors, pads, fluid of course), tranny fluid. I think the washer fluid is old. That's next ;-)

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Not sure if they fail often, but the low temp one allows engine to run cooler. That helps with health of the m96 according to the info I've gleaned from this board.

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I read that too. I get the IMS problem and I get why someone would be proactive in replacing it with an after market product. Things like a thermostat that opens at a lower temperature or a spin-on oil filter adapter as being improvements I'm not so sure about. No one seems to have a real answer on why either of those would be a benefit. If the standard thermostat keeps the engine at the temperatures Porsche designed it to operate, why would I change it. Same thing for the spin on oil filter without bypass. I just don't see the benefit. Besides, cutting open a spin on oil filter is a pain. I like having a canister type filter with a replaceable element.

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Up to you. I have the original one in my car. You are right there, though, the thermostat hose is right next to the water pump there, you already might have to pull the hose off to drain the old coolant, etc. If you do replace the thermostat, the low temp one is what I would get despite the possibility that it may not lower your temps in extreme hot weather (if the thermostat is already wide open because it�s hot outside or you don�t have much air moving through your from radiators - such as traffic - your temps are going to be more related to the front radiator efficiency). It�s not going to do any harm, unless you always take short trips and only in the winter without frequent oil changes. In general, the low temp stat will protect your engine against a certain type of thermal condition whereby you accelerate hard and fast when the engine is already quite hot. The low temp stat gives you about 20 degrees of margin in that condition, which could be priceless.